Yield of hop cones and alpha acid in relation to the age of plants and weather conditions in year 2015, 2016, 2017

Summary

An analysis of weather conditions (precipitation, temperature, solar irradiance) in southern Saaz hop production area – weather station Heřmanov for years 2015, 2016 and 2017, data compared with long term average from same station (time period 1975 - 2016). On hop producing area of 33 ha in years 2015, 2016, in southern Saaz hop production area a statistical research regarding: a) relation of hop yield on age of hop culture growth
b) relation of alpha acid content on hop culture growth age c) relation of alpha acid yield on hop culture growth age. In year 2015, 2016, 2017 further statistical examination carried out regarding: d) relation of solar irradiance and hop yield, e) solar irradiance and alpha acid content.

Key words: Hops, yield, alpha, age, weather

1.         Sources and methods

Yield of hop cones was measured by weighing dry cones after drying in the hop kiln-dryer while separating individual blocks of trellises made of a single or multiple trellis constructions in the same location if the harvest could not be separate due to the technology of the harvest. The measuring was carried out in the year of 2015, 2016, and 2017. The weather data were compared always from the months of January through September because the data for months 10, 11, 12 were not yet available in the time of the analysis. Next reason for the exclusion of said months is that this period is not relevant due to the fact that further research discovered low impact of weather conditions of these months on yield. The data from months 1-9 are crucial for the formation of yield and quality of hops. Yields were measured exclusively from the Saaz variety of hops mostly originating from meristematic culture with the exception of a single location which had plants from classical vegetative multiplication of plants. (Without viral treatment). The age of plants range from 1 year to 40 years.

Content of alpha acid compounds was measured always immediately before harvest, the dates are 22. 8. 2017, 22. 8. 2016 and 20. 8. 2015. Alfa acids are measured in 100% dry plant matter. The determination of alpha acids and gathering of samples was carried out by ZKULAB s. r. o. The samples were taken from the height of approximately 5 metres above ground.

Correlation and regression – dependencies and relations between measured elements were examined using methods of correlation and regression. It was investigated whether the two examined elements link to each other and to what extent. The value of the correlation coefficient determined the greater of lesser relation of said elements. For clarity the data of Alfa acid content and yield were transferred into percentage.

Climatic and weather data – were used from the years 2015, 2016, 2017 from station Heřmanov ČHMÚ for precipitation and temperature and from station Kralovice ČHMÚ for solar irradiance. This data was compared with long term average of precipitation and temperature from the 1975 – 2016 time period from Heřmanov-Kněževes station and solar irradiance was compared to long term average of Kralovice station from 1975- 2016 period.

Yearly total solar irradiation. - An evaluation of the locations (hop trellis blocks) was carried out to establish the relative value of the yearly total solar irradiation of the location. Firstly, the area of these individual locations was measured (m²) (inside of a single hop trellis block) with regards to its orientation in the cardinal directions and its inclination in degrees. An average value of the yearly total solar irradiation was then calculated for each individual location. Finally, a weighted average of the yearly total solar irradiation was calculated for each hop trellis block.

The source for the quantification of the inclination and orientation data was the digital topographical model of the Czech Republic of the fourth generation (DMR 4 G). The model originated from the aerial laser altimetry of the Czech Republic, which was carried out in years 2009 – 2013. DMR 4 represents the projection of the earth surface in digital form of a set of X, Y and H coordinates of discrete points in a regular lattice of 5 x 5-meter squares. The H coordinate represents the altitude above sea level with a mean error of 0,3 meters for bare terrain and 1m for covered terrain.

2.         Goal

2.1.     Analysis of the course of precipitation, temperatures and solar irradiance in years 2015 – 2017 and their deviance from long term average 1975 - 2016

2.2.     To carry out a statistical examination between:

1)    Age of hop fields and hop yield, alpha acid content and yield

2)    Solar irradiance a hop yield, alpha acid content and yield

2.3.     Outcome of this paper should be outside of the above mentioned goals a basic direction and content of further analyses.

3.         Results and discussion

3.1.     Weather conditions in year 2015, 2016, 2017 in comparison with a long term average of Heřmanov - Kněževes station from 1975-2016 period.

3.1.1.  Analysis of temperature and precipitation of year 2015-2017

Tab. 1 – Monthly precipitation in years 2015 – 2017

Precipitation of year 2016 were only 68,53 % of the long term average in the 1.-9. Month period with only just 127,6 mm in comparison with the average. The year 2016 with 376,3 mm was 92,79 % of long term average with the deficit of 29,2 in comparison with long term average. In the year 2017 the precipitation was 365,6 mm which is 90,16 % of long term average with the deficit of 39,9 mm. (Chart No. 1).

Tab. 2 –Deviance of  monthly precipitation from 1975 – 2017 average

Tab. 3 – Average montly temperatures for years 2015 – 2017

Average of examined months 1.-9. Compared to long term average of 1975-2016 showed the difference of 110,39% for year 2015, 109,45 % for year 2016 and 103,11 % for year 2017. There is an interesting trend where the averages start to near the long term average (chart No. 2).

Tab. 4 – Deviance of monthly temperature averages from long term average of 1975 – 2016

3.1.1.1.           Deviance of temperature and precipitation (months 1-9) of year 2015 from long term average of Heřmanov station 1975-2016

Chart 1 – Deviance of monthly precipitation and monthly temperature average of year 2015 from long term average 1975 – 2016 from Heřmanov station

Year 2015 (graph no. 1) was characteristic by its accumulation of above average temperatures, especially in the for hop growth crucial time period of July to and at same time accompanied by cumulated deficit of precipitation in the time period of May to September. The development of temperature and precipitation during the year of 2015 could serve as a typical combination of weather effects that predict a minimal yield in hops.

3.1.1.2.           Deviance of temperature and precipitation (months 1-9) of year 2016 from long term average of Heřmanov station 1975-2016

Chart 2 – Deviance of monthly precipitation and monthly temperature average of year 2016 from long term average 1975 – 2016 from Heřmanov station

Year 2016 (graph no. 2) can be characterized as above average regarding temperature especially in months of February, May, June and September and average in the time period of months March and August. When looking at precipitation there was a deficit detected in the time period of March-May, which was 42,3mm under long term average, which was mostly mitigated by above average precipitation of June, which was 28,7 mm above long term average. Further deficit was detected in the month of August which was 35 mm under long term average which was only partially softened by above average precipitation in the month of September which was 18,1 above long term average.

3.1.1.2.1.       Optimal weather conditions for hop yield

Development of temperatures and precipitation of year 2016 could serve as a typical combination of weather effects which predicts a maximal yield of hops.

Similar effects of similar weather conditions on hop yield is also described in further literature (Pejml, 1967), which show on a basis of a long term hop yield and weather correlation analysis from Saaz hop growing region 1871-1964 following requirements for high yields are required:

1)        Above-normal precipitation in May,

2)        Above average temperature in May,

3)        Above average temperature in June,

4)        Precipitation normal should not drop below 40mm in June,

5)        Precipitation cannot drop below 40 – 60mm in July.

According to empirical knowledge if 3, 4 or 5 conditions are met the hop yields are expected to be higher. If only 2 or fewer conditions are met the yields are lower. Contemporary papers for Moravia hop growing region present similar conclusions (Lednický, 1982).

3.1.1.3.           Deviance of temperature and precipitation (month 1-9) in year 2017 in relation to 1975-2016 average.

Chart 3 - Deviance of monthly precipitation and monthly temperature average of year 2017 from long term average 1975 – 2016 from Heřmanov station

Year 2017 (graph no. 3) was very imbalanced regarding temperature with an above average month of February, March, May and especially June. July and August are also above average temperature. Regarding precipitation April was above average with 25,1 mm above long term average when May was on the opposite side with a significant deficit of 31,8 under long term average. Total precipitation of following time period of June – September could not balance out the deficit completely. The presence of lower than average temperatures and above average precipitation in the month of April is interesting because the combination of high precipitation, low air and soil temperature caused the slow growth of hop plants after growth retardation procedures especially with older cultures of plants.

It is therefore necessary when these conditions are present (especially in April or alternatively months 3-5) to choose an earlier term of plant growth retardation measures especially in older hop plant cultures, which were in the year of 2017 poorer in the terms of both quality and yield.

During the compilation and basic analysis of weather data of the reference time sequence 1975-2016 a trend of deviance of both precipitation and temperature from long term average was indicated. This fact deserves a further analysis. It would therefore be possible to predict with a certain level of certainty the long term development of weather elements or at least their monthly averages and their effect on hop yield. A cyclical fluctuation of climate and weather in 16th and 17th with great influence on hop yield are shown in further sources (Pejml, 1965).

Temperature and precipitation development of year 2017 could serve as a combination of weather effects that predicts more of an average yield of hops.

3.1.2.  Analysis of solar irradiance of years 2015, 2016, 2017

Tab. 5 – Monthly data of solar irradiance in years 2015 – 2017

The total of solar irradiance of examined time period of 1st-9th month (chart no. 5) in individual years compared with a long term average 1976-2016 as such: 112,71 % for year 2015, 100,62% for year 2016 and 108,95 % for year 2017.

It is interesting to note that a low yield is related to an above average irradiance (2015) very high yield is linked to a somewhat average irradiance (2016) and a small increase in solar irradiance (up to 110 %) results in an average yield (2017).

Tab. 6 – Deviance of monthly solar irradiance from long term average 1975 – 2016

3.1.2.1.           Deviance of solar irradiance (1st- 9th month) in year 2015 in comparison to 1975 – 2016 long term average

Chart 4 – Deviance of monthly solar irradiance in year 2015 in comparison to 1975 – 2016 long term average from Kralovice station

Solar irradiance in year 2015 (graph no. 4) showed above average values in all months except for May and July which could not mitigate the low solar irradiance of May and June. It is therefore probable that a low yield of hop is predictable by an above average time of solar irradiance in time period of 1st – 9th month.

3.1.2.2.           Deviance of solar irradiance (1st- 9th month) in year 2016 in comparison to 1975 – 2016 long term average

Chart 5 – Deviance of monthly solar irradiance in year 2016 in comparison to 1975 – 2016 long term average from Kralovice station

The development of solar irradiance in a yield record year of 2016 is interesting (graph no. 5) where beginning with February and August the solar irradiance is below average.

3.1.2.3.           Deviance of solar irradiance (1st – 9th month) in year 2017 from 1975-2016 long term average.    

Chart 6 – Deviance of monthly solar irradiance in year 2017 in comparison to 1975 – 2016 long term average from Kralovice station

Development of solar irradiance of year 2017 (graph No. 6) was a combination of years 2015 and 2016 where the months May, June and August are similar to year 2015 and April and July are similar to year 2016.

3.2.     Yield of hops in relation to hop field plant age in years 2015, 2016, 2017

Tab. 7 – Dependence of hop yield on age of hop field plants (2015 – 2017)

3.2.1.  Dependence of hop yield on age of hop field plants in year 2015

Chart 7 – Dependence of hop yield on age of field plants in year 2015 (100 % = hop field yield average)

In the year 2015 (graph No. 7) no statistically conclusive link was found between yield and age of plants with regards to a dominant influence of a combination of negative yield affecting factors (high temperatures, low precipitation, above average solar irradiance).

3.2.2.  Dependence of hop yield on age of hop field plants in year 2016

Chart 8 – Dependence of hop yield on age of field plants in year 2016 (100 % = hop field yield aveerage) – including newly planted cultures

In year 2016 (graph no. 8) a statically significant link was found between the age of hop plant cultures and its yield which was that with increasing age of the culture the yield also increased. It was caused by a combination of optimal temperatures; precipitation and a deficit of solar irradiation which was a clear limiting factor for year 2016 (graph no. 5). The deficit of solar irradiance had an influence especially on younger intensively growing but shadowed cultures. This combination of factors caused that he older more sun permitting cultures yielded more hops in year 2016 than younger cultures which did not permit as much sunlight.

3.2.2.1.           Epigenetical adaptation in older hop cultures

What also deserves attention and further research are both virally cured (meristematic multiplication) and uncured hop cultures aged 20 – 40 years (graph no. 8, hop field no. 9) which are able (even on worse locations) to provide stable although not excellent but still above average yields and content of alpha acids. At least for their genetic material which could be very suitable as a source for further breeding. It is a material, which adapted over the time at least to course of the climate of the whole examined time period of 1975 – 2016 using a not very well known mechanism of viral resistance and stability. It is probable that the older cultures are a mixed population of genotypes. The resulting population is probably very non-uniform, variable and stable.

It is likely that this is not just a random interaction of environment and of a given genotype, as is this fact presented in (Krofta, 2010), where this state is described as an isolated long term stabilized chronical disease.

Above mentioned state of unexpected productivity of old hop cultures can be explained by epigenetical adaptation.  Epigenetical adaptation is a way of adaptation of an organism which forms sometimes non-hereditary but mostly sometime hereditary changes of the phenotype, which are not included in the DNA sequence (Letzel, 2015). Mechanism of this phenomenon lies in the regulation of activating (demethylation of DNA) and disabling (methylation of DNA) of one or more genes – group of genes using derivates of RNA. Methylation is also used for suppression of the negative influences of transposons – foreign segments of DNA – which can move in the genome. Methylation achieves their successful silencing. Epigenetical processes are very important in the interaction of the organism with its environment where they play a role in the regulation of expression of genes in relation to environmental conditions.

Many organisms (typically hops) are reproduced using clones (vegetative reproduction). Using this method meiosis never takes place unlike in sexual reproduction where during meiosis - which also changes the genome - most epigenetical markers are removed especially the methylation of cytosine.

In other words during sexual reproduction most of the epigenetical variability is deleted and is not carried over to the next generation. Whilst in vegetative reproduction using clones this barrier is missing.  This means the longer the clones life (including its vegetative ancestors) without an interruption of sexual reproduction the better is the clone adapted (a non-genetical memory of DNA activation regulators is created) to the environmental conditions where the clone or its ancestors exists/existed.

3.2.3.  Yield of hop in relation to age of hop cultures in year 2017

Chart 9 – Dependence of hop yield on age of field plants in year 2017 (100 % = hop field yield average)- exluding newly planted cultures

In year 2017 a relation was found between age of cultures and their age (graph no. 9) (R² = 0,41802) where with older cultures the yield continuously decreased. The course of function of statistical relation is typical and we can observe a continual decrease of yield up to the 20th year of age after which an exception and an opposite trend was found in cultures with age of around 40 years which already gained the ability of adaptation thanks to the epigenetical interaction to its environment.

3.2.3.1.           Viral infections, decrease of yields over time

Differences of yields between older and younger cultures (graph no. 9) can be explained in the most part by a viral infection in older cultures thanks to mechanical plant growth retardation measures (Krofta, 2010), which is the cause of a certain genetical modification and the slowing of the growth intensity. Similar findings are presented by other sources as well (Pluháčková, 2011).

This question deserves great attention then especially interesting are the new, non-destructive and perspective plant growth retardation measures or at least combinations of mechanical destructive and above-mentioned measures. The mechanical method can transmit viruses while the non-destructive chemical variant of plant growth regulation methods doesn’t. With the combination of both ways the decreasing yield can be delayed with rising age or entirely eliminated if the mechanical measures are excluded entirely.

3.2.3.2.           Hop plant growth retardation measures

Another cause for lower yield in older cultures can be seen in year 2017 can be seen in a later date of retardation measures in these cultures therefore in retardation measures that were carried out later than they should where there should be a significant difference in the date of retardation in older cultures (early date of retardation) and younger cultures (later date of retardation).

The results of year 2017 should not be extensively generalized because these are the results of a single year and should not be a basis for the acceptance of revolutional decisions. (one-off replacement of all older cultures with newly planted ones for example) that in particular if looking at year 2016 where the results are contradictory. It is firstly necessary to deplete all agro technical ways of utilising of the yield potential of a given hop culture including older cultures. After using up all corrective measures should the culture be terminated and replanted.

3.2.3.3.           Influence of weather effects on yield of hops in year 2017

     Another cause of great yield differences between older and younger cultures was the course of weather in year 2017 such as average temperatures and precipitation (graph no. 3) Year 2017 was distinctive by its high precipitation and lower temperatures especially in the month of April. Combination of both of these factors caused a longer cool period for the topsoil thus causing lower intensity of growth especially in older hop cultures after the mechanical plant growth retardation measures.

     It would be interesting to predict the development of this weather situation even into future growing seasons followed with an appropriate measure (especially the date of plant retardation measures) for its mitigation.

3.3.     Content of alpha acids in relation to age of cultures in years 2015, 2016, 2017 in ZS Chráštany s.r.o. 

Tab. 8 – Relation of alpha acids content to hop culture (plant) age (2015 – 2017)

3.3.1.  Content of alpha acids in relation to age of hop cultures in year 2015

Chart 10 – Relation of alpha acids to the age of hop culture in year 2015 (100 % = average of alpha acids)

During the analysis of the relation of alpha acid content on the age of cultures in year 2015 (graph no. 10) a statistical relation was found with a medium correlation (R² = 0,52433), which in the first part of the regression function signalizes a decrease of alpha acids with increasing age, which is in accordance with findings of other authors (Krofta, 2010). However, an opposite trend is present in the following part of the regression function. (Graph no. 10, hop trellis no. 6), where we must refer to the already discussed hypothesis in this paper about the high degree of epigenetical adaptation to the environment, stability of yields (Chapter 3.2.2.1) in clonally long existing hop cultures.

3.3.2.  Content of alpha acids in relation to age of hop cultures in year 2016

Chart 11 – Relation of alpha acids tothe age of hop culture in year 2016 (100 % = average of alpha acids) – including newly planted cultures

In year 2016 a statistically significant relation between alpha acids content and age of cultures was not found (R² = 0,18433). This reality was based on the course of weather in year 2016 where the limiting factor was the deficiency of solar irradiation (chapter 3.1.2.2.). A great yield and quality instability occurred especially in younger cultures with respect to their extensive habitus alongside solar irradiation deficiency. Excellent contents of alpha acids of older cultures (graph no. 11, hop culture no. 6) can be again explained epigenetical adaptation (chapter: 3.2.2.1).

3.3.3.  Content of alpha acids in relation to age of hop cultures in year 2017

Chart 12 – Relation of alpha acids to the age of hop culture in year 2016 (100 % = average of alpha acids) – including newly planted cultures

In year 2017 a statistically very significant relation (R² = 0,73763) between alpha acid content and age of hop cultures was found (graph No. 12) where 20 year of age can be designated as a point where the viral curing effect is depleted. The effect is greatest at age of around 10 years which is in line with other sources (Krofta, 2010). Older cultures (graph No. 12 hop culture no. 11) were not able to reach the results of younger cultures despite their adaptation, especially in the age of 5 – 7 years.

3.4.     Yield of alpha acids in relation to age of hop cultures in years 2015, 2016, 2017

Tab. 9 – Relation of alpha acid yield to age of hop (2015 – 2017)

3.4.1.  Yield of alpha acids in relation to age of hop cultures in year 2015

Chart 13 – Relation of alpha acid yield to age of hop cultures in year 2015 (100 % = average alpha acid yield

In year 2015 no statistically significant relation was found (Chart no. 9) between alpha acid yield and age of hop culture (graph no. 13) especially regarding bordering weather conditions of year 2015 (chapter 3.1.1.1). There is nothing more to point out than the results of very old hop cultures (graph no. 13, hop field no. 6).

3.4.2.  Yield of alpha acids in relation to hop culture age in year 2016

Chart 14 – Relation of alpha acid yield on age of hop culture in year 2016 (100 % = average alpha acid yield

In year 2016 a statistically significant link was found (chart no. 9) (R² = 0,8983) between hop culture age and yield of alpha acids (graph no. 14) where the older was the culture the greater was the yield of alpha acids from one hectare. Unusual result can be explained by very low solar irradiation of younger cultures in otherwise optimal temperatures and precipitation (graph no. 2) with a parallel deficit in solar irradiance (chapter no. 3.1.2.2, graph no. 5) especially in months 5 – 7. It is necessary to ask several questions. What is the meaning for the hop to produce the complex of alpha acids, whether it is not a specific way of establishing a species competition advantage, which is modified in time and space following certain ruleset.

3.4.3.  Yield of alpha acids in relation to hop culture age in year 2017

Chart 15 – Relation of alpha acid yield on age of hop culture in year 2017 (100 % = average alpha acid yield) – including newly planted cutures

In year 2017 no statistically significant relation was found (R² = 0,14547) between alpha acid yield and age of hop cultures (chart no. 9) and (graph no. 15).

3.5.     Yield of hops in relation to solar irradiance in years  2015, 2016, 2017

The analysis of the relation between solar irradiance and hop yield, alpha acid yield and alpha acid content was added as a goal at a later date. The neccesity of the analysis was obvious only after analysing the year of 2016 in which a significant positive influence of solar irradiance on hop yield was found. However this influence was also negative to some extent in years 2015 and 2017. It is interesting to note that the correlation of discovered yearly links exressed using a correlation coefficient is not strong. However  the R² value constantly oscilates in around 0,35 – 0,41 which can be evaluated as weak to medium strength link. Therefore the relation is yearly very stable from which a constant influence of solar irradiation on hop yield can be deducted. However other influence also take part. (i. e. temperatures, precipitation, etc.)

Tab. 10 – Relation of hop yield on yearly total solar irradiance of the hop field slope (%)

Chart 16 - Relation of hop yield on yearly total solar irradiance of hop field slope (%)  in year 2015

In year 2015 a statistically significant link between solar irradiance and hop yield was found (graph no. 16) with a medium correlation of (R² = 0,41), where the limit of the function was at around 101,7% of total solar irradiance. It can be explained by the specific nature of year 2015 which had higher solar irradiance (graph no. 4) in comparison with a long term average of years 1975 – 2016.

3.5.2.  Yield of hop in relation to solar irradiance in year 2016

Chart 17 - Relation of hop yield on yearly total solar irradiance of hop field slope (%)  in year 2016

In year 2016 yield depended (among other factors – i. e. temperatures and precipitation) also on solar irradiance because a statistically significant link with a medium correlation (R² = 0,3957) was found between hop yield and yearly total solar irradiance. The yield grew with more irradiance (graph no. 17) but after reaching a limit of 103% of yearly total solar irradiance after which it started to slightly drop

The effect of solar irradiance can be judged as follows: Above average solar irradiance is not favourable for hop yield. Especially so for year with an average abundance of solar irradiation such as years 2015 and partially 2017. In years with an insufficient solar irradiation such as year 2016 it becomes a limiting factor for yield (yield difference up to 30%).

3.5.2.1.           Solar irradiance – a limiting weather factor for hop yield in year 2016

After analysing the weather data of year 2016 (optimal temperatures and precipitation and below average solar irradiance) a hypothesis was stated that the limit for yield in year 2016 which was otherwise optimal was the sufficiency or deficiency of solar irradiance which was also hinted by the basic analysis of sunlight hours (graph no. 5). This is reflected in year 2016 especially in the yields of older cultures which have a lower growth vitality and thinner growth and therefore accessible growth for sunlight.

This fact was confirmed for year 2016 from other locations in EU too, especially then central and western Europe (Schönberger H., 2016), where unusually low solar irradiance was a limiting factor and a cause of totally unexpected low yields in crops such as winter rape and winter wheat because of low solar irradiance a higher shadowing which caused (in otherwise very promising cultures) a lower photosynthetical assimilation.

Younger cultures, which are far more vital and dense, are far less accessible for sunlight, which causes (with optimal precipitation and temperatures) a lower yield compared to older cultures.

In order to verify the above-mentioned hypothesis for year 2016 we proceeded to the taxation of the individual hop trellis (blocks) in order to establish the relative value of total yearly solar irradiance of the location.

The hypothesis was verified and confirmed (graph no. 17). For a further more detailed analysis a multifactorial statistical regressive analysis would be appropriate.

3.5.2.2.           Regulation of solar irradiance in the trellises

It is therefore necessary to better regulate solar irradiance in terms of microclimate of the trellis especially with younger cultures. It is possible to regulate this using the spacing of plants (wider spacing meaning more light), also by training fewer shoots to the guide wire and especially by the time of the mechanical plant growth retardation measures or other means of plant growth retardation where it is necessary to choose a later or a very late date with younger cultures with a great vitality and intensity of growth (around o rafter 25. 4. of a regular year).

3.5.2.3.           Regulation of solar irradiance by choice of a location and orientation of rows

With some crops the key requirement for their growth is the orientation and slope of the growing location. (grape production). There is a justification for this practice in hop production also because the difference in a yearly total solar irradiance reaches in relation to the location slope (i.e. up to 10°) and its exposition (north south orientation) up to 15 % (Čvančara, 1962).

 Other and substantially greater differences can arise in the intensity of the reception of solar irradiance in relation to the exposition and slope of the location in individual months where especially outside the main vegetation period of hops (4th – 8th month) can the differences reach up to 50-80% (Pavloušek, 2011).

3.5.3.  Yield of hops in relation to solar irradiance in year 2017

Chart 18 - Relation of hop yield on yearly total solar irradiance of hop field slope (%)  in year 2017

A statistically significant link was found in year 2017 between hop yield and yearly total solar irradiance of the hop field (graph no. 18) which had a weak correlation (R² = 0,35647) which means that the yield was also formed by other factors as well (precipitation etc.) Nonetheless the shape and course of the functional relation is similar to other examined years. The function reached its limit at 102% after which started to decline. It can be explained by the total time of solar irradiance which was above average for year 2017 in comparison with 1975-2016 long term average (chart no. 5) and also by the course of sunlight in seperate months of year 2017 (graph no. 6)

3.6.     Content of alpha acids in relation to solar irradiation in years 2015, 2016, 2017

We were also interested in the influence of solar irradiation on the content of alpha acids in hops. It was a very interesting discovery that solar irradiation has more influence on alpha acid content than hop cone yield (chart no.10) in some years (2015, 2016). This can be deduced from the tighter nature of discovered links and values of the correlation coeficcients (Chart. no. 11). It is at the same time neccesary to state that there are also years where the influence of solar irradiation on hop yield is very small (2017) and the content of alpha acid is therefore caused by multiple factors (age of cultures, genotype, soil properties etc.).

Tab. 11 - Relation of alpha acid content to yearly total solar irradiance of hop field slope (%)

3.6.1.  Content of alpha acids in relation to solar irradiation in year 2015

Chart 19 – Ralation of alpha acid content on yearly total solar irradiance of hop field slope (%) in year 2015

One of the interesting things of year 2015 was a positive influence of solar irradiation on the alpha acid content (graph no. 19). Statistical significant link was found with a strong correlation. (R² = 0,93047). However the solar irradiation effect had its limit around a value of 102% of yearly total solar irradiation after which the content of alpha acids declined. This limit is the same as in the yield of year 2015 (graph no. 16)

3.6.2.  Content of alpha acids in relation to solar irradiation in year 2016

Chart 20 – Ralation of alpha acid content on yearly total solar irradiance of hop field slope (%) in year 2016

A statistically significant link was found with a strong correlation (R² = 0,72089) between yearly total solar irradiance and alpha acid content where with increasing solar irradiation the content starts to increase also and increases up until the value of 101% and then sharply decreases its content of alpha acids (graph no. 20). The point of interest of year 2016 was a higher value of solar irradiation caused an increase in yield of cones (graph no. 17) but caused a decrease in alpha acid content.

3.6.3.  Content of alpha acids in relation to solar irradiance in year 2017

Chart 21 – Relation of alpha acid content on yearly total solar irradiance of hop field slope (%) in year 2017

In year 2017 only a weak statistical link was found with a low correlation (R² = 0,18376) between yearly total solar irradiation of the hop field and alpha acid content. The trend of the relation hints that with increasing value of yearly total solar irradiation of the location the content decreased in a linear fashion.

3.7.     Yield of alpha acids in relation to solar irradiance in years 2015, 2016, 2017

There are two points of interests of the discovered statistical relations between yield of alpha acids and total solar irradiance. Firstly they are very similar links woth similar course (2015, 2017) year 2016 is their inversion. Secondly they are relations with a more of a strong correlation (2015, 2017) however sometimes the correlation is sometimes weak (2016).

The total course of the relations (yield of alpha acids) in noble aromatic hops is probably set by yield of cones in a given year because its variability is greater than the yield of AA). In Saaz this value is not economically interesting. Research was carried out for the sake of completeness

Tab. 12 – Relation of alpha yield and yearly total solar irradiance of field slope (%)

3.7.1.  Yield of alpha acids in relation to solar irradiance in year 2015

Chart 22 – Relation of alpha yield and yearly total solar irradiance of hop field slope (%) in yeard 2015

In year 2015 a statistically significant link was found with a strong correlation (R² = 0,98988) between alpha acid yield and yearly total solar irradiation (graph no.22). This relation has a course identical to the relation between cone yield and yearly total solar irradiation in year 2015 (graph no. 16)

3.7.2.  Yield of alpha acids in relation to solar irradiance in year 2016

Chart 23 – Relation of alpha yield and yearly total solar irradiance of hop field slope (%) in yeard 2016

In year 2016 practically no statistical relation was found (R² = 0,10177) between yield of alpha acids and the value of yearly total solar irradiation of the location (graph no. 23). The shape of the found trend can be interpreted as a certain kind of inversion of the relation of hop yield on total solar irradiation also found in year (graph no. 17)

3.7.3.  Yield of alpha acids in relation to solar irradiance in year 2017

Chart 24 – Relation of alpha yield and yearly total solar irradiance of hop field slope (%) in yeard 2017

In year 2017 a statistically relevant link was found with a medium correlation (R² = 0,43229) between alpha acid yield and yearly total solar irradiation (graph no. 24). This relation has a course identical to the relation between cone yield and yearly total solar irradiation in year 2017 (graph no. 18)

4.         Conclusion

4.1.     Weather conditions of years 2015, 2016, 2017

The very low yield of hops in year 2015 was caused by above average solar irradiation (112,71 % of normal), below average precipitation (68,53 % of normal) with and absolute deficit of 127,6 mm in comparison to normal and above average temperatures (110,39 % of normal) in the time period of 1st – 9th months in comparison to 1975 – 2016 long term average.

The above average yield of hops in year 2016 was accompanied by average to above average temperatures (109,45 % of normal), above average precipitation in June and average precipitation in the whole time period (92,79 % of normal) with an absolute deficit of 29,2 mm. This year was very specific by its below average solar irradiance in the months of 2nd – 7th and a more of an average irradiance in the year as a whole (100,62 % of normal).

Year 2017 with an average hop yield was accompanied by above average temperatures (109,45 % of normal) outside of the 4th month which was very cold and had above average precipitation while the whole year of 2017 only was below average to average regarding precipitation (90,22 % of normal) with an absolute deficit of 39,9 mm. Solar irradiance in the year 2017 was above average (108,95 % of normal).

4.2.     Yield of hop and age of hop cultures

In year 2015 no statistical relation between age of cultures and their age was found (R² = 0,002). In year 2016 a relevant statistical relation was found between age of cultures and their yield (R² = 0,503) with the conclusion that their yield increased with rising age. In year 2017 only a slight statistical link was found between age of hop cultures and their yield (R² = 0,418). With rising age the yield decreased.

4.3.     Alpha acid content and age of cultures

In years 2015, 2016, 2017 a statistical relation was found between alpha acid content and age of cultures.  In year 2015 it was discovered that up to 20 years of age the content decreases but after that it increases. It is a correlation of a medium correlation (R² = 0,524). In year 2016 the same breaking trend was discovered but with increasing age of cultures the content also increased. However this relation had a weak correlation (R² = 0,184). In year 2017 a relation with a strong correlation was discovered (R² = 0,738), where increasing age of cultures caused a decrease in alpha acid content where the peak age was 20 years after which no further decrease occurs. The highest content was discovered in cultures which were up to 10 years of age.

4.4.     Yield of alpha acids and age of hop cultures

A statistical relation was found between age of cultures and yield of alpha acids in years 2015, 2016, 2017. The value of the correlation coefficient was weak in year 2015 (R² = 0,236). In year 2016 the found statistical relation had a very strong correlation (R² = 0,898) between age of cultures and alpha acids yield where with increasing age the yield of AA also increased. The greatest yield was achieved in year 2016 in the oldest culture with the age of approximately 35 – 40 years. In year 2017 the found statistical relation had a weak correlation (R² = 0,146) between age of cultures and alpha acids yield

 4.5.            Yield of hops and solar irradiance

In years 2015, 2016, 2017 a statistically relevant link was found between yield of hop cone and solar irradiance. In year 2016 which was characteristic by its yearly average and monthly below-average solar irradiance a statistical relation was found (R² = 0,39575) between hop yield and yearly total solar irradiance of the location of the hop culture. With increasing solar irradiation the yield also increased. Therefore the limiting factor in year 2016 for yield was solar irradiation. In years 2015 and 2017 the discovered statistical relation strength expressed in a correlation coefficient was similar. Correlation in year 2015 (R² = 0,412) and year 2017 (R² = 0,3564) oscillate around a similar value. This relation is therefore yearly stable from which we can deduce a constant influence of solar irradiation on the formation of yield in hops. In years with above average solar irradiation (2015, 2017) solar irradiation causes an increase in yield but after reaching a certain point the effect reverses and starts to decrease the yield.

4.6.      Content of alpha acids and solar irradiation

A statistical relation was found between alpha acid content and solar irradiation in years 2015, 2016, 2017. In year 2015 it was a very strong correlation (R² = 0,93047) in year 2015 a strong correlation in year 2016 (R² = 0,721). But in year 2017 only a weak correlation was discovered (R² = 0,18376). In year 2015 higher solar irradiation caused a higher content of alpha acids but after reaching a certain point a decrease took place In years 2016 and 2017 only a small increase of solar irradiation above the average level caused a decrease in alpha acid content

4.7.     Yield of alpha acids and solar irradiation

A statistical relation was discovered between solar irradiation and yield of alpha acids. In year 2015 the correlation was very strong (R² = 0,99), in year 2016 it was weak (R² = 0,102) and in year 2017 the discovered correlation was medium (R² = 0,432).

4.8.     Further areas for research and analysis

One of the goals of this paper was to establish further areas for research. They are areas of analytic or research goals:

4.8.1.  Weather conditions

Influence of weather conditions on yield and quality of hops in a long time period. Development of weather conditions and its dynamics in a long time period, possible cycles and their possible prediction, adaptation of hops agro technics to climate change. Influence of solar irradiance and Sun activity on hop yield and quality.

4.8.2.  Agro technics of hops

Regulation of solar irradiance in hop growths. Spacing of hop plants, location choice for hop production in relation to tilt and exposition. Non-destructive hop growth retardation measures, combination of non-destructive measures with mechanical measures as a major factor for the slowing of viral infections in hops in order to increase the yields of hop cones and alpha acids.

4.8.3.  Breeding and hop genetics

Epigenetical adaptation and heritability in hops. Old and very old cultures as valuable genetical source. Clonal selection with a focus on epigenetically acquired traits of hops – stable yield, content of alpha acids, longevity, resistances against pests, diseases and weather irregularities, adaptation to the environment.

5.         Literature

ČVANČARA F. - Zemědělská výroba v číslech, Státní zemědělské nakladatelství, 1962, Praha.

KROFTA, Karel – Dlouhodobý vývoj obsahu alfa kyselin v českých odrůdách chmele, Sborník Bioklima 2010, Mezinárodní konference 7. - 9.9. 2010, Praha,

Available at: https://www.forim.sk/index_soubory/Fabrika_Merganic_2010b.pdf

KROFTA K., Mikyška A., Tichá J. – Ročníkové prognózy obsahu alfa kyselin v českých chmelech, Kvasný průmysl, 2012 (roč. 58), č. 9, Výzkumný ústav pivovarský a sladařský, a.s.,

Available at: https://www.kvasnyprumysl.cz/en/artkey/kpr-201209-0002_Rocnikove_prognozy_obsahu_alfa_kyselin_v_ceskych_chmelech.php?back=%2Fsearch.php%3Fquery%3DSlada%F8sk%FD%20%FAstav%20in%3Aauth%20name%20key%20abstr%26sfrom%3D180%26spage%3D30

LATZEL Vít - Epigenetická dědičnost v ekologii a evoluci rostlin, Vesmír 94, 105, 2015/2, Praha,

Available at: https://casopis.vesmir.cz/clanky/clanek/id/10917

LATZEL Vít a kol. - Epigenetic diversity increases the productivity and stability of plant populations, Nature Communications 4, Article number: 2875, 23.6. 2013,

Available at: https://www.nature.com/ncomms/2013/131128/ncomms3875/full/ncomms3875.html

LEDNICKÝ V., Doležel F., Klapal I. – Vliv teploty vzduchu  a srážek na výnos a kvalitu chmele v moravské chmelařské oblasti, Meteorologické zprávy 35, 1 (1982), s. 24-28, 3 obr., 5 tab., lit. 13, Praha.

MOŽNÝ M., Havlík V. – Studie kolísání atmosférických srážek v české chmelařské oblasti, Meteorologické zprávy 45, 5 (1992), s. 148-152, 8 obr., 3 tab., lit. 11. Praha.

PAVLOUŠK P. - Pěstování révy vinné, moderní vinohradnictví, Grada a.s., 2011, Praha.

PEJML, Karel - Příspěvek ke studiu vlivu počasí na průběh fenologických fází chmele a na jeho výnosy, Meteorologické zprávy 24, 5-6 (1971), 123-126, Praha, 1971.

PEJML, Karel – Kolísání klimatu v 16. a v 17. stol. v české vinařské a chmelařské oblasti, Meteorologické zprávy 18, 6 (1965), 164-167, Praha, 1965.

PEJML, Karel – Příspěvek ke studiu srážek v české chmelařské oblasti, Meteorologické zprávy 26, 2 (1973), 42 – 5O, Praha, 1973.

PEJML, Karel – Vliv meteorologických faktorů na výnosy chmele, Meteorologické zprávy 20, 1 (1967), 24-27, Praha.

PLUHÁČKOVÁ H., Ehrenbergerová J., Kretek P., Kocourková B. – Chmelové silice ve vybraných odrůdách z různě starých chmelnic, Kvasný průmysl, 2011 (roč. 57), č. 7-8, Výzkumný ústav pivovarský a sladařský, a.s.,

Available at: https://kvasnyprumysl.cz/artkey/kpr-201107-0016_Chmelove_silice_ve_vybranych_odrudach_z_ruzne_starych_chmelnic.php?l=EN&l=CZ&l=EN&l=CZ

POKORNÝ, Jaroslav Ing. – Vliv lokality a ročníku na produkci a kvalitu chmele. Doktorská disertační práce, Česká zemědělská univerzita v Praze, katedra rostlinné výroby. Praha, 2011,

Available at: https://www.agrobiologie.cz/pds/dp/pokorny.pdf

SCHÖNBERGER H. – N.U. Agrar GmbH – poradenství mezi výzkumem a praxí, Info 15/2016 z 1. srpna 2016, Studená – Jindřichův Hradec.

ŠTRANC J., Zelenka V. – Vliv teploty ovzduší na růst, vývoj a výnosy chmele, Chmelařství 6/1989. Vydavatel Petr Svoboda, Žatec.

ŠTRANC P., Štranc J., Jurčák J., Štranc D., Pázler B. – Řez chmele – odrůdy Žatecký poloraný červeňák v podmínkách ČR, Praha, 2007,

Available at: https://katedry.czu.cz/storage/4463_metodika_rch.pdf

Thanks:

The authors thank the associates from the Agricultural Society Chrášťany s. r. o. for help with data processing, for valuable suggestions and comments.