The effect of timing of stripping on hop production under South African conditions

Thesis (MSc)--Stellenbosch University, 2015.

ENGLISH ABSTRACT: Hops is a small, but important agricultural commodity in South Africa. The hop cones contain secondary metabolites responsible for the flavour, aroma and bitterness in beer. The George and Waboomskraal area are the best suited for growing hops due to climate and geographical location. The optimum latitude for hops growing is 45° - 54° north and south of the equator, while these areas are at 34° S. International hop varieties are not adapted to the South African climate. Winters are too warm and summer days too short, thus all the varieties grown in South Africa have been bred for these specific conditions. The Hop breeding and research program strives to develop internationally competitive varieties with higher yields, desirable brewing characteristics, reducing input costs and increased productivity. To evaluate the current agricultural practices and to determine whether some of these practices negatively affect the plant health and yield, a study was conducted to investigate the effect of the time of stripping. Stripping refers to the practice of removing basal growth of the lower laterals and untrained bines. The study was done in two different microclimates, namely George and Waboomskraal. The aim was to determine the effect of the time of stripping on the vegetative growth, light interception, biomass, soft resins (alpha and beta acids), yield, rootstock weights and carbohydrate concentrations. The effect of stripping was also evaluated on different planting systems and plant maturity, namely Tram lines and conventional planting and 3 years old versus 5 year old plants. The effect of early stripping in terms of dry weight on mature plants is considerably less than the effect on younger plants. Young plants delivered lower dry weights, but accumulated higher carbohydrate reserves in the early stripping treatment. On young plants early stripping showed a significant difference favouring yield on tramlines. There is a general trend across all sites of higher crop efficiencies in the early stripped treatments. Growers could significantly increase yields without negatively affecting the quality (alpha and beta acids) by applying early stripping, especially in the George area. The effect of early stripping becomes more apparent in consecutive years as the time of defoliation affects foliage, carbohydrate concentration, canopy microclimate and light interception resulting in increased yields, especially on tramlines. From this study it can be concluded that early stripping appeared to have an influence on the dry root weight, carbohydrate concentration, light interception, crop efficiencies, yield and biomass. This practice is not only environmentally friendly by using less herbicide, it also delivers an economic gain. The effect of the time of stripping in consecutive years and different varieties deserves further study.

AFRIKAANSE OPSOMMING: Hops is 'n klein, maar belangrike landbou kommoditeit in Suid-Afrika. Die hopkeëls bevat sekondêre metaboliete wat verantwoordelik is vir die smaak, aroma en bitterheid van bier. Die George en Waboomskraal gebied is die beste geskik vir die kweek van hops, a.g.v. die klimaat en geografiese ligging. Die optimum breedtegraad vir hops verbouing is tussen 45° - 54° noord en suid van die ewenaar, waar hierdie gebiede by 34° S lê. Internasionale hops varieteite is nie aangepas by die Suid-Afrikaanse klimaat nie, want die winters is te warm en die daglengte in die somer is te kort. Al die variëteite in Suid Afrika is geteel vir dié spesifieke klimaatstoestande. Die Hopsteling en -navorsingprogram streef daarna om internasionaal mededingende varieteite met hoër opbrengste, verminderde insetkoste, verhoogde produktiwiteit en wenslike broueienskappe te ontwikkel. 'n Studie is gedoen om die effek van stropingstyd te evalueer en te identifiseer of hierdie praktyk negatiewe invloede op die gesondheid van plante en opbrengste het. Stroping verwys na die praktyk van die verwydering van basale groei van die laer laterale en onopgeleide ranke. Die studie is gedoen in twee verskillende mikroklimate, naamlik George en Waboomskraal. Die doel was om te bepaal wat die effek van stropingstyd op die vegetatiewe groei, lig onderskepping, biomassa, alfasure en betasure, opbrengs, wortelmassa en koolhidrate konsentrasies is. Die effek van stropingstyd was ook geëvalueer op verskillende plantsisteme en plant volwassenheid, naamlik Tramlyne en Konvensionele -plantsisteem, en 3 jaar oue teenoor 5 jaar oue plante. Die effek van vroeë stroping in terme van droë gewig op volwasse plante is aansienlik minder as die effek op jonger plante. Jong plante het laer droë gewig, maar hoër koolhidraat reserwes in die vroeë stroping behandeling gelewer. Vroë stroping van jong plante het ’n beduidende verskil getoon ten opsigte van opbrengste op die Tramlyn-plantsisteem. Daar is 'n algemene tendens op alle lokaliteite van hoër opbrengs doeltreffendheid in die vroeë stropings behandelings. Produsente, veral in die George area, kan opbrengste aansienlik verhoog sonder om die kwaliteit (alfa en beta sure) negatief te beïnvloed, deur die toepassing van vroeë stroping. Die effek van vroeë stroping word meer duidelik in agtereenvolgende jare, omdat die stropingstyd die koolhidrate konsentrasie, mikroklimaat binne in die hopranke en lig onderskepping wat lei tot verhoogde opbrengste, veral op die Tramlyn- plantsisteem, beïnvloed. Uit hierdie studie kan afgelei word dat vroeë stroping 'n invloed op die ligonderskepping, plantgewas doeltreffendheid, biomassa, opbrengs, droë wortelgewig en koolhidraatkonsentrasie het. Hierdie praktyk is nie net omgewingsvriendelik nie, maar gebruik minder onkruiddoder en lewer ekononomiese voordele vir die produsent. Die effek van stropingstyd op agtereenvolgende jare en verskillende varieteite verdien verdere studie.