Influence of grazing management and drought on white clover population performance and genotypic frequency

Abstract

Between 1984 and 1990, the performance of 'Tahora', 'Huia', 'Pitau' and 'Kopu' white clover cultivars were evaluated in self-contained farmlets under rotational grazing and set stocking with sheep. A severe late spring-early summer drought in the third year from sowing (1986-87), caused a marked interaction between grazing management and cultivar performance. The drought caused no losses under set stocking, but large losses of stolon (75-90%) and a reduction in white clover content from 15% to 2% under rotational grazing. All cultivars were equally affected and recovery over the next 3 years was slow, although the small-leaved Tahora recovered faster than the large-leaved Pitau and Kopu. It was hypothesised that this would have caused differences in the preferential loss of genotypes with specific characteristics between grazing managements. Populations of white clover sampled from the field in 1989 and established from the original seedlines, were grown in common conditions for a year and compared for leaf size and cyanogenesis content. There had been a significant loss in mean leaf size in all cultivars with time, irrespective of drought or grazing management. It was shown that a general decline in stolon biomass commenced in all treatments 18 months after sowing, suggesting either, (1) a preferential loss of large-leaved genotypes with time irrespective of grazing management, or (2), a general change in per-formance of all genotypes, which may coincide with the change from dependence on seminal taproot of the seedling, to nodal rooting on free growing stolons. Keywords: cultivars, drought, genotypic change, grazing management, populations, Trifolium repens, white clover