Effects of improved, adapted white clover (Trifolium repens L.) germplasm on the productive properties of a hill pasture

Abstract

White clover (Trifolium repens L.) herbage accumulation rate, nitrogen (N) fixation rate and efficiency, and plant growth characteristics were compared in pastures based on eight white clover lines on north‐ and south‐facing sites under sheep grazing in southern North Island hill country during years 4–7 following oversowing. The lines compared included three cultivars ('Grasslands Tahora’, ‘Grasslands Prestige’, and Prop) bred for hill country and/or sheep grazing systems with (notionally, at least) a moderate‐high degree of adaptation to the environment, and two cultivars ('Grasslands Huia’ and ‘Grasslands Kopu') known to have moderate—low adaptation. The control treatment comprised pastures based on the resident clover ecotype of the site, seed of which was sown at the same rate as the other lines. Two of the adapted lines (Tahora, Prestige) yielded significantly more white clover herbage than the resident ecotype over 3 full years of measurement. Prestige displayed consistently greater cool‐season growth in late winter‐early spring than all other lines. All three adapted cultivars fixed significantly more N than the resident ecotype over 12 months (mean for resident = estimated 64 kg N/ha per year compared to mean for three cultivars = 100 kg N/ha per year). There was a close linear relationship between white clover herbage accumulation rate and clover stolon density in most years and on both aspects. The adapted lines developed greater stolon densities through superior branching ability (higher frequency of occurrence of branches at stolon nodes). There was evidence that the adapted clover lines were able to compete more effectively with grasses for growth resources at some times of the year, e.g., for mineralised N in late winter‐early spring, resulting in lower N fixation efficiencies for Tahora and Prestige (mean 43 kg N fixed/t clover DM grown, versus 122 kg N/t clover DM for the resident ecotype). There were no statistically significant effects of clover line on total annual pasture herbage accumulation, but a difference between Prestige and all other lines in total pasture growth in winter gradually strengthened over 4 years of measurement so that, in the final year measured (year 7 after sowing), a statistically significant effect of line was recorded. In this winter, Prestige‐based pastures grew 1130 kg DM/ ha over 65 days from early June to late August, whereas resident pastures grew 570 kg DM/ha. The results are discussed within the context of pasture improvement strategies in hill country. Several important effects of adapted germplasm on the productive properties of hill pastures are identified.