Perennial ryegrass breeding and the scaling issue: a review of system experiments investigating milk production and profit differences among cultivars

Abstract

Results of studies designed to determine whether or not differences measured among perennial ryegrass cultivars in small-plot studies translate into differences in milk production and profit in dairy whole-system studies were reviewed. Only three experiments were identified that met the criteria for fully self-contained systems repeated over multiple years required to account for annual feed supply–demand balance, its interaction with animal intake and production, and the influence of inter-annual climate variability on these processes. Collectively, these studies provide evidence of improvement in animal production, associated with genetic gains from ryegrass breeding, albeit through shifts in factors such as heading date (as it affects herbage quality and grazing efficiency) and host plant by endophyte interactions, rather than through gains in dry-matter yield. The latter remains unresolved, despite substantial evidence for gains from small-plot trials of dry-matter yield increases in the order of 0.5% per annum. These studies also highlighted the number of factors that have to be taken into account in the design and conduct of such studies, including gaining clarity about the size of the differences that can be expected and ensuring sufficient statistical power. Implementing objective management rules that allow cultivars to express their potential and capture differences through the grazing animal will ensure sufficient measurement intensity to enable differences (if observed) in milk production and profit to be explained. This should guard against confounding factors such as the differential effects of insect pests on plant performance, and consequent changes in pasture botanical composition mediated by ryegrass endophyte strains. Despite these difficulties, more experiments of this type are required to quantify and, ultimately, increase the value being delivered by ryegrass breeding to pasture-based dairy production systems in temperate regions. Therefore, there is a need for whole-system studies to be undertaken to provide valuable new information and give farmers the confidence to invest in the use of new cultivars.