The contribution of perennial ryegrass (Lolium perenne L.) breeding to whole pasture productivity under dairy cattle grazing in New Zealand. 2. Rates of gain in production traits and economic value

Perennial ryegrass (Lolium perenne) plant breeding has delivered several high performing and broadly adapted cultivar-endophyte (Epichloë) combinations for the New Zealand pasture-based livestock industries. However, the trends in key traits and the potential economic value being added to farm systems have not previously been addressed systematically. Rates of gain in yield, nutritive value, phenological and morphological traits, and the economic value of those traits, were estimated among and within ryegrass functional groups (heading date, ploidy) traded commercially between 1973 and 2012. Whole pasture dry matter yield improvements of c. 80 and 60 kg DM/ha per year were identified in in late-season heading diploids and tetraploids respectively. Trends in metabolisable energy were negative within functional groups, but positive when all cultivar-endophyte combinations were included in the analysis due to the addition of tetraploids and late-season heading diploids from the mid-1990s. The estimated trend in overall economic value (all cultivar-endophyte combinations included) was $7/ha per year based on DM yield only, about half the rate of gain previously estimated using small-plot merit testing data. The trend increased to $15/ha per year when ME was included due to the emergence of commercial tetraploids from the early 2000s onwards. Estimated economic gains were high within late-season heading diploids and tetraploids but not mid-season diploids. These trends suggest that, if high performing cultivar-endophyte combinations had been adopted in New Zealand dairy systems, then pasture utilization should have increased by ~1 t DM/ha above the current industry average. Reasons for this apparent discrepancy are discussed.