The effects of genetic merit and management on pasture performance traits of perennial ryegrass swards under grazed conditions : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

The DairyNZ Forage Value Index (FVI) was developed to help dairy farm businesses select suitable ryegrass cultivar-endophyte combinations. The FVI value of a cultivar-endophyte combination is given in $/ha/year; this value is an estimate of the expected contribution to the business’ operating profit. The FVI $ value and ranking position for a specific cultivar-endophyte combination is calculated using trait performance values from small plot trials, and the expected economic value of differences in these performance traits. A potential limitation of the performance values is that small-plot trials are managed to a standard that is higher than what could realistically be achieved on a typical farm. The objective of this research was to close the knowledge gap around the validity of the FVI model for perennial ryegrass under different management conditions in terms of nitrogen fertiliser and stocking rate. This was done by testing pasture performance traits using large-scale paddocks (~2.9 hectares). This research was conducted on the Southern Dairy Hub, Wallacetown, Southland using a 2 x 2 factorial design replicated eight times. The design included two FVI factors - low FVI ranked and high FVI ranked, and two management factors - lower impact (up to 50 kg N/ha/year and 2.8 cows/ha) and standard impact (up to 200 kg N/ha/year and 3.4 cows/ha). Between the 2nd of October 2020 and 31st of May 2021, measurements of pre- and post-grazing herbage accumulation, leaf stage, sward composition, nutritive value, and persistence were taken. Total herbage accumulation was greater in the high FVI treatment compared with the low FVI treatment (11.6 versus 10.7 t DM/ha/year, p = 0.005). The difference in yield due to FVI was largely the result of improved herbage yield of the high FVI treatment during summer (p = 0.002). Using a standard impact management approach increased pasture growth relative to the lower impact management approach (11.8 versus 10.4 t DM/ha/year). The effect of management influenced herbage yield in the summer (p = 0.001) and autumn (p = 0.028) where the standard impact had a greater yield than the lower impact treatment. The average leaf stage at sampling, leaf emergence rate, and days to sampling were not significantly different between treatments. Therefore, in the treatments with higher herbage accumulation, the effect was not due to faster leaf emergence rates or differences in regrowth interval, but some other factor that was not investigated in this trial. There were limited significant interactions between FVI and management, meaning that for the most part, the effect of FVI ranking on pasture performance traits did not depend on management. Where differences did occur, there was a scaling interaction where the yield advantage of high FVI was significantly greater under the standard impact management than under the lower impact management. The results suggest the differences in genetic expression could be due to optimum growing temperatures, nitrogen use efficiency, leaf size versus leaf emergence rates, and/or endophyte. The overall economic advantage to the high FVI treatment was determined to be $109.30/ha/year. This value is 22% of the difference of $503/ha/year predicted by the FVI. The fact the high FVI treatment out-grew the low FVI treatment suggests the model is valid, however, the model appears to be over-estimating the $ value.