Nitrogen partitioning and milk production from dairy cows grazing simple and diverse pasture mixtures: A dissertation submitted in partial fulfillment of the requirement for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Milksolids production and nitrogen partitioning was examined in late lactation for dairy cows grazing a range of simple and complex pasture mixtures. A total of 48 Friesian x Jersey cows (LW = 495kgLW ± 18.76) were arranged in groups of four cows and allocated to two replicates of six pasture treatments. The treatments were factorial combinations of three base pastures (perennial ryegrass, high sugar perennial ryegrass and tall fescue) sown either in simple mixtures (with white clover) or diverse mixtures (with a combination of white clover, chicory, plantain, lucerne, red clover and prairie grass). Cows were allocated a fresh break of 14kgDM/cow/day above a grazing residual of 1480kgDM/ha following each afternoon milking session over the eight day trial period. There was no significant effect of base pasture or pasture diversity on milk yield (x = 14.9kg/cow/day), milksolids yield (x = 1.46kgMS/cow/day), N intake (x = 462g/cow/day) and faecal N (x = 74g/cow/day). There was a highly significant decrease in urine N concentration (Xsimple = 0.676 %, Xdiverse = 0.506 %) (P < 0.001), urine N excretion (Xsimple = 152.4g/cow/day, Xdiverse = 114g/cow/day) (P = 0.009) and urine N % of intake N (xsimple = 32.4 %, Xdiverse = 24.7 %) (P < 0.001) found for cows grazing diverse mixtures compared to cows grazing simple pastures. There was a significant effect of base pasture on milk N % of intake N (XHS = 23.6 %, XRG = 22.2 %, XTF = 19.4 %)) (P = 0.048). It is proposed this may reflect differences between base pastures in crude protein content and the WSC:CP ratio, and level of diversity in changes in ERDP and RUP, water content, lower NDF and subsequent increase in DMD and ROP, and increased protein deposition within the body. This research provides evidence of benefits for farmers to adopt diverse pasture mixtures into their systems for reducing N losses. Since milksolid production was similar but the N concentration in urine lower, the potential for N loss as nitrate may be decreased by grazing diverse pasture mixtures. Further research is required to quantify these results at other stages of lactation, plus also dry matter production levels of these pasture mixtures.