The effect of the level of dietary inclusion of grape marc on nitrogen partitioning in the lactating dairy cow: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Environmental sustainability of the New Zealand dairy industry is becoming increasingly important. The typical pasture-based diets fed to New Zealand dairy cattle provide an excess of crude protein in the diet which leads to high urine nitrogen (N) concentrations. The main source of on-farm N leaching and nitrous oxide (N₂0) emissions is from urine patches deposited by grazing animals. Dietary inclusion of grape mare, a by product of the viticulture industry, contains condensed tannins and low crude protein concentrations, properties which are known to alter N partitioning in ruminants, and improve environmental sustainability. The objective of this experiment was to determine if the dietary inclusion of grape mare altered N partitioning in dairy cows fed a typical pasture-based diet. The experiment was arranged in a 2 x 4 factorial design, including four levels of grape mare supplementation (0, 2, 4, 6 kg DM/d) and two levels of polyethylene glycol (PEG; 0, 350 g/d) supplementation. Forty Friesian x Jersey crossbred multiparous, lactating cows were evenly divided into one of eight treatment groups. Supplementing grape mare to cows receiving a typical pasture-based diet reduced dietary N (g/kg DM) intake by up to 10.2% and increased dietary condensed tannin concentrations. The dietary inclusion of PEG decreased fecal N % (P < 0.001), while the supplementation with grape mare significantly lowered urine N % (P = 0.005) and urine urea concentration (P = 0.003). Cows fed grape mare also had significantly lower plasma urea (P = 0.024) and MUN (P = 0.026) concentrations. These results indicate that the dietary inclusion of grape mare is effective at increasing fecal N and reducing urine N concentrations, which will improve the environmental sustainability of pastoral-based dairy farming systems in New Zealand.