The effects of whey supplementation on nitrogen partitioning in dairy cows grazing perennial ryegrass pastures: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Evaluation of the effects of supplementation acid whey permeate (whey) to pasture based diets was completed with the aim of increasing nitrogen (N) partitioning into milk and faeces and reducing N losses in urine for 28 days during mid lactation at Lincoln University Research Dairy farm. Friesian x Jersey cows (n=6/treatment) were allocated to one of three treatments being whey (whey 20 L, molasses 250 ml and pasture), molasses (250 ml and pasture) and control (pasture only). The pasture allowance offered was 14 kg dry matter (DM)/cow/d above a grazing residual of 1500 kg DM/ha, but cows in the whey, molasses and control groups all grazed to lower residuals and consumed a total diet of 20 kg DM/cow/d, 21.6 kg DM/cow/d and 19.4 kg DM/cow/d respectively. Lower pasture intakes occurred for the whey treatment due to substitution. Metabolisable energy (ME) and crude protein (CP) content of dietary intake were similar in all treatment groups, being approximately 12.1 MJ ME/kg DM and 19.6% of DM respectively. Although dietary components were similar N partitioning in milk, plasma, faeces and urine differed. Increased protein (P = 0.028) in milk for cows supplemented with whey (0.63 kg/day) was observed relative to the control (0.55 kg/day). Milk yield and other milk components were not significantly different. Plasma urea N (BUN), milk urea N (MUN) and urea excreted in urine all decreased in the whey treatment group, resulting in a less loss of N in urine (g N/day; P = 0.003) in the whey treatment group (211 g N/day) relative to the control (319 g N/day). Increased %N (P = 0.006) in faeces was observed in the whey treatment group (3.77%) relative to both the molasses (3.44%) and control (3.35%). There were no significant differences in faecal DM. Increased microbial crude protein synthesis can account for increased milk protein and reduced PUN, MUN and urea in urine. It was concluded that this was due to the combination of highly fermentable lactose and rumen passage rate. Increased rumen flow rate from whey supplementation caused the increased N% in faeces, which is emphasised by its significant difference to both the molasses and control treatments.