Nitrogen fertiliser effects on milk yield and composition, pasture intake, nitrogen and energy partitioning, and rumen fermentation parameters of dairy cows in early lactation

The effects of nitrogen (N) fertiliser on milk production and composition, voluntary dry matter intake (DMI), and aspects of rumen fermentation and N and energy partitioning were studied in early lactation. Twenty Jersey and 12 Friesian cows were individually offered freshly cut pastures that had received either 100–150 kg N/ha (HN) or 25 kg N/ha (LN) as urea, either ad libitum or restricted to 70% of ad libitum. Treatments were imposed for two consecutive 14‐day periods during September and October 1993. The main effects of higher rates of N fertiliser (LN versus HN) were to decrease milk protein yield (0.58 versus 0.56 kg/cow per day; P < 0.05), milk protein content (36.0 versus 35.1 g/1000 g; P < 0.01), and DMI for cows on ad libitum pasture allowance (14.6 versus 14.0 kg DM/cow per day; P < 0.01). Dry matter and organic matter digestibilities, microbial N production, and N and energy balance were unaffected by N fertiliser. The proportion of ingested N retained in milk was reduced slightly (19.8 versus 21%; P < 0.01) whereas milk urea N and milk non‐protein N as a proportion of total N were increased by HN (0.21 versus 0.19 g/kg, P < 0.0001 ; and 7.0 versus 6.6%, P < 0.05). HN tended to increase rumen ammonia concentration and pH levels while foremilk urea levels were unchanged. There were no clear effects of N fertiliser on rumen volatile fatty acid concentrations. Effects of N fertiliser on the chemical and botanical composition of pasture were minor. We conclude that the application of N fertiliser in winter/early spring had only minor effects on pasture intake and milk composition in early lactation.