Effects of level of nitrogen fertilizer application and various nitrogenous supplements on milk production and nitrogen utilization of dairy cows given grass silage-based diets

Sixteen Finnish Ayrshire dairy cows were used in a cyclic change-over exper iment with four 28-day experimental periods and a 4 x 2 factorial arrangeme nt of treatments to evaluate the effects of dietary nitrogen (N) source on dry-matter (DM) intake., diet digestibility, milk production, blood metabol ite concentrations and N utilization. Experimental treatments consisted of two silages prepared from timothy and meadow fescue swards fertilized with either 52 or 104 kg N per ha (S-L and S-H, respectively) offered ad libitum supplemented with one of four concentrate supplements offered at 10 kg/day . A basal concentrate (control) was formulated from (g/kg on an air-dry bas is) barley (307), oats (460), molassed sugar-beet pulp (200) and vitamins a nd minerals (33). Three isonitrogenous concentrates were prepared by replac ement of basal concentrate ingredients (g/kg) with urea (14.4), wheat-glute n meal (WGM; 57.2) and heat-moisture treated mechanically expelled rapeseed cake (RSC; 188), respectively. Harvesting of secondary growths receiving h igher applications of fertilizer N increased silage N content (19.2 and 23. 6 g/kg DM, for S-L and S-H, respectively) but decreased water-soluble carbo hydrate concentrations (respective values 149 and 93 g/kg DM). Increases in N fertilizer had no effect on silage DM intake, output of energy-corrected milk, milk fat or milk protein secretion, while provision of additional N in concentrate supplements improved all of these parameters. However, the m agnitude of silage DM intake responses to additional N in concentrates was higher (P < 0.05) for S-L than S-H silage-based diets. Increases in dietary N content derived from silage or inclusion of urea, WGM or RSC in concentr ate supplements led to a decline in the proportion of N intake secreted in milk. Relative to N derived from silage or urea, WGM and RSC supplements ha d beneficial effects on milk production. Both the quantity and quality of d ietary N sources should be considered in future attempts to improve the eff iciency of on-farm N utilization.