THE INFLUENCE OF GENETIC INDEX FOR MILK-PRODUCTION ON THE RESPONSE TOCOMPLETE DIET FEEDING AND THE UTILIZATION OF ENERGY AND NITROGEN

Thrity-six Holstein/Friesian cows were used in a 3 X 2 factorial desig n randomized-block experiment to evaluate the production and nutrient utilization responses of animals of three genetic indices (cow genetic index 90 (CGI); 950, 650 and 550), each given either a complete diet (CD) or concentrate separate from grass silage through out-of-parlour feeders (OPF). The experiment included days 11 to 160 of lactation. On the CD treatment the diet was offered ad libitum with a concentrate p roportion of 0.64 (dry matter (DM) basis), while on the OFP treatment the grass silage was offered ad libitum and the allowance of concentra te was mad equal to the mean concentrate intake of the CD treatment du ring the previous week. The concentrate was based on barley, maize glu ten, malassed sugar-beet pulp, citrus pulp, soya-bean meal, fish meal and protected fat. During the experiment eight blocks each of six anim als were used in metabolism studies to determine total ration digestib ility, nitrogen balance and energy utilization. No significant feeding method X genetic index interactions were found in terms of food intak e, milk production or nutrient utilization. Although CGI had no signif icant effect on total DM intake, silage DM intake increased linearly a s the cow CGI increased (P<0.01) across the CD and OPF treatments. The high CGI cows produced respectively 6.60 and 8.25 kg/day more milk (P <0.001) than the medium and low CGI animals without altering milk conc entrations of fat and protein, but with on average a negative live-wei ght change with the high CGI cows. Although nitrogen digestibility was significantly higher with the low than medium CGI cows (P<0.05), cow CGI had no significant effects on DM and energy digestibilities, daily methane energy output, heat production or the efficiency of utilizati on of metabolizable energy for lactation (K-l) in the metabolism study . The results indicated that higher milk production with the high CGI cows was mainly attributed to an alteration in nutrient partitioning b etween milk energy and body energy retention. Across the three genetic indices, feeding method had no significant effect on total DM intake, although silage DM intake was 0.46 kg/day higher (P<0.05) on the OPF treatment. However feeding the complete diet resulted in 3.04 kg/day m ore milk (P<0.05) than feeding concentrate separate from silage withou t altering milk concentrations of fat and protein. In the nutrient met abolism study whole tract digestibilities of DM (P<0.001), nitrogen (P <0.05) and energy (P<0.01) were higher on the OPF treatment, but metha ne energy output and heat production were similar between the two trea tments.