Thirty-six Holstein/Friesian cows were used in a 3 × 2 factorial design 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-ofparlour feeders (OPF). The experiment included days 11 to 160 of lactation. On the CD treatment the diet was offered ad libitum with a concentrate proportion of 0.64 (dry matter (DM) basis), while on the OPF treatment the grass silage urns offered ad libitum and the allowance of concentrate was made equal to the mean concentrate intake of the CD treatment during the previous week. The concentrate was based on barley, maize gluten, molassed sugar-beet pulp, citrus pulp, soya-bean meal, fish meal and protected fat. During the experiment eight blocks each of six animals were used in metabolism studies to determine total ration digestibility, nitrogen balance and energy utilization.
No significant feeding method × genetic index interactions were found in terms of food intake, milk production or nutrient utilization. Although CGI had no significant effect on total DM intake, silage DM intake increased linearly as 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 fP < 0.001) than the medium and low CGI animals without altering milk concentrations of fat and protein, but with on average a negative live-weight 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 utilization of metabolizable energy for lactation fk,) 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 between 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 more milk CP < 0.05) than feeding concentrate separate from silage without altering milk concentrations of fat and protein. In the nutrient metabolism study whole tract digestibilities of DM (F < 0.001), nitrogen (P < 0.05) and energy (P < 0.01) were higher on the OPF treatment, but methane energy output and heat production were similar between the two treatments.