The effect of differing forage:concentrate ratio and restricting feed intake on the energy and nitrogen utilization by beef cattle

Abstract

Six Charolais cross steers (mean live weight 459 kg) were used in a partially balanced changeover design experiment with four 4-week periods. The energy and nitrogen utilization by cattle offered diets differing in forageiconcentrate ( F: C) ratio, but of equal metabolizable energy intake (MEI), were measured. The three diets were (1) high digestibility (D) grass silage offered ad libitum; (2) low D silage offered ad libitum and supplemented with sufficient concentrates to provide the same MEI as (1); (3) high D silage supplemented with the same F: C ratio as (2) and with intake restricted to the same MEI as (1) and (2). These three diet types were offered at two levels of energy intake, the higher level of energy intake having been achieved by adding further concentrates to diets 1 to 3. Gross energy intake (GEI) and faecal, urinary and methane outputs and heat productions were measured in respiration chambers for three days at the end of each period. When values were adjusted to equal MEI across the three diets, animals offered the restricted diet produced significantly less methane, expressed as a proportion of GEI ( P < 0.001), than those offered the other two diets. However, there was no difference in energy retention or the efficiency of utilization of energy for growth between the three diet types used in this study.