The effect of date of cut and barley substitution on gain and on the efficiency of utilization of grass silage by growing cattle

A primary growth of perennial ryegrass (Lolium perenne) was cut early or late to produce silages of high and low digestibility. The crops were wilted for 2-4 h and preserved with formic acid at 2.4 litres/t fresh weight. The resulting silages were well preserved with a pH of 3.9 and 3.8, lactic acid content of 108 and 73 g/kg dry matter (DM) and total nitrogen content of 24.6 and 18.4 g/kg DM for early- and late-cut silage respectively. Forty-two British Friesian male castrates (steers) initially 12 months of age and 305 kg live weight (LW) were used, of which ten were slaughtered at the start of the experiment. The remaining steers were divided into four groups of eight animals and were given the early cut-silage alone (H) or the late-cut silage alone (L) or with barley at either 280 (LC1) or 560 (LC2) g DM/kg total DM. The intake of total DM was restricted to a daily allowance of 18 g DM/kg LW and the steers were slaughtered in two groups after 119 and 140 d on experiment. Both earlier cutting of herbage and substitution of late-cut silage with barley significantly (P less than 0.001) increased the apparent digestibility of gross energy (H0.748, L0.619, LC10.668, LC20.705), whereas earlier cutting increased the digestibility of acid-detergent fibre from 0.638 (L) to 0.777 (H) and substitution with barley resulted in a significant (P less than 0.001) depression to 0.595 (LC1) and 0.519 (LC2). Sustitution of late-cut silage with barley significantly (P less than 0.001) increased metabolizable energy (ME) intake from 58.9 (L) to 69.5 MJ/d (LC2) and crude protein (N X 6.25; CP) intake from 668 (L) to 779 g/d (LC2), but the highest intakes of ME and CP (73.5 MJ/d and 952 g/d respectively) were achieved with the early-cut silage. Earlier cutting resulted in significant (P less than 0.001) increases in body-weight gain from 292 to 696 g/d, fat gain from 121 to 260 g/d, protein gain from 31.1 to 86.9 g/d and energy retention from 5.5 to 12.2 MJ/d for silages L and H respectively. However, substitution of the late-cut silage with barley increased gains to a greater extent. Thus, empty-body gain was increased to 552 and 800 g/d, fat gain to 189 and 302 g/d, protein gain to 76 and 116 g/d and energy retention to 9.2 and 14.6 MJ/d for diets LC1 and LC2 respectively. The difference in gains between diets H and LC2 achieved significance (P less than 0.05) for all components except fat. It is concluded that although earlier cutting of herbage for silage results in increased gains of protein and energy, the amounts retained are less than those from a similar increment of ME and CP achieved by substituting a late-cut silage with barley.