Fish-meal supplementation of grass silage offered to young steers: Effectson growth, body composition and nutrient efficiency

Growth and efficiencies of nitrogen and energy utilization for growth by 72 young British Friesian steers (initial live weight (LW) 110 kg) offered a well preserved, formic acid-treated, perennial ryegrass silage with and wit hout supplements of fish meal were examined. Silage was offered either alon e or mixed with 50, 100 or 150 g fish meal/kg silage dry matter (DM) and ea ch diet was offered either ad libitum or intakes were restricted to 16, 19 or 22 g dietary DM/kg LW/day. Treatments were imposed over a period of 132 days, Body component weight gains were determined by comparative slaughter. Increasing the level of either feeding or fish meal increased rates of empt y body weight gain linearly (P < 0.001) and curvilinearly (P < 0.05) respec tively. Fish-meat supplementation increased rates of ash and crude protein gain (P < 0.001) but, in comparison with the curvilinear response to increa sing level of feeding (P < 0.001), had small linear effects on fat gain (P > 0.01). Consequently, in terms of whole body composition, animals given fi sh meal were leaner than animals offered silage alone. Fishmeal supplementa tion had no significant effect on the composition of the carcass but increa sed the concentration of protein in the liver and gastrointestinal tract. The increase in nitrogen intake associated with feeding fish meal resulted in a reduction in the efficiency of nitrogen utilization as level of fish m eal increased. Nitrogen intake required for maintenance was estimated to be 1.054 g/kg LW0.75. In spite of marked differences in the composition of th e empty body-weight gain, there was no evidence to support an effect of fis h meal on the efficiency of metabolizable energy (ME) utilization for growt h (k(f)) which was estimated to be 0.346 on the basis of data scaled by LW0 .75. ME intake required for maintenance (MEm) was estimated to be 0.536 and 0.502 MJ/kg LW0.75] for silage alone and the 150 g fish-meal level respect ively.