Utilization of grass silage by cattle: effect of supplementation with different sources and amounts of protein

Forty-eight crossbred beef steer calves, initial live weight of 260 kg, were used to examine production responses when grass silage was supplemented with fish meal (FM) or soybean meal (SBM) during a 210-d feeding period. The silage was available on an ad libitum basis either alone or supplemented with 100, 225, 350, or 500 g of FM or 600 g of SBM/d. Increasing levels of FM resulted in a linear increase in live weight gain (P < .001) and gain:feed ratio (P < .001). The FM supplementation had no effect (P = .79) on silage DM intake. The FM supplement increased plasma albumin concentration (P < .05) but did not affect plasma glucose (P > .20) The steers were shipped to market when they had 8 mm of fat over the longissimus muscle as determined by ultrasound. Sensory evaluation of roasts from steers fed 0, 225, 350, or 500 g/d of FM indicated a positive linear (P < .01) increase in flavor and decrease in juiciness as FM level increased. There were no differences (P > .15) in live weight gain, intake, or gain:feed ratio between isonitrogenous supplements of FM and SBM. However, the additional gain resulting from SBM and FM indicated that, on a crude protein basis, SBM was approximately 80% as effective as FM in increasing live weight gain in steers fed grass silage.