NITROGEN-UTILIZATION BY RUMINANTS DURING RESTRICTED INTAKE OF HIGH-CONCENTRATE DIETS

A series of experiments was conducted to determine the effects of limit-feeding high-concentrate (LFHC) diets on dietary CP requirements of steer calves. When steer calves were fed 80% concentrate diets at 78 g/kg of BW.75, increasing dietary CP resulted in increased ADG (P < .001). Average daily gain was increased in steers as daily monensin dosage increased from 120 to 180 mg (P < .05). Increasing the daily monensin dosage to 240 mg did not increase ADG further. There were no (P > .10) CP x monensin interactions, suggesting that the monensin response was caused by improved energy utilization and not be the possible protein-sparing effects of ionophores. Steer calves in the second feedyard experiment expressed similar ADG when provided equal NE(g) as limit-fed, high-moisture ear corn (HMEC) or when given ad libitum access to corn silage. The basal diet did not affect the steers' daily N requirement for growth. Gain per unit of protein intake declined quadratically (P < .05) with increasing CP intake, indicating that CP requirements were near NRC estimates on both diets. The corn silage-based diet was less digestible (70.3 vs 77.4%; P < .01) than the HMEC diet when fed to lambs. Fecal output differed (P < .10) substantially (342 g/d of corn silage vs 205 g/d of HMEC), whereas fecal N output was only slightly higher (6.97 vs 6.34 g/d, respectively; P < .10). Limited feeding of higher-concentrate diets to steer calves seemed to be an effective management procedure and did not cause acute digestion upset. Increasing the CP intake (g/d) may increase performance when LFHC diets are used. Monensin improved the efficiency of utilization of limit-fed, high-concentrate diets.