Twenty-seven Hereford weaner steers (mean age 11 months, and live weight 142±17 kg) were used to study the effect of supplementation of an energy-rich grain (sorghum), and/or a protein meal on the live-weight change of cattle on a basal diet of low-quality grass hay over 48 days (Expt 1). The basal hay had an estimated energy value of 6·2 MJ metabolizable energy (ME)/kg dry matter (D.M.) and a nitrogen content of 6·2 g/kg D.M. Corresponding values for the sorghum supplement were 12·4 and 14·4, and for the protein meal 9·6 and 65. The daily rates of supplementation in Expt 1 were (g/head) 0 (nil), 600 (P1) or 1200 (P2) of protein meal pellets or 0 (nil), 560 (S1) or 1120 (S2) of sorghum and all combinations of P1 or P2 with S1 or S2. The carry-over effect of such supplementation on subsequent growth was recorded in Expt 2 when the steers grazed a kikuyu grass pasture for 100 days.
In Expt 1, organic-matter intake of hay (HOMI) was increased (P < 0·01) by protein but decreased (P < 0·01) by the sorghum supplements. By 48 days, P1-supplemented steers were eating 29·5% more hay than non-supplemented steers, and P2 steers were eating 43·5% more hay than the non-supplemented steers. S1 supplemented steers were eating 29·5% less, and S2 steers 23% less hay than non-supplemented steers. Steers lost weight on the basal hay diet, and at the lower level of sorghum (S1) supplementation, but were able to maintain weight at the higher level (S2) of supplementation. In contrast, steers supplemented with protein gained weight (P < 0·01): 430 g/day on P1 and 700 g/day on P2 treatments. None of the steers offered combinations of P and S had live-weight changes as high as those offered only protein supplements. There was no evidence of compensatory gain by the smaller steers in Expt 2 when grazing and unsupplemented.
Glucose kinetics in plasma were studied in each steer on day 35 of Expt 1 by means of a single intravenous injection of [2-3H]glucose. The quantity of glucose in the sampled pool and the total rate of glucose entry into the pool (GER) were higher (P < 0·01) in steers that were protein supplemented, with GER being related to both ME intake and to N intake per unit ME intake.
Urea pool size and entry rate were also estimated on day 35 of Expt 1 using a single intravenous injection of [14C]urea. The quantity of urea in the sampled pool, and entry rate into the pool were increased significantly (P < 0·01) by protein, but reduced (P < 0·05) by sorghum, supplementation. Rumen ammonia concentrations were low in steers given the hay alone, and when supplemented with sorghum (10–15 mg N/l), but were significantly higher (70 mg N/l; P < 0·01) during protein supplementation.
It was concluded that growth of cattle on the basal ration was restricted by low voluntary intakes of hay. Intake was increased substantially by protein supplementation. The consequent increase in live weight was due to a greater intake of organic matter which was accompanied by a greater synthesis of glucose and availability of amino acids in the animals.