Responses to Protein Meal Supplements by Lactating Beef Cattle Given a Low-Quality Pasture Hay

Abstract

A feeding experiment (47 days) was undertaken to evaluate the role of protein meal supplements for Hereford cows given a low quality subtropical grass hay and suckling calves (mean age 37 ± 2.2 days). The rates of supplementation were 0, 5.25, 10.5, 15.75, and 21.0 g pelleted protein meal /kg liveweight W⁰·⁷⁵. The consumption of grass hay, and the estimated total metabolizable energy intake (MEI) by cows, were significantly (P < 0.01) increased by supplements of protein meal, and their rate of liveweight loss was reduced (P < 0.01). Unsupplemented cows lost 2.56 kg/day compared with a 0.15 kg gain/day for cows given the highest rate of supplementation. Daily milk production and yields of milk protein, lactose, solids-not-fat and fat increased linearly (P < 0.01) with increasing amounts of protein meal. Throughout lactation, daily milk yield declined most rapidly (P < 0.01) in cows given the lower rates of protein supplementation: on day 33 the milk yield in the cows given 21 g protein meal/kg W⁰·⁷⁵ was 83% higher than that of the unsupplemented cows. In a second period the rate of supplementation of cows was altered on day 34 from 0 to 21.0 g/kg W⁰·⁷⁵. Milk yield at day 47 increased to the same output as that of cows supplemented at 21.0 g/kg W⁰·⁷⁵ from day 1. Growth rates of calves, over both periods, tended to be higher in those calves whose dams received the higher rates of supplementation, but the differences did not become significant (P < 0.05) until day 47. There were no significant differences (P < 0.05) in voluntary intakes of hay by the calves whose dams received the various rates of protein supplementation. The concentration of total volatile fatty acids (VFA) was higher (P = 0.06) in the rumen of supplemented cows, but the molar proportions of VFA did not differ between treatments, viz. acetate, 0.71; propionate, 0.16; butyrate, 0.09; other VFA, 0.04. Rumen ammonia concentration was higher (P < 0.01) in cows supplemented with protein meal at the higher rates. The kinetics of plasma glucose and its products in the cows were studied on day 28 by means of a single intravenous injection of [2-³H] and [U-¹⁴C] glucose, and on day 29 by means of an intravenous injection of [2-³H] glucose and [¹⁴C] sodium bicarbonate. The rates of irreversible loss of glucose C and of bicarbonate C were higher in all supplemented cows. The results are discussed in relation to the availability of substrates for glucose synthesis, glucose oxidation and utilization for synthesis of lactose in milk.