METABOLIC CHANGES IN BRANGUS STOCKER CALVES GRAZING WHEAT PASTURE

Authors: APPEDDU, LISA - SW OKLA. STATE UNIV.; Brown, Michael; Phillips, William

Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/20/2003
Publication Date: 8/15/2003
Citation: Appeddu, L.A., Brown, M.A., Phillips, W.A. 2003. Metabolic changes in Brangus stocker calves grazing wheat pasture [abstract]. Western Section of Animal Science Proceedings. 81(1):91.

Interpretive Summary: Wheat pasture plays a major role in the stocker industry in the Southern Great Plains. Previous work has substantiated a period of adaptation when gains are minimal or negative for the first 21 days after stocker calves are first placed on wheat pasture. During this adaptation time, stocker operators are not only losing per diem costs, but inadequate nutrition during this time exacerbates the stresses in the stockers and increases the probability of sickness, morbidity, and death losses. Shortening the adaptation period could result in substantial increases in net income for stocker operators. Research at the USDA-ARS Grazinglands Research Laboratory suggested that a portion of the poor performance for the first 21 days on wheat pasture may be attributable to energy costs required to metabolize the high levels of protein in wheat pasture. Strategies to facilitate metabolism of protein to spare energy in stocker cattle may be useful in improving performance during the first 21 days on wheat pasture.

Technical Abstract: Previous research suggests stocker calves do not effectively convert wheat forage to gain when first placed on pasture. The objective of this research was to investigate metabolic changes in stockers over the first 21 to 49 d on fall wheat pasture (39% CP, 35% NDF) by evaluating serum metabolites and rumen function in Brangus x Hereford steers. Calves were weaned (n = 24; 268 ± 36.4 kg), and offered prairie hay ad libitum (6% CP, 76% NDF) and 40% CP supplement daily (1362 g/hd/d). Calves were sorted into three groups to be placed on wheat at successive 10 to 14 d intervals. Calf weights and serum samples were taken prior to and after grazing wheat. Rumen fluid was taken from two cannulated steers over 72 h on 10 d prior to and 4 d after being placed on wheat to determine ammonia levels. Additional fluid was taken to evaluate potential changes in 48-h in vitro digestibility of wheat forage prior to and on d 6, 13, and 21 after grazing wheat. As expected, calves did not achieve a positive weight gain until after grazing wheat for 14 d. Calves gained 1.7 kg/d from d 28 to 49. Serum non-esterified fatty acid levels did not change in steers prior to or after grazing wheat for 6 or 13 d (295 vs avg 294 ± 23.7 mg/dl); however, levels decreased (P < 0.001) by d 20 and 27 (avg 213 mg/dl). Serum glucose levels increased (P < 0.01) after d 20 (87 vs 100 ± 5.0 mg/dl). Serum urea nitrogen was higher (P < 0.001) during the first 21 d on wheat regardless of sample day (9 vs 23 + 0.7 mg/dl). Rumen ammonia levels also increased (P < 0.001) after cannulated steers were placed on wheat (2.5 vs 22.7 ± 3.74 g/dl). Serum glucose and urea nitrogen remained elevated through d 49 (117 ± 5.0 and 21 ± 1.2 mg/dl). Day of rumen fluid collection did not change wheat forage in vitro digestibilities (90 ± 1.1%). From January 2 to April 9, 2002, calves gained 1.0 +/- 0.05 kg/d, and wheat quality declined to 26% CP, 51% NDF, and 83% in vitro digestibility. Serum urea nitrogen and glucose levels remained above 21 ± 0.8 and 83 ± 7.1 mg/dl. While potential digestibility of wheat forage remains high, results suggest stocker calves adjust metabolically when first introduced to wheat pasture before positive weight gains can be achieved.