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a Estación Experimental Coronel Suárez, MAA, CC 204, 7540 Cnel. Suárez (BA) Argentina
b Comisión de Investigaciones Científicas de la Provincia de Buenos Aires
c Departamento de Agronomía CERZOS, Universidad Nacional del Sur, 8000 Bahía Blanca (BA) Argentina
* Corresponding author (marzadun{at}faa.unicen.edu.ar).
Received for publication December 17, 2001. The effects of three different grazing pressures on beef (Bos taurus) and wheat (Triticum aestivum L.) grain production were studied at Pasman (Argentina) during a 3-yr study. Thirty hectares of the variety ‘Pincen’ wheat were grazed with Angus heifers during 3 yr (1995–1997) until apex differentiation. The different grazing pressures were obtained through different stocking rates, adjusted every 21 d according to forage mass, predicted forage production rate, and an estimated forage allowance of 10, 15 and 20 kg dry matter (DM) heifer-1 d-1 for high, medium, and low pressures, respectively. The three treatments were randomly allocated within a complete block design with two replications. Every 21 d forage samples were clipped and forage mass and plant composition were recorded and in vitro dry matter digestibility (IVDMD), crude protein (CP), and neutral detergent fiber (NDF) were determined. Spike density and grain production were measured at harvest. Average daily gain (ADG) was obtained from 10 heifers per plot, present throughout the entire grazing period. Gain per hectare (GH) was obtained in each plot from the mean ADG and animal days per hectare computed during stocking rate adjustments. During grazing period the levels of CP and IVDMD decreased and NDF content increased, and these trends were more pronounced in 1996 and 1997 because of rust infection. Increased grazing pressure caused increased GH and reduced ADG, forage mass at the end of the grazing period, and grain production. Means of treatments over the years for GH were 283, 225, and 176 kg ha-1, and for grain production were 955, 1150, and 1351 kg ha-1 for high, medium, and low pressures, respectively. The increase in grazing pressure (from low to high treatment) produced an increase of 107 kg ha-1 in beef production and a 396 kg ha-1 reduction in grain production. Taking into account the traditional price of beef with respect to grain in Argentina, this beef gain over grain relation at increased grazing pressure is economically profitable for farmers.
Abbreviations: ADG, average daily gain • CP, crude protein • DM, dry matter • DP, dual purpose • GH, animal weight gain ha-1 • IVDMD, in vitro dry matter digestibility • NDF, neutral detergent fiber
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