Intake and Diet Selection by Protein Supplemented Grazing Steers

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Abstract
Nine esophageal-fistulated and 12 rumen-cannulated steers were randomly allotted to 3 equal supplement groups: cottonseed cake (CSC), ground, pelleted alfalfa hay (ALF), or no supplement (CON). Supplements were individually fed at isonitrogenous levels (1.7 kg .cntdot. hd-1 CSC vs 3.5 kg .cntdot. hd-1 ALF) every other day. Animals were maintined on treatment from January through April 1983 while grazing blue grama (Bouteloua gracilis) rangeland in southcentral New Mexico. Esophageal samples were collected at 2 times during this period: early February and late March; intake was estimated from total fecal collections in late February and early April. Esophageal samples were analyzed in the laboratory for nitrogen (N) components, in vitro digestibility, fiber, and botanical composition. Animal selection for total grass content of the diet was not influenced (P > .10) by supplementation but declined from February to March (31.8% vs. 21.2%, respectively). Blue grama and mat muhly (Muhlenbergia richardsonii) comprised over 66% of the grass component. Total forbs selected was not influenced by supplementation but increased (P < .05) from February (68.2%) to late March (78.8%), which may be the result of declining quantities of grass and emergence of forbs because of spring moisture. Dietary N components were not affected by supplementation. Soluble N, insoluble unavailable N, and crude protein were higher (P < .05) in February than March as a result of increasing forb consumption and possible N leaching from grass. Fiber components, acid detergent fiber and acid detergent lignin, as well as organic matter digestibility of the diet, were not influenced by supplementation, and increased from February to March but only organic matter digestibility was higher (P < .05) in late March than in early February. Forage organic matter intake was not influenced by supplementation (P > .49), type of supplement consumed, or sampling period (P > .90). Total organic matter intake however, differed (P < .05) between treatment groups because of addition of supplements. Results of this study indicate protein supplementation of wintering steers does not influence botanical or chemical composition of their diets or amount of forage consumed.