Influence of protein supplementation on forage intake and dietary selectivity of grazing steers

Research was conducted on tobosagrass (Hilaria mutica) range during April through July in 1985 and 1986 to quantify the effect of feeding three levels (0.00, 0.34, and 0.68 kg/hd/d) of cottonseed meal on dietary nutritional and botanical constituents, and forage intake of grazing steers. Esophageally fistulated steers collected samples during three week-long periods within each season that correspond to early, mid and late phenological stages of tobosagrass. Extrusa samples were analyzed for crude protein (CP), neutral detergent fiber (NDF), and in vitro digestible organic matter (DOM). Extrusa samples were also systematically scanned at 15x magnification using the microscope point technique. Material beneath each of 100 points/sample tray was identified as tobosagrass, alkali sacaton ^Sporobolus airoides), composited other grass, composited forbs or browse, as well as live vs. dead and whether the material was leaf, stem or flower. Organic matter intake (OMI) during 1985 was determined from fecal output and DOM. Digestible energy intake (Mcal/d) during 1985 was calculated as the difference between gross energy consumed and gross energy excreted. During 1985, steers receiving 0.34 and 0.68 kg/d CSM selected diets higher (P<0.05) in CP (12.0 and 10.5%, respectively) than unsupplemented steers (9.4%).

Steers fed 0.68 kg/d during April 1986 consumed diets higher (P<0.05) in CP (10.3%) than those receiving 0.34 kg/d (7.1%) or no supplement (8.2%). In late June, steers receiving 0.00 and 0.34 kg/d CSM consumed diets higher (P<0.05) (13.9 and 14.4%, respectively) in CP than steers fed 0.68 kg/d. Neither dietary NDF or the digestibility of organic matter were affected (P>0.05) during 1985 or 1986. Steers fed 0.68 kg/d CSM consumed more (P<0.05) organic matter (60.0 g/kg BW^-^S) than unsupplemented animals (4 8.4 g/kg BwO-75) in 1985 and had a higher (P<0.05) digestible energy intake (7.68 Mcal/d) than steers fed 0.34 kg/d CSM (6.67 Mcal/d) and unsupplemented steers (6.34 Mcal/d).

Supplemental CSM did not alter (P>0.05) the percentage of tobosagrass, alkali sacaton, other grasses, forbs, browse, live leaf, live stem, live flower, dead flower or the leaf/stem ratio selected by steers in 1985. Steers supplemented during 1985 with 0.68 kg/d selected diets lower (P<0.05) in dead leaf (10.5%) and dead stem (5.4%) than unsupplemented (16.4 and 7.7%, respectively) or low supplemented animals (13.4 and 6.9%, respectively). Unsupplemented steers during 1986 chose diets higher (P<0.05) ii-^ alkali sacaton (41.4%) and live leaf (49.2%) than steers fed 0.34 kg/d (32.1 and 43.7%, respectively) and 0.68 kg/d (35.4 and 45.5%, respectively). Dietary leaf/stem ratio was unaffected (P>0.05) by supplemental CSM in 1985 and 1986. Data suggest that except for possible increases in dietary CP, dietary nutrient and botanical composition will not generally be altered by supplemental protein but intake of organic matter and digestible energy may increase as a result of cottonseed meal supplementation.