Submitted to: Journal of Animal Science Supplement
Publication Type: Proceedings
Publication Acceptance Date: 7/6/1995
Publication Date: N/A
Citation: Interpretive Summary: Genetic selection and breed choice are 2 means used by the industry to alter growth rate in cattle. The impact that increased growth rate may have on efficiency of production depends upon the growth stage of the animal and the environment under which efficiency is measured. Cattle grazing on rangeland are subjected to changes in nutrient quantity and quality throughout the year. This potentially limiting environment may affect the animal's ability to express genetic differences in growth potential & may, in turn, affect efficiency of production. One component of efficiency is food intake and a number of factors may influence level of intake in grazing cattle. The objectives of this study were to evaluate the effect of sire growth potential on food intake in grazing steers. Steers sired by either high indexing (Charolais-sired) or moderate indexing (Hereford-sired) bulls were used in a 4 year study conducted at the USDA-ARS laboratory near Miles City, MT. Forage, but not milk, organic matter intake by suckling calves was influenced by sire growth potential. Milk organic matter intake decreased from 1026 g/d in June to 902 g/d in July & 698 g/d in Sept. Forage organic matter intake for suckling calves increased from 340 g/d in June to 819 g/d in July and to 2358 g/d in Sept. Sire growth potential had no effect on intake by 7-mo-old or yearling steers, expressed as either kg/d or g/kg BW. We conclude that while growth potential of sire affects forage intake of suckling calves it does not affect intake of 7-mo-old or yearling steers grazing rangeland in summer. Summer grazing of 7-mo-old and yearling steers may not provide the nutrients required to allow for expression of potential genetic differences for growth.
Technical Abstract: Steers sired by either high indexing (Charolais-sired) or moderate indexing (Hereford-sired) bulls were used in a 4-yr study to evaluate the effect of sire growth potential on intake in grazing cattle. Intake was estimated using sustained release Cr2O3 boluses (Captec , Nufarm, Auckland, NZ) as an external marker to determine fecal output and diet digestibility as determined by in vitro digestibility of esophageal masticate. Milk intake of calves was determined by weigh-suckle-weigh. June collections were made in a 68 ha pasture seeded to Russian wildrye, July collections were made in a 80 ha pasture that had been contour furrowed and interseeded with alfalfa, and September collections were made in a 265 ha pasture of native rangeland. Forage, but not milk, OM intake by suckling calves was influenced by sire growth potential. Forage OM intake averaged 1237 g/d for high and 1106 g/d for moderate growth potential calves. Milk, forage, and total intakes were affected by mo of sampling. There were significant effects of yr and yr by mo interactions for forage and total intakes. Milk OM intake decreased from 1026 g/d in June to 902 g/d in July and 698 g/d in September. Forage OM intake for suckling calves increased from 340 g/d in June to 819 g/d in July and to 2358 g/d in September. However, sire growth potential had no effect on intake, expressed as either kg/d or g/kg BW. Seven-mo-old steers ate less forage (4.3 kg/d) than yearlings (5.7 kg/d) when expressed as kg/d, but more when expressed as g/kg BW (7-mo-old, 15.9 vs yearling, 14.5 g/kg BW). We conclude that while growth potential of sire affects forage intake of suckling calves it does not affect intake of 7-mo- old or yearling steers grazing rangeland in summer.