EFFICIENCY OF PRODUCTION IN CATTLE OF 2 GROWTH POTENTIALS ON NORTHERNGREAT-PLAINS RANGELANDS DURING SPRING-SUMMER GRAZING

A study was conducted to evaluate the effect of sire growth potential, steer age, and cow size on intake, growth, and production efficiency in grazing cattle. Data were collected on 24 cow-calf pairs during eac h of four summers (1989 to 1992) and on 12, 7-mo-old and 12 yearling s teers during three summers (1990 to 1992). Suckling calves and older s teers were sired by either high- (Charolais with high EPD for yearling weight) or moderate-growth-potential (Line 1 Hereford with average ye arling weight ratios) bulls. Cow size was defined by principal compone nt scores developed from cow weights, condition scores, and hip height s. Forage, but not milk, OM intake by suckling calves was influenced ( P < .05) by sire growth potential. Yearling high-growth-potential stee rs tended to consume more OM than yearling moderate growth potential s teers (P < .10) when expressed as kilograms/day but not when expressed as grams/kilogram BW (P > .10). Seven-month-old steers ate less (P < .01) forage (4.3 kg/d) than yearlings (5.6 kg/d) when expressed as kil ograms/day but more (P < .01) when expressed as grams/kilogram BW (7-m o-old, 15.9 vs yearling, 14.4 g/kg BW). Cow OM intake was affected by cow size and milk production but not calf growth potential. Milk produ ction but not cow size was a significant covariate for cow efficiency (grams of calf BW gain/kilogram of forage OM intake by cow-calf pair). Calf sire growth potential did not affect cow efficiency. We conclude that growth potential of sire for suckling calves and steers and cow size for cows affected intake of rangeland forage in summer but did no t affect efficiency of production from Northern Great Plains rangeland s.