Ad Libitum Phosphorus Supplementation of Range Cows in the Northern Great Plains

Abstract

Phosphorus (P) supplementation of range cows is practiced in the Northern Great Plains, but animal performance data to support this practice are limiting. The performance of range cows and calves grazing native pastures with and without ad libitum supplemental P was compared during the June through October grazing seasons of 1978, 1979, and 1980. Pastures contained primarily western wheatgrass [Pascopyrum smithii (Rydb.) A. Lötve], needleandthread (Stipa comata Trin. and Rupr.), green needlegrass (S. viridula Trin.), blue grama [Bouteloua gracilis (H.B.K.) Griffiths], and upland sedges (Carex spp.). Phosphorus levels in extrusa and standing crop forage were monitored and compared to U.S. National Research Council (USNRC requirements for lactating beef cows (Bos taurus). Mean overall supplemental P consumption was 0.14, 0.09, and 0.02 oz/cow/d, which represented only 19, 12, and 3% of USNRC requirements for 1978, 1979, and 1980, respectively. Cow liveweight gains and calf adjusted weaning weights (205 d) were not affected (P > 0.05) by ad libitum supplemental P. Final serum P levels were higher (P < 0.05) in P supplemented cows compared to nonsupplemented cows in 1978 and 1979, but not in 1980. Ad libitum supplemental P consumption was roughly inverse to forage P levels in 1978 and 1979. However, for the 3 yr a low correlation coefficient (r = −0.57) was obtained between extrusa forage P and ad libitum supplemental P consumption. The poor relationship between forage P and supplemental P consumption, plus a lack of animal liveweight response any of the 3 yr, suggests that either forage P levels were adequate for animal needs or that animals did not consume adequate amounts of supplemental P as offered in the salt mixture. Further P supplementation studies are in progress to determine the effect of higher P levels fed over a longer period of time.