Performance and Phosphorus Status of Range Cows with and without Phosphorus Supplementation

Abstract

Performance and phosphorus (P) status were studied in 2 groups of range cows, one of which had free access to salt alone (control) and the other to a mineral mix (P-supplemented). The mineral mix contained 50% dicalcium phosphate, 45% salt, and 5% cottonseed meal. Performance traits (birth date, calving interval, weaning weight, suckling gain, and percent calf crop) were compared over 5 years (1979-83), one of which was considered a drought year (1980). Cows involved in the study received no supplemental protein or energy during the experiment. Lack of P supplementation had a detrimental effect on cow performance only when coupled with the effects of drought. This apparent combined effect delayed postpartum estrus in control cows during the 1980 drought, thus causing them to calve later (7 April vs. 11 February; P<.05) and wean lighter (226 vs. 253 kg; P<.05) calves in 1981 as compared to P-supplemented cows. Percent calf crop did not differ (P>.05) between the 2 groups during any year of the study, although in both groups, this trait was considerably lower in 1981 than in other years because of the 1980 drought. These results suggest that rainfall or P supplementation before and during the breeding season may be critical in maintaining early calving dates and heavier weaning weights but, even with P supplementation, lower conception rates may occur under drought conditions. Phosphorus status of cows was estimated from fecal, saliva, and rib bone biopsy samples collected at 6 intervals from April 1981 to January 1982. Fecal P varied (P<.05) among sampling dates and was higher (P<.05) for P supplemented cows than for control cows when averaged over sampling dates. Levels of fecal P were higher (P<.05) during the period of active forage growth than during dormancy. Salivary P peaked concurrently with fecal P; however, across sampling dates, response to supplemental P was inconsistent as evidenced by a treatment × date interaction (P<.05). During lactation, bone P levels were higher (P<.10) in P supplemented cows than in control cows. After lactation, bone P did not differ (P>.10) between groups and was higher (P<.001) than during lactation, which indicates bone P levels can be replenished following lactation without P supplementation.