PSVIII-8 - Impacts of Phosphorus Intake on Beef Heifer Growth Performance and Conception Rates

Abstract: The objective of this experiment was to further investigate the effects of phosphorus intake on beef heifer growth performance and conception rates. An increase in phosphorus soil concentrations from use of livestock manure as fertilizer in Northwest Arkansas has led to greater phosphorus concentrations available in forages. This study was designed to determine if phosphorus supplementation is warranted when adequate phosphorus soil concentrations exist. This experiment was conducted over 2 years using two separate groups of weaned crossbred Angus heifers (n=72/year). Approximately 30 d after weaning, heifers were stratified by body weight (average initial weight 262kg) and allocated randomly to 14 groups (8 in Year 1, 6 in Year 2). Groups were assigned randomly to 1 of 2 treatments: 1) a free-choice-mineral mix that contained no supplemental phosphorus (CON), or 2) a free-choice-mineral mix with 4% supplemental phosphorus and identical concentrations of other supplemental minerals (4PMIN). Heifers grazed 2.24 ha mixed grass pastures with a history of livestock manure application and were supplemented with soy hulls (0.5% of body weight) daily. On d 112, heifers > 273 kg body weight had an ultrasound evaluation of reproductive tracts (1= infantile, 5= cyclic). Heifers were determined pregnant or open via rectal ultrasonography. Data were analyzed using the MIXED or GLIMMIX procedures of SAS 9.4 with group within year as the experimental unit. There were no differences in gain for either treatments for the 224-day period (P ≥ 0.14). Reproductive tract scores did not differ (P = 0.95). There were no differences for conception rates (AI or natural bred) (P ≥ 0.55). Overall pregnancy was 79% for CON and 83% for 4PMIN. Heifers grazing pastures with a history of livestock manure application did not benefit from adding supplemental phosphorus in the free choice mineral offered.