Phosphorus supplementation improves the growth and reproductive performance of female Brahman cattle grazing phosphorus-deficient pastures in the Victoria River District, Northern Territory, Australia

Abstract

Context There is a paucity of information on the responses to phosphorus (P) supplementation of breeder herds grazing P-deficient rangelands in northern Australia. Aims A study examined the production and economic benefits of P supplementation of Brahman females grazing P-deficient pastures in the Northern Territory (NT) of Australia. Methods Two treatment groups (initially n ~ 90) grazed two similar P-deficient paddocks for 5 years after weaning (at approximately 6 months of age). Management was the same except that the loose lick mineral supplement either contained P (+P) or did not contain P (−P). Performance attributes were measured from commencement until after the cows had the opportunity to calve during three successive annual cycles. Key results The cows in the +P treatment were overall heavier (P < 0.001), taller (P < 0.001) and had higher plasma inorganic P (PiP) concentrations (P < 0.001) than did those in the −P treatment at every measurement date after the first wet season. Pregnancy rates in lactating cows were 25%, 39% and 63% units higher (P < 0.01) in +P in 2017, 2018 and 2019 respectively and overall average weaner weight was 33 kg heavier (P < 0.001) in +P. Due to higher weaning rates and heavier weaning weights, +P produced about AU$39 365 more value of weaners per 100 heifers allocated to each treatment than did −P over the first three calf crops. The cumulative mortality rate was 12% higher in the −P over the 5 years (14.8% vs 2.2%, P < 0.01), and would have been even higher if at-risk −P cows had not been fed additional supplements. Return on investment calculations indicated that each AU$1 spent on P supplement returned AU$4.98 in value of extra calves weaned and AU$7.88 in extra total liveweight increase over the 5 years of the study. Conclusions The large production and financial benefits from P supplementation observed in this study are relevant to most northern Australian properties where soils are P deficient, although the magnitude of response is likely to vary depending on the level of P deficiency. Implications The large production and economic benefits from P supplementation in this study demonstrated the importance of P supplementation for breeder herds grazing P-deficient pastures.