Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.

Mobilisation and replenishment of phosphorus reserves in Bos indicus cows. 2. Mature lactating cows fed diets deficient or adequate in phosphorus

Robert Dixon , Marcelo Benvenutti , Kerry Goodwin, Stephen Anderson, Bob Mayer, Peter Isherwood, Lisa Kidd, Mary Fletcher

Abstract

Context: Pastures growing on low phosphorus (P) soils are often P deficient, particularly for lactating cows. Aim: To examine the effects of P-deficient diets on the performance of lactating multiparous cows. Methods: From parturition mature Bos indicus cross cows (n=32, initially 474 kg liveweight and body condition score 3.5) were fed ad libitum for 14 weeks one of four diets: (i) high P (HP-HCa), (ii) low P with low calcium (LP-LCa), (iii) low P with high Ca (LP-HCa), or (iv) the LP-LCa diet with ammonium chloride (LP-LCa+ac). Results: Voluntary feed intakes were higher for cows fed the HP-HCa than the three LP diets (23.1 versus 17.4–19.6 g DM/kg liveweight (LW).day) (P<0.001). The HP-HCa cows gained 0.15 kg LW/day while LP cows lost 0.14–0.51 kg LW/day; P<0.05). The cows fed LP diets generally maintained milk production and calf growth comparable to that of HP-HCa cows (5.8 kg/day and 0.70 kg/day). The HP-HCa cows retained 6.0–6.4 g P/day from weeks 1-8, but P retention was negligible at week 14. The LP cows mobilised 4.9–9.1 g body P/day. Rib cortical bone shortly after parturition was 634 ±127 (337 – 848) µg P/mm2 in the 12th rib, and indicated that the bone P reserves were deficient, marginal and adequate in 27, 37 and 37% of the cows, respectively. During lactation rib bone P increased 23% in HP-HCa cows, changed little in the LP-LCa and LP-HCa cows (-4 and +7%), and decreased 13% (P<0.05) in LP-LCa+ac cows. The change in rib cortical bone P during lactation was correlated with this measurement at parturition; the PSACB of cows with low bone P at parturition did not change but in cows with high bone P decreased up to 16%. Plasma inorganic P (Pi) averaged 1.49 mmol/L in the HP-HCa cows and <0.8 mmol/L in the LP cows. Plasma Ca, Ca/Pi ratio, 1,25-dihydroxy vitamin D3, bone alkaline phosphatase (BAP), and carboxy-terminal telepeptides of type 1 collagen (CTX-1) increased during lactation (P<0.05 to P<0.001) in the LP cows. Conclusions: When fed acutely P-deficient diets mature cows in moderate liveweight and BCS at parturition generally maintained milk secretion and calf growth by mobilisation of body reserves. Implications: The capacity of beef cows to maintain lactation when ingesting P deficient diets provides opportunities for improved herd management.

AN24216  Accepted 10 December 2024

© CSIRO 2024