Management of phosphorus nutrition of beef cattle grazing seasonally dry rangelands: a review

This review examines the effects of phosphorus (P) deficiency as a major constraint to productivity of cattle grazing rangelands with low-P soils. Nutritional deficiency of P may severely reduce liveweight (LW) gain of growing cattle (e.g. by 20–60 kg/annum) and the productivity of breeder cow herds as weaning rate, mortality and calf growth. In seasonally dry tropical environments, the production responses to supplementary P occur primarily during the rainy season when the nutritional quality of pasture as metabolisable energy (ME) and protein is high and pasture P concentration is limiting, even though the P concentrations are higher than during dry season. When ME and nitrogen of rainy-season pasture are adequate, then P-deficient cattle typically continue to gain LW slowly, but with reduced bone mineralisation (i.e. osteomalacia). In beef breeder herds when diet P is insufficient, cows with high bone P reserves can mobilise bone P reserves during late pregnancy and early lactation. Mobilisation may contribute up to the equivalent of ~7 g diet P/day (one-third of the P requirements) in early lactation, and, thus, allow acutely P-deficient breeders to maintain calf growth for at least several months until depletion of cow body P reserves. However, severe P deficiency in cattle is usually associated with reduced voluntary intake (e.g. by 20–30% per kg LW), severe LW loss and poor reconception rates. When P intake is greater than immediate requirements, breeders can replenish bone P. Replenishment in mature cows occurs slowly when ME intake is sufficient only for slow LW gain, but rapidly at ME intakes sufficient for rapid LW gain. Bone P replenishment also occurs in late-pregnant heifers even when losing maternal LW. Intervals of mobilisation and replenishment of body P reserves will often be important for P nutrition of beef breeder cows through annual cycles. Diagnosis of P deficiency in grazing cattle is difficult and must encompass estimation of both diet P intake and availability of P from body reserves. Cattle behaviour (e.g. pica, osteophagea), low soil P concentrations and low herd productivity provide valuable indicators. Some constituents of blood (plasma inorganic P, calcium, plasma inorganic P:calcium ratios and endocrine markers) are valuable indicators, but the threshold values indicative of P deficiency at various ME intakes are not well established. It is evident that knowledge of both the nutritional physiology and requirements for P provide opportunities to better manage P nutrition to alleviate production losses in low-input systems with beef cattle grazing rangelands.

Evaluation of an inexpensive needle test for the diagnosis of phosphorus deficiency and management of phosphorus supplementation for cattle: A multiple case study

The needle test (NT) is a point-of-care test developed in Brazil to evaluate the Phosphorus (P) status in cattle. Based on bone resistance, the NT is a very inexpensive method which allows the diagnosis of any degree of P deficiency in a fast and simple way in vivo and directly on farm. The NT measures three levels of resistance in the transverse process (TP) of the lumbar vertebrae: a) TP that are impenetrable and result in warping of the needle (P healthy animals); b) TP offering some resistance to the penetration (animals with subclinical P deficiency); and c) TP which has minimal resistance to penetration (clinical P deficiency). This manuscript presents results from a series of case studies to evaluate the hypothesis that the NT could be used to assess P status in cattle and assesses the usefulness of results to support decision making on mineral supplementation strategies for grazing cattle. The NT was able to detect the changes in the resistance patterns of the TP, as there was reduction or elevation of P levels in the mineral mixtures. The NT was useful to assist in decision-making for adoption of mineral supplementation strategies better suited for each farm, helping farmers to save money and avoid unnecessary waste of P.

Alternative rib bone biopsy measurements to estimate changes in skeletal mineral reserves in cattle

Rib bone biopsy samples are often used to estimate changes in skeletal mineral reserves in cattle but differences in sampling procedures and the bone measurements reported often make interpretation and comparisons among experiments difficult. 'Full-core' rib bone biopsy samples, which included the external cortical bone, internal cortical bone and trabecular bone (CBext, CBint and Trab, respectively), were obtained from cattle known to be in phosphorus (P) adequate (Padeq) or severely P-deficient (Pdefic) status. Experiments 1 and 2 examined growing steers and Experiment 3 mature breeder cows. The thickness of cortical bone, specific gravity (SG), and the amount and concentration of ash and P per unit fresh bone volume, differed among CBext, CBint and Trab bone. P concentration (mg/cc) was closely correlated with both SG and ash concentrations (pooled data, r=0.99). Thickness of external cortical bone (CBText) was correlated with full-core P concentration (FC-Pconc) (pooled data, r=0.87). However, an index, the amount of P in CBext per unit surface area of CBext (PSACB; mg P/mm2), was more closely correlated with the FC-Pconc (pooled data, FC-Pconc=37.0+146×PSACB; n=42, r=0.94, RSD=7.7). Results for measured or estimated FC-Pconc in 10 published studies with cattle in various physiological states and expected to be Padeq or in various degrees of Pdefic status were collated and the ranges of FC-Pconc indicative of P adequacy and P deficiency for various classes of cattle were evaluated. FC-Pconc was generally in the range 130 to 170 and 100 to 120 mg/cc fresh bone in Padeq mature cows and young growing cattle, respectively. In conclusion, the FC-Pconc could be estimated accurately from biopsy samples of CBext. This allows comparisons between studies where full-core or only CBext biopsy samples of rib bone have been obtained to estimate changes in the skeletal P status of cattle and facilitates evaluation of the P status of cattle.

Bone mineral density in the tail-bones of cattle: effect of dietary phosphorus status, liveweight, age and physiological status

In three grazing experiments in the seasonally dry tropics of Australia, growing steers (Experiment 1), first-calf cows (Experiment 2) and mature breeder cows (Experiment 3), ingested diets for 12–17 months, which were either adequate or severely deficient in phosphorus (P) (Padeq and Pdefic, respectively). Bone mineral density (BMD) at the proximal end of the ninth coccygeal vertebra (Cy9) was measured at intervals using single photon absorptiometry (SPA). Liveweight (LW) and plasma inorganic phosphorus (PIP) concentrations were monitored at intervals and rib-bone cortical bone thickness (CBT) of biopsy samples was measured at the end of Experiments 1 and 3. Measurements of LW change, PIP concentrations and CBT confirmed that diet P intakes of cattle in the Padeq treatments were adequate whereas there was severe and chronic P deficiency in the Pdefic treatments. In Experiment 1 BMD in Padeq steers increased with LW and age from ~0.25–0.27 g/cc (8 months, 200 kg LW) to ~0.34 g/cc (32 months, 490 kg LW), whereas in Pdefic steers BMD decreased progressively to ~0.23–0.24 g/cc. Although BMD decreased in the Pdefic steers bone volume of Cy9 (calculated from tail-bone thickness) increased, and some net bone deposition in the Cy9 continued. Rib-bone CBT and tail-bone BMD at the end of Experiment 1 were closely correlated (r = 0.93). In Experiment 2 BMD was initially 0.33 g/cc (~25 months, 400 kg LW) and did not change through pregnancy and lactation in Padeq cows. However, in the Pdefic cows there was a gradual decline in BMD to ~0.25 g/cc. There was no change in dimensions of the Cy9 so the decreases in BMD involved net demineralisation of bone. In Experiment 3 BMD was less responsive to P deficiency than in Experiments 1 and 2. Only after ~11 months was BMD reduced (P < 0.05) in the Pdefic cows, and then only by 15%. In contrast, rib-bone CBT decreased by 30% due to P deficiency, and BMD was poorly correlated with CBT (r = 0.4). The effects of animal weight, age and maturity on tailbone BMD of P-adequate animals, and the different responses to P deficiency observed in young growing steers, first-calf cows and mature breeders are discussed in relation to the use of SPA measured tail-bone BMD to diagnose P deficiency in grazing cattle.

Utilising mobilisation of body reserves to improve the management of phosphorus nutrition of breeder cows

Phosphorus (P) deficiency is a major constraint to the productivity of breeder herds grazing low-P rangelands due to adverse effects on growth and fertility. However, P supplementation during the wet season, when additional dietary P is most needed, is often difficult due to practical constraints. Body P reserves in breeders can be mobilised and alleviate dietary P deficiency within an annual cycle. Approaches to estimate bone P reserves and net mobilisation or replenishment of P from the analysis of rib and hip (tuber coxae) biopsies are discussed. In at least some circumstances, breeder cows grazing P-deficient pastures mobilise bone P to alleviate the effects of diet P deficiency. Recent experiments with breeders have investigated mobilisation of body P to alleviate the adverse effects of dietary P deficiency during pregnancy and early lactation, and subsequent replenishment of body P reserves. Both mature cows and first-calf cows (FCC) calving in a high P status and fed severely P-deficient diets during lactation were able to mobilise sufficient body P reserves to provide milk for moderate calf growth (viz. 0.6–0.8 kg liveweight (LW)/day for 3 months), but this was associated with rapid cow-LW loss and markedly decreased bone P content. First-calf cows appear to have lesser capacity than mature cows to mobilise body P. FCC fed P-adequate diets during late pregnancy maintained high voluntary intakes and had higher LW and bone P reserves at calving. When fed a P-deficient diet during early lactation, these greater bone P reserves were utilised. Bone P reserves can be replenished by P-adequate diets fed post-weaning. Breeder management that relies more on mobilising body P reserves when P demands are high and on replacing these body P reserves when P demands are lower offers an opportunity to alleviate the effects of dietary P deficiencies during the early wet season in situations where P supplementation is not possible.