Blood, rib bone and rumen fluid as indicators of phosphorus status of grazing beef cows supplemented with different levels of phosphorus at Armoedsvlakte

Abstract

Sixty-six grazing beef cows at Armoedsvlakte were supplemented per fistulam with different levels of phosphorus (P) from 1984 to 1989. Supplementation was provided according to a 3 × 2 factorial of three levels of P (Low, Medium and High), in the periods of either all year round (LP 12, MP 12 and HP 12) or six months only (LP 6, MP 6 and HP 6), i.e. September to February (late gestation to late lactation). Blood, rib bone and rumen fluid were evaluated as indicators of the P status of cattle. Animals which did not receive P supplement from March to August (LP 6, MP 6 and HP 6), had lower inorganic phosphorus (Pi) levels in blood plasma than the 12-month groups (LP 12, MP 12 and HP 12) by late pregnancy (August). By late lactation (March), after having received P supplement for six months, there was little difference between treatments. This pattern was repeated during each of the successive calving seasons, indicating that plasma Pi level reflected the P intake of animals, not necessarily their P status. There was a tendency for an increased plasma calcium (Ca) concentration to compensate for a low Pi level. At the end of the first year (August 1985), differences in bone P concentration were apparent, with the HP 6, MP 12 and HP 12 groups showing their superiority; by late lactation of the first calving season (March 1986), only the MP 12 and HP 12 cows were able to maintain bone P reserves. The rib bone P levels of the other treatments were considerably lower in March 1986, falling well short of the accepted 'normal' level of 140 mg P/cm3 fresh bone, and even below that which is classified as deficient, i.e. 120 mg P/cm3 fresh bone. In contrast to plasma Pi levels, bone P levels of the 6-month groups did not recover after six months of P supplementation. The pattern in bone Ca concentration was similar to that of bone P, as might be expected given the composition of the bone crystal. The Ca:P ratio in rib bone varied little from 2:1. In general, slightly wider ratios were apparent when cows were only supplemented for 6 months of the year. Bone density (SG, specific gravity) followed the same trends as observed for the individual bone minerals and may be used as a measure of bone mineralisation, although it is not as sensitive an indicator as a single mineral, e.g. P, nor does it provide information concerning the bone reserves of a specific mineral. In the 6-month groups, a sharp decline in rumen Pi level was observed during those months without any P supplement, followed by a rapid increase in rumen Pi once P supplementation recommenced in September. Rumen fluid Pi concentration was insufficiently sensitive to distinguish at all times between the different levels of supplementation (LP, MP and HP) but it readily distinguished between unsupplemented and supplemented (6- or 12-month) groups.