Relationship between phosphorus intake and blood or fecal phosphorus in gestating cows

Abstract

The relationship between fecal, serum or plasma phosphorus (P), and P intake was examined with 10, crossbred, 5-year-old, gestating cows (avg wt 475 kg) in an individual feeding study using 2 orthogonal 5 X 5 Latin squares. All cows received 9.07 kg of meadow grass hay which contained 7.4% crude protein (CP) with an in vitro dry matter digestibility (IVDMD) of 56%, and received .5 kg of 1 of 5 supplements which resulted in P intakes of 10.3, 12.4, 14.3, 16.1, or 18.4 g/day. Fecal and blood samples were collected for 5 days after a 14-day dietary adjustment period. Fecal grab samples were taken twice daily (hour 0800 and 2000). Blood samples were taken at 0800. Statistical analysis included analysis of variance and regression analysis. A linear response to P intake was observed for both plasma and serum P, however, with regression of P intake, the R2 for plasma P was .06 and for serum P was .10. Evaluation of morning and evening fecal P levels with regression resulted in different equations. The morning equation (Y = .055 + .212X) had a larger intercept and a smaller slope coefficient than the evening equation (Y = -.781+ .310X). Morning and evening R2 were .69 and .78, respectively. To examine the predictive ability of the P intake equations, a validation trial was conducted with 20 4-to-gyear-old cows individually fed (4/treatment). Daily P intakes were 10.0, 12.4, 15.3, 20.4, and 22.6 g. Management and sampling procedures were the same as used in the previous trial except blood samples were not collected. There was no difference (P>.05) in the equations established with regression from the morning and evening samples. The combined regression equation was (Y = .306 + .219X). This equation was not different (P>.05) from the equations established from either the morning or evening samples in the previous trial. These data indicate that fecal P is related to P intake; however, the extent that this relationship is influenced by the availability of dietary P may limit the usefulness of this association.