title: Phosphorus partitioning during early lactation in dairy cows fed diets varying in phosphorus content.
abstract: The effect of dietary P content on P partitioning and excretion during early lactation was evaluated in 13 cows fed diets containing 0.34 (no supplementary P), 0.51, or 0.67% P. All cows were fed a common pre-partum total mixed ration (TMR) (0.28% P), followed by common TMR (0.51% P) for 7 d post-partum. On day 7, cows were randomly assigned to one of the three dietary P treatments. All treatment diets contained 16.6% CP, 15.2% ADF, 26.3% NDF, and 0.74% Ca. Total collections of milk, urine, and feces were conducted during weeks 3, 5, 7, 9, and 11 of lactation. Average body weight (591 kg), milk yield (47.9 kg/d), and DMI (25.2 kg/d) throughout the 10-wk trial were not affected by dietary P content. With increasing dietary P, however, linear increases in P intake (84.7, 135.2, and 161.5 g/d), fecal P (42.3, 87.5, and 108.6 g/d), urinary P (0.32, 1.28, and 3.90 g/d), and total P excretion (42.6, 88.8, and 112.5 g/d) were observed. Apparent P digestibility (49.0, 34.4, and 32.8% of P intake) decreased quadratically with increasing dietary P. Phosphorus balance was highly variable, but cows fed the 0.34% P diet were in negative P balance longer than were cows fed diets containing 0.51 or 0.67% P. With increasing dietary P, serum concentrations of inorganic P increased linearly, but serum Ca and Mg concentrations decreased. Increasing dietary P increased fecal and urinary P excretion in early lactation cows. Increased duration of negative P balance and changes in blood mineral concentrations suggest that cows fed low P diets mobilized more P from body reserves than cows on medium- and high-P diets.
authors.affiliation: Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg, USA. knowlton@vt.edu
authors.forename: K F
authors.initials: KF
authors.lastname: Knowlton
authors.forename: J H
authors.initials: JH
authors.lastname: Herbein
mesh_terms: D000818:Animals; D002118:Calcium / Q000097:blood; D002136:Calcium, Dietary / Q000008:administration & dosage / Q000493:pharmacokinetics; D002417:Cattle / Q000378:metabolism*; D004032:Diet; D004043:Dietary Fiber / Q000008:administration & dosage; D004044:Dietary Proteins / Q000008:administration & dosage; D004063:Digestion; D005243:Feces / Q000737:chemistry; D005260:Female; D007774:Lactation; D008274:Magnesium / Q000097:blood; D008892:Milk / Q000737:chemistry; D016226:Phosphorus, Dietary / Q000008:administration & dosage* / Q000032:analysis / Q000493:pharmacokinetics*
chemical_list: D002136:Calcium, Dietary; D004043:Dietary Fiber; D004044:Dietary Proteins; D016226:Phosphorus, Dietary; D008274:Magnesium; D002118:Calcium