Utilization of phosphorus in lactating cows fed varying amounts of phosphorus and sources of fiber.

This study was undertaken to determine the effect of dietary P content and fiber source on P utilization. Four dietary treatments were formed in a 2 x 2 factorial arrangement. The P content was 0.32 or 0.44%, and the fiber source was varied by substituting 10% soyhulls for 6% alfalfa hay on a dry matter (DM) basis. Diets also contained approximately 50% corn silage and alfalfa silage for all treatments. The diets were fed to 32 early to midlactation Holsteins for 10 wk. Fecal P excretion was estimated using indigestible acid detergent fiber marker determined with 12-d in situ incubation and grab sampling. Milk yield was high, averaging 43 kg/d across treatments, and 42.1 and 44.0 kg/d for the 0.32 and 0.44% P diets, respectively. Milk fat content was also high, averaging 3.68 and 4.12% for the 0.32 and 0.44% P diets, respectively. Milk protein yield averaged 1.240 and 1.323 kg/d. Differences in milk production were associated with 1.5 kg/d less DM intake for the lower P diets on average. Based on lactation performance, 0.32% P appeared inadequate for this level of production, whereas the calculated (National Research Council) requirement was 0.37%. Fecal P concentration increased linearly with P intake, and based on this relationship, reducing dietary P from 0.44 to 0.37% would reduce fecal P excretion by 12%. Partial substitution of soyhulls for alfalfa hay did not affect feed intake or milk production, but reduced fecal P excretion, partially because of increased P apparent digestibility. The reduction in fecal P excretion resulting from reduced P intake or substitution of soyhulls for alfalfa hay was apparently through reductions in the regulated portion of fecal P. Cows producing 43 kg/d of milk appeared to need > 0.32% P, whereas the requirement assessed from National Research Council data was 0.37%. Using highly digestible nonforage fiber sources in place of forage fiber sources in the diet may allow less P to be fed while still meeting the requirement.