Nitrogen and phosphorus partitioning in lactating Holstein cows fed different sources of dietary protein and...

To evaluate dietary N and P partitioning, 36 Holstein cows grouped by parity were assigned at calving to diets supplemented with soybean meal (S) or a combination of S and blood meal (B). Diets S and B were formulated to contain 16.2% CP and 0.35% P using mono- and dicalcium phosphate (PM) or wheat bran (WB) as the supplemental source of P. Actual dietary P contents were 0.38, 0.36, 0.34, and 0.34% for SPM, BPM, SWB, and BWB. Two-day total collections of feces, urine, and milk were conducted between 30 and 45 d in milk (DIM), then all cows were fed a control diet until 120 DIM. Between 120 and 150 DIM, cows were again fed the diet assigned at calving, then 2-d total collections of feces, urine, and milk were conducted. Milk production was similar for cows fed diets containing WB (SWB or BWB) when compared with cows fed PM. However, DMI tended to be lower, and P intake and total P excretion were lower in response to WB (20.7 kg/d, 71.9 g/d, and 40.3 g/d) compared with cows fed PM (23.0, 86.7, and 46.8 g/d). Apparent digestibility of dietary P did not differ due to source of supplemental P, averaging 45% across diets. The lower P intake by cows fed WB resulted in lower absorbed P and lower retained P (32.2 and 7.5 g/d) compared with those fed PM (40.6 and 13.4 g/d). Apparent N digestibility, urinary N, and N retention were not affected by P source. Blood meal decreased apparent N digestibility and absorbed N, and also decreased P retention compared with S. In later lactation, cows retained proportionately more absorbed N and P in body tissue and secreted less in milk than they did in early lactation. Results indicated the organic source of P (phytate-P) in WB can be used to provide a substantial portion of the P needed in dairy cattle diets after peak lactation, but the amount of WB in the diet during early lactation should be limited to prevent suppression of DMI and P retention.