Ruminal Phosphorus Disappearance from Corn and Soybean Feedstuffs1

Byproducts of corn and soybeans have high phosphorus (P) content, but little is known about their P disappearance in the rumen of lactating dairy cows. In situ disappearance of P from corn and soybean feed-stuffs was determined in 2 experiments. In the first experiment, 3 ruminally cannulated lactating dairy cows were used to estimate in situ P disappearance of 9 feed ingredients that included 3 sources of dried distillers grains with solubles (DDG; DGa, DGb, DGc), corn, corn germ, solvent-extracted soybean meal, (44% CP; SBM), expeller soybean meal (SoyPlus; SP), extruded soybeans (ES), and soyhulls (SH). Nylon bags were incubated in the rumen of each cow for 2, 6, 12, 18, 24, 36, and 48 h. The water-soluble fraction of P (AP) was greatest in DDG (mean 82.1%) followed by corn germ (77%), with SH having the least AP among all feedstuffs (45%). The remaining feedstuffs (SBM, SP, ES, and corn) were similar in AP (64.2%). The slowly available fraction of P (BP) was greatest in SH (45.6%), lowest in DDG (13.5%), and intermediate, averaging 31.4%, in SBM, ES, SP, and corn. The effective disappearance of P (EDP) was greatest for DDG (93.5%), whereas corn germ, ES, SBM, and SP followed with an EDP of 93.3, 88.0, 87.5, and 87.0%, respectively. The EDP was less for corn and SH than for the other feedstuffs at 83.3 and 69.1%, respectively. Rate of P disappearance was similar for all feedstuffs (16.2%/h). In the second experiment, 3 new sources of DDG (DG1, DG2, and DG3), and one wet distillers grains with solubles (WDG) source were incubated for 3, 6, 12, 24, and 36 h on replicate days in the rumen of 2 cannulated lactating dairy cows. Fraction AP varied from 82.7 to 90.3%, with that in WDG being the least soluble. The WDG source had a greater BP fraction (15.8%) compared with the DDG sources (9.5%). The WDG had the lowest EDP (88.1%), whereas the DDG varied from 89.7 to 92.7%. Corn and soybean byproducts tested, with the exception of SH, have high ruminal P disappearance as measured with the Dacron bag technique.