INTERACTIONS BETWEEN PARTIAL REPLACEMENT OF BARLEY GRAIN WITH DRIED WHEY PERMEATE AND DIETARY RUMINALLY-DEGRADABLE PROTEIN LEVEL ON RUMINAL FERMENTATION CHARACTERISTICS, NITROGEN UTILIZATION, UREA-N RECYCLING, AND PRODUCTION PERFORMANCE IN DAIRY COWS

The objective of this experiment was to delineate the interactions between partial replacement of barley grain with dried whey permeate (DWP; a source of lactose) and dietary rumen-degradable protein levels on feed intake, milk production, ruminal fermentation characteristics (ruminal pH, and SCFA and ammonia-N concentrations), whole-body urea-N kinetics, nitrogen balance, and total-tract nutrient digestibility. Eight lactating Holstein cows (738 ± 98 kg BW; 93 ± 39 DIM at the beginning of the experiment) were used in a replicated 4 x 4 Latin square design experiment with a 2 x 2 factorial arrangement of dietary treatments and 28-d periods (18 d of dietary adaptation and 10 d of data and sample collection). One square had four ruminally-cannulated cows for measurement of ruminal fermentation characteristics, whole-body urea-N kinetics, nitrogen balance, and total-tract nutrient digestibility. The dietary treatment factors were: 1) two levels of dietary inclusion of DWP (0 vs. 11.2%; dry matter [DM] basis): and 2) two levels of dietary RDP (9.5% vs. 11.5%; expressed as % of DM). Dietary RDP level was manipulated by inclusion of untreated or heat-treated soybean meal. Dietary addition of DWP increased water-soluble carbohydrate (WSC) content from 4.8% (range = 4.61 to 5.04%) in the diet without added DWP to 10.7% (range = 10.4 to 11.0%) in the diet with added DWP. The dietary addition of DWP tended to increase DMI (P = 0.09) but did not affect milk yield, or fat content and yield (P > 0.05), while milk protein content (P = 0.01) and yield increased (P = 0.01). Milk lactose content was similar in cows fed 9.5 and 11.5% RDP without added DWP, but milk lactose content was greater in cows fed 9.5% RDP compared to those fed 11.5% RDP with added DWP (P 0.05) when ruminal pH was less than 5.8. Cows fed the low RDP diet had a greater maximum ruminal pH compared those fed the high RDP diet (P = 0.03). Dietary treatments did not affect ruminal concentrations of acetate, propionate, and ammonia-N (P > 0.05), whereas the dietary inclusion of DWP decreased ruminal concentrations of isobutyrate, isovalerate, total BCFA, and total SCFA (P 0.05). Increasing dietary content of RDP tended to increase N intake (P = 0.08) but decreased fecal N excretion (% of N intake), total N excretion (% of N intake) (P 0.05) on urea-N returned to the ornithine cycle (ROC), urinary urea-N excretion (UUE), urea-N utilized for anabolism (UUA), and all fractional urea-N transfers. Increased dietary RDP content decreased UER (P = 0.03), and urea-N gut entry rate (GER) tended to be higher in cows fed the low RDP diet compared to those fed the high RDP diet (P = 0.10), and the low RDP diet had a higher urea-N loss to feces compared to the high RDP (P = 0.02). The results show that the partial substitution of barley grain with DWP does not increase the susceptibility of cows to ruminal acidosis and has no impact on animal productivity. Increasing dietary RDP content reduced overall N excretion; however, the dietary RDP changes in this experiment were likely not large enough to stimulate changes in animal productivity or change urea-N recycling