The effect of the type of protein supplementation on splanchnic and mammary metabolism of dairy cows fed grass silage-based diets.

The current experiment studied 2 situations proposed to lead to decreased milk protein yield (MPY). First, the feed evaluation system of the NRC predicts that substitution of soybean meal (SBM) with canola meal (CM) decreases MP supply, thereby negatively affecting MPY. Second, excess RDP supply has a negative effect on post-liver AA supply, and hence, on MPY. These 2 potentially negative situations were tested in 6 multiparous multicatheterized Holstein cows through (1) substitution of SBM with CM in a grass silage-based diet and (2) with an excess RDP supply added to a CM and grass silage-based diet. Using a double 3 × 3 Latin square balanced for residual effects, with three 21-d periods, each cow was offered a fixed amount of 3 grass silage-based diets, including the following protein sources: (1) SBM (SBM_diet; 14.5% DM); (2) canola meal (CM_diet; 21.1% DM); or (3) CM_diet plus 12 g of urea per kilogram DM (CMU_diet). Effects on MPY and trans-organ net fluxes of nutrients were examined. The SBM_diet was balanced to meet MP, RDP, and NEL requirements. The CM_diet was balanced to be iso-N and iso-NEL with the SBM_diet. The SBM_diet and CM_diet provided sufficient RDP, whereas the CMU_diet supplied a large excess of RDP. On the last 4 d of each period, DMI and milk production and composition were determined. On the last 3 d (2 cows per day), 6 blood samples were collected from arterial, portal, hepatic, and mammary vessels, along with 3 spot samples of feces and urine. Data were analyzed using the MIXED procedure of SAS, and preplanned contrasts were used to compare the CM_diet and SBM_diet, and the CM_diet and CMU_diet. Despite lower estimated MP supply with CM_diet versus SBM_diet (2,591 vs. 2,779 g/d), MPY tended to be higher in CM_diet than in SBM_diet. The CM_diet also tended to increase the efficiency of N utilization and to decrease the proportion of urinary urea-N relative to total N compared with the SBM_diet. Of the EAA, only Met arterial concentration tended to increase with CM_diet compared with SBM_diet; however trans-organ net fluxes across the splanchnic tissues and the mammary gland were not affected. The net portal appearance of propionate tended to be greater in CM_diet than in SBM_diet. None of the net flux measurements support the lower NRC predictions of MP and energy supply when CM replaces SBM, indicating that CM is an adequate protein source in dairy rations. On the other hand, excess RDP supply decreased the efficiency of N utilization and increased plasma urea-N concentration and urinary urea-N excretion, in agreement with increased net portal absorption of ammonia and hepatic and splanchnic release of urea-N. Although excess of RDP increased hepatic removal of ammonia, it did not affect hepatic removal of EAA and therefore did not negatively affect milk and MPY. Therefore, although RDP supply was progressively increased to be 40% higher than recommended, this level of excess supply had no effect on net liver removal and splanchnic net flux of EAA, so that elevated rumen-derived ammonia production incurs no penalty on EAA availability and animal performance.