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The effect of the type of protein supplementation on splanchnic and mammary metabolism of dairy cows fed grass silage-based diets
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DOI：10.3168/jds.2026-28332.png)
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Abstract
En 中文
The current experiment studied 2 situations proposed to lead to decreased milk protein yield (MPY). First, NRC (2001) predicts that substitution of soybean meal (SBM) by 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 through 1) substitution of SBM by CM in a grass-silage based diet, and 2) with an excess RDP supply added to a CM-grass silage-based diet, in 6 multiparous multicatheterized Holstein cows. Using a double 3 × 3 Latin square, balanced for residual effects, with 3 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/kg DM (CMU_diet) on MPY and trans-organ net fluxes of nutrients. The SBM_diet was balanced to meet MP, RDP and NEL requirements (NRC, 2001). The CM_diet was balanced to be iso-N and iso-NEL with the SBM_diet (NRC, 2001). 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 d), 6 blood samples were collected from arterial, portal, hepatic and mammary vessels plus 3 spot samples of feces and urine. Data were analyzed using the Mixed procedure of SAS, and pre-planned contrasts were used to compare CM_diet to SBM_diet and CM_diet to CMU_diet. Despite lower estimated MP supply with CM_diet vs. SBM_diet (2591 vs. 2779 g/d), milk true protein yield 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 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 supports the lower NRC (2001) predictions of MP and energy supply when CM substitutes SBM, indicating that CM is an adequate protein source in dairy ration.
Keywords:
soybean meal
canola meal
rumen degradable protein
urea
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