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Relationship Between Digestibility and Residual Feed Intake in Lactating Holstein Cows Fed High and Low Starch Diets
We determined if differences in digestibility among cows explained variation in residual feed intake (RFI) in 3 cross-over design experiments. Lactating Holstein cows (n=89; 107±27 DIM) were fed diets high (HI) or low (LO) in starch. LO diets were ~38% NDF and ~14% starch; HI diets were ~26% NDF and ~32% starch. Each experiment consisted of two 28-d treatment periods, with digestibility measured during the last 5 d. Individual DMI and milk yield were recorded daily, BW was measured 3-5 times per wk, and milk components were analyzed twice weekly. DMI was modeled as a function of milk energy output, metabolic BW, body energy gain, and fixed effects of parity and cohort, with the residual being RFI. High RFI cows are less efficient because they eat more than expected for a given multiple of maintenance based on requirements. RFI was negatively correlated with digestibility of starch for both HI (r=-0.25; P=0.02) and LO diets (r=-0.20; P=0.07), and with digestibility of DM (r=-0.30; P<0.01) and NDF (r=-0.24; P=0.03) for LO diets but not HI diets (P>0.4). Cows with the highest RFI (HiRFI) and the lowest RFI (LoRFI) were those greater or less than 1 SD of the mean for each cohort. For HI diets, LoRFI cows tended to have greater starch digestibility than HiRFI (96 vs. 94%; P=0.08), but similar digestibilities (P>0.2) of DM (67%) and NDF (37%). For LO diets, LoRFI cows tended to have greater DM digestibility (66 vs. 64%; P=0.08), but similar starch (95%; P>0.2) and NDF (50%; P>0.3) digestibilities. Apparent NEL concentrations for HI and LO diets, based on cow performance, were 21 and 12% greater (P<0.01), respectively, for LoRFI cows than HiRFI cows. LoRFI cows had 3% greater DM digestibility than HiRFI cows for LO diets, which accounted for 25% of their greater ability to extract NELfrom the same diet. Although digestibility differed for LoRFI and HiRFI cows, some of the differences were expected because high RFI cows eat at a higher multiple of maintenance, which should depress digestibility. Based on these data, we conclude that a cow's digestive ability explains none of the variation in RFI for cows eating high starch diets but may explain as much as 25% of the variation in RFI among cows eating low starch diets.
Keywords: Residual feed intake, digestibility, dietary starch