Residual feed intake is repeatable for lactating Holstein dairy cows fed high and low starch diets.

Residual feed intake (RFI) is a tool to quantify feed efficiency in livestock and is commonly used to assess feed efficiency independent of production level, body weight (BW), or BW change. Lactating Holstein cows (n=109; 44 primiparous and 65 multiparous), averaging (mean ± standard deviation, SD) 665±77kg of BW, 42±9kg of milk/d, and 120±30 d postpartum, were fed diets of high (HI) or low (LO) starch content in 4 crossover experiments with two 28-d treatment periods. The LO diets were ~40% neutral detergent fiber (NDF) and ~14% starch and the HI diets were ~26% NDF and ~30% starch. Individual dry matter intake (DMI) of a cow was modeled as a function of milk energy output, metabolic BW, body energy change, and fixed effects of parity, experiment, cohort nested within experiment, and diet nested within cohort and experiment; RFI for each cow was the residual error term. Cows were classified as high (>0.5 SD of the mean), medium (±0.5 SD of the mean), or low (<-0.5 SD of the mean) RFI. On average, for the linear model used to determine RFI for individual cows, each unit increase in milk energy output, metabolic BW, or body energy gain was associated with 0.35, 0.09, or 0.05kg increase in DMI, respectively. When compared with LO diets, HI diets increased energy partitioning to body energy gain and tended to increase DMI. The correlation between RFI when cows were fed HI diets and RFI when cows were fed LO diets was 0.73 and was similar across each parity and experiment. Fifty-six percent of cows maintained the same RFI classification (high, medium, or low RFI) and only 4 of 109 cows changed from high RFI to low RFI or vice versa when diets were changed. Milk:feed, income over feed cost, and DMI were also highly repeatable (r=0.72, 0.84, and 0.92, respectively). We achieved significant changes in milk yield and component concentration as well as energy partitioning between HI and LO diets and still determined RFI to be repeatable across diets. We conclude that RFI is reasonably repeatable for a wide range of dietary starch levels fed to mid-lactation cows, so that cows that have low RFI when fed high corn diets will likely also have low RFI when fed diets high in nonforage fiber sources.