The objectives of this study were: (1) to evaluate feed efficiency indexes and their relationships with body measurements and blood and ruminal metabolites in the pre-weaning period; (2) to determine if such measurements can be used as feed-efficiency markers during the pre-weaning period. Holstein–Gyr heifer calves (n = 36), enrolled between 4 and 12 weeks of age, were classified into two residual feed intake (RFI) and residual body weight gain (RG) groups: high efficiency (HE; RFI, n = 10; and RG, n = 9), and low efficiency (LE; RFI, n = 10; and RG, n = 8). Calves were fed whole milk (6 L/day) and solid feed ad libitum. Body developments were measured weekly and feed intake (milk and solid feed) daily during the whole period. Blood samples were collected at 12 weeks of age and analyzed for glucose, insulin and β-hydroxybutyrate (BHB). Samples of ruminal content were collected on the same day and analyzed for pH, NH₃-N, and volatile fatty acids (VFA). Among the growth characteristics, only the initial hip width differed between the RFI groups, and withers height differed between the RG groups. Concentration of BHB was greater and glucose: insulin ratios tended to be greater in LE-RG animals. Butyric acid proportions were similar among RFI groups, but tended to be greater for HE-RG than for LE-RG. Overall, correlation coefficients between RFI or RG and blood, rumen, or morphometric markers were low. Thus, it is unlikely that measurements of metabolic indicators, per se, will be useful in the early identification of more efficient animals. Understanding the underlying physiological basis for improved feed efficiency in dairy heifers requires further investigation.

Dairy cattle systems have targeted improvements in feed efficiency by selecting animals that can convert less feed into more products. Residual feed intake (RFI) has been the index of choice when selecting dairy cattle for feed efficiency. Nonetheless, RFI studies have focused on lactating cows, and the crucial importance of pre-weaning efficiency on farm profitability and cow productivity has been mostly neglected. This review discusses the current knowledge of how RFI divergence relates to nutrient metabolism in pre-weaning dairy calves, including the advantages and limitations of evaluating RFI in this phase. Existing literature indicates that nutrient utilization, energy metabolism, protein metabolism, vitamin metabolism, intestinal development, and hindgut bacterial populations may be implicated in RFI divergence between pre-weaning calves. Techniques developed to date to evaluate RFI in this phase are still evolving to better adapt to the unique characteristics of this phase, and more research is needed to fill in the gaps in our current understanding of early-life feed efficiency divergence in cattle. However, current results suggest great potential for selecting high-efficiency calves while in pre-weaning to accelerate the progress of genetic selection in dairy cattle.

The aim of this study was to characterize dry and fresh period mammary metabolism as estimated via mammary plasma flow of the milk vein (MV; MPF_MV) and mammary acetate and glucose (glc) metabolism and to determine potential cow- and mammary-level factors that influence colostrum production. A total of 21 Holstein cows (n = 11 parity 2 [P2] and 10 parity 3 [P3] cows) were dried off at d-58 ± 3.4 relative to calving and followed until 7 ± 1 d in milk (DIM). All cows were provided the same dry and lactating diets supplying 46.4 and 54.4 Mcal ME/d, respectively. Starting at d-49 relative to expected calving, estimation of MPF_MV via Doppler ultrasonography, and mammary acetate and glc arteriovenous difference, uptake and clearance, were determined every other wk until wk-1 before expected calving, then every 4 d until actual calving, at calving, and at 7 ± 1 DIM by collection of frequent pairs of coccygeal vessel and MV blood samples. Colostrum was collected within 1.4 ± 0.76 h of calving, components were determined via mid-infrared spectroscopy, and IgG was determined by radial immunodiffusion. Mammary metabolism as estimated via MPF_MV and glc and acetate uptake was lowest from wk-6 to wk-4 relative to calving. Total MPF_MV began to steadily increase starting at wk-2, while mammary acetate and glc uptake only began to increase at wk-1 and d-4, respectively. The earlier increase in mammary acetate uptake may suggest its role in providing energy for mammogenesis and lactogenesis I, while mammary glc uptake may coincide with lactogenesis II and copious milk production. Assuming the complete oxidation of mammary acetate and glc uptakes, the average daily rate of energy expenditure of the dry mammary gland was 0.93 Mcal/d and equated to a total energy expenditure of 56.1 Mcal throughout the entire dry period. Correlation and regression analyses highlighted the importance of achieving adequate dry-off during the far-off period of high-producing cows in promoting colostrum production, as greater previous lactation milk yield and far-off mammary acetate uptake were negatively associated with colostrum yield. In summary, the mammary gland appears to achieve a non-secretory, low metabolic state between wk-6 to wk-4 relative to calving, and this period appears to be of greater importance in driving colostrum production than the close-up period. Future work should aim to uncover the relationships between dry period mammary metabolism, especially the non-secretory state during the far-off period, and prepartum mammogenesis and colostrogenesis.