Abstract
Simple SummaryImprovements in dairy cattle feed efficiency have substantial effects on economic efficiency and can reduce environmental impacts through lower feeding costs and fewer emissions associated with dairy farming. The efficiency of an animal for converting feed into products is influenced by genetic, physiological, and environmental factors that result in individual variations. The utilization of feed efficiency indexes aims to identify and select animals with great economic value in a production system. Associations between morphometric indicators, hormone concentrations, and blood parameters may assist in the identification of differences in the efficiency of feed utilization and in understanding the physiological bases linked to animals’ metabolic responses, thus helping to identify more efficient animals. In our study, it is unlikely that measurements of blood, rumen, or morphometric 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.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.
Highlights
Improvements in dairy cattle feed efficiency (FE) have substantial effects on economic efficiency and the reduction of environmental impacts through lower feeding costs and less emissions associated with dairy farming
The residual feed intake ranged from −0.14 to 0.13 kg/day (p < 0.01) for high efficiency (HE) and low efficiency (LE), respectively, equating to a difference of 0.27 kg of DMI/day between the groups ranked high and low RFI
Differences were not detected between groups in the RFI test for withers height, hip height, and heart girth (p > 0.05; Table 3), and between groups in the residual body weight gain (RG) test for hip height, hip width, and heart girth (p > 0.05; Table 3)
Summary
Improvements in dairy cattle feed efficiency (FE) have substantial effects on economic efficiency and the reduction of environmental impacts through lower feeding costs and less emissions associated with dairy farming. To increase the biological and economic efficiencies of cattle production, animal selection strategies need to focus on improving FE without compromising performance. Breeding for improved residual feed intake (RFI) can enhance feed efficiency without increasing the animal’s mature size [2]. This has obvious positive ramifications for the improvement of the FE of growing and adult cattle. There is limited published information on phenotypic RFI and the residual body weight gain (RG) of dairy heifers, as well as on its potential impacts on milk production and the biological factors contributing to variation in these traits
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