Effects of genetic capacity on milk production and on plasma metabolites in dairy cows during post-ruminal or intravenous carbohydrates or amino acid infusions

The objectives of this study were to investigate the effects of genetic capacity on abomasal and intravenous infusions of wheat starch or glucose (CHO) or a mixture of amino acids (AA) on milk production, nitrogen utilization efficiency, plasma metabolites and hormones of dairy cows in early and late lactation. Eight cows from two genetic lines selected for low (L) and high (H) milk production were used in a 4 × 4 Latin square design. The mean differences in pedigree index between the two groups were 1639 kg milk and 55 kg protein yield based on 305 days lactation. Infusions were: 1) starch in the abomasum (SP), 2) glucose in the blood (GB), 3) AA in the abomasum (AP), and 4) AA in the blood (AB). The experiment was conducted in early lactation (start: 57 ± 4 and 52 ± 2 days postpartum, 31.3 ± 2.8 and 34.7 ± 1.4 kg milk for L and H cows, respectively) and repeated with the same animals and treatments in late lactation (start: 168 ± 4 and 163 ± 2 d postpartum, 21.0 ± 1.9 and 23.8 ± 0.7 kg milk for L and H cows, respectively). Daily amounts infused were on average 354 and 258 g in early and late lactation, respectively. The cows were restrictively fed a basal diet consisting of concentrate mixture and grass silage (55:45 on DM basis). Differences in milk yield and ECM between the genetic groups were 3.7 and 3.3 kg in early lactation and 2.9 and 2.0 kg in late lactation, respectively, but the difference was not significant (P > 0.10). Minor effects of genetic group were found in milk production and plasma metabolite concentrations. However, the extraction rates of EAA, BCAA, NEAA and TAA were higher (P < 0.05) in H cows than in L cows in early lactation but not in late lactation. OM and CP digestibility and hormones were affected by genetic group or genetic group × treatment interaction. It is concluded that genetic capacity is important for digestion and metabolism of nutrients, and particularly, how hormone levels are influenced by different nutrient supply.