Effect of stage of lactation and dietary starch content on endocrine-metabolic status, blood amino acid concentrations, milk yield, and composition in Holstein dairy cows.

Discipline: nutrition; Keywords: dairy cow, starch level, metabolism, milk composition.

Milk yield and composition are modified by level and chemical characteristics of dietary energy and protein. Those factors determine nutrient availability from a given diet, and once absorbed, they interact with the endocrine system and together determine availability of metabolites to the mammary gland. However, the mechanisms in early and late lactation are not well described. Therefore, the objective of the study by F. Piccioli-Cappelli and co-workers was to investigate in early- and late-lactation dairy cows the effects of diets with similar crude protein, rumen undegradable protein and energy content, but different rates of rumen carbohydrate (starch or fermentable fibre) fermentation rates on (1) endocrine-metabolic conditions; (2) AA metabolism, and (3) composition and renneting properties of milk at each stage of lactation. Their results were published in the Journal of Dairy Science, Volume 105 of 2022, page 1131 to 1149, the title being: Effect of stage of lactation and dietary starch content on endocrine-metabolic status, blood amino acid concentrations, milk yield, and composition in Holstein dairy cows.

Four multi-lactation dairy cows in early lactation and subsequently in late lactation were fed two diets for 28 days in a changeover design that provided, within the same stage of lactation, similar amounts of rumen fermentable feed with either high (HS) or low starch (LS). All diets had similar dietary crude protein and rumen-undegradable protein content. Amino acid profiles were similar to that of casein. On day 28, an isotope of the amino acid leucine was infused into one jugular vein with blood samples taken at 0, 2, 4, 6, and 8 hours, and cows milked at 0, 2, 4, 5, 6, 7, and 8 hours from the start of infusion. Isotopic enrichments of plasma leucine, keto-isocaproic acid, and milk casein were determined for calculation of leucine kinetics.

Dry matter intake within each stage of lactation was similar between groups. Feeding LS resulted in lower blood glucose and a greater ratio of growth hormone to insulin. This response was associated with a greater blood concentrations of non-esterified fatty acids and β-hydroxybutyrate, which might have contributed to greater milk fat content in LS-fed cows. Except for histidine, the average concentrations of all amino acids in the blood were higher in late than early lactation. Diet did not alter the average plasma concentrations of amino acids. However, for most of the essential amino acids (particularly the branched-chain ones), the HS diet led to a marked decrease in concentrations after the forage meal, resulting in significant differences between dietary groups in early lactation. In early-lactating cows fed HS, a greater reduction in plasma concentrations at 8 hours relative to pre-feeding values (time zero) was observed for methionine, lysine and histidine, resulting in decreases of 27.9%, 33.6%, and 38.5%, respectively. A higher growth hormone-insulin ratio in early lactation and in cows fed LS could possibly have led to greater breakdown and, consequently a higher amino acid flux from the peripheral tissues. In LS-fed cows, higher mobilization of body fat and protein was confirmed by the greater body weight loss in both stages of lactation. Higher irreversible loss of the leucine isotope in early lactation suggested lower protein retention in the peripheral tissues during early compared with late lactation. Milk yield, protein output and milk composition were similar between the groups at both stages of lactation, whereas milk coagulation was faster (lower curd firming rate) and with higher curd firmness in response to feeding HS in late lactation.

Overall, the data indicated that rate of carbohydrate fermentation in the rumen can modify the availability of metabolites to the udder and consequently modify milk protein coagulation, with probable effect to cheese manufacturing.