Basal levels and diurnal variations of some hormones and metabolites in blood of dairy cows treated daily with rbST in early and late lactation

Abstract

The variations of basal value (before morning feeding at 3-4 days interval) and daily pattern (on 4th and 18th day of treatment with 8 blood collections within 24h) of hormones and metabolites were studied in four dairy cows; 2 in early and 2 in late lactation, which were alternatively injected daily for a period of 21 days with 26.3 mg rbST or saline. The rbST injection significantly increased the basal and daily levels of GH, IGF-I in both stages of lactation. Daily levels of NEFA were significantly increased by rbST in both stages of lactation and their variations, due to the effect of meals, were counteracted by the rbST; the higher level of NEFA allowed a greater sparing of glucose, the blood level of which resulted higher (P<0.05) in late lactation (mainly for less lactose yield), as well as a sparing of amino acid that resulted in a lower blood urea level (P<0.05, only in late lactation). Moreover, these metabolic changes in late lactating cows, and in particular the high availability of glucose, determined a significantly higher blood level of insulin and T3, and lower level of glucagon. Despite the similar GH, IGF-I and NEFA increases observed in the cows treated in early and late lactation, other metabolic and endocrine consequences resulted more evident in late lactation. Furthermore, some of these variations were affected by the forage meals. It can be concluded, therefore, that the changes occurring in the blood subsequent to a rbST treatment are not equal to those which occur at the start of lactation; moreover, they are affected by the daily feeding pattern and perhaps by the stage of lactation.