Age, gestational and heat stress effects on ghrelin secretion in dairy cattle.

It is known that heat stress decreases dry matter intake in cattle with impacts on milk production and fertility. Ghrelin is an orexigenic hormone with suppressive effects on reproduction. In this study, we investigated the effects of heat stress and gestational status on ghrelin secretion and its possible associations with DMI in Holstein cattle. The study was conducted in a dairy farm without any artificial cooling measures. The animals were fed a total mixed ration twice daily; each morning the leftovers were removed and weighted. Lactating cows and heifers were used during the winter and the summer; in each season 8 groups were formed as following: non-pregnant cows (n = 10) and non-pregnant heifers (n = 10) and pregnant cows (3 groups, each n = 8) and heifers (3 groups, each n = 10), being at the 1st (days 65-90), the 2nd (days 114-144) and the 3rd (dry cows, days 198-220; heifers, days 192-230) trimester of gestation. In each season the blood samples were collected from all groups on the same day, 1 h prior to morning feeding. In the winter, the Temperature Humidity Index (THI) was 58 in the winter and 73 in the summer. Normal and acidified sera were stored at -20 °C and analyzed for cortisol, total and acylated ghrelin concentrations, respectively. T-Test and Welch-Satterthwaite were performed for continuous data comparison, while two-way ANOVA to test for differences between gestation and season. Feed refusals were higher (p < 0.01) during the summer compared to the winter. In cows, total ghrelin levels differed between gestation stages in winter and summer(p < 0.04), while acylated ghrelin levels differed by gestation stage in winter (p < 0.001) but not in summer. There was an effect of season by the gestational stage in the pattern of acylated (p < 0.001) but not of total ghrelin. In heifers, the pattern of total and acylated ghrelin secretion was not affected by season or gestation stage (p > 0.05). Both in cows and heifers, acylated ghrelin levels were lower in summer compared to winter, (p < 0.002). During the summer months the low ghrelin levels might explain the reduced feed consumption of heat stressed animals. We infer that the lactation-induced altered metabolic status of the animals governed the different ghrelin levels at various gestational stages in cows and heifers.