استرس مزمن، ویژگی های هورمون و شدت استروس در گاوهای شیری

The objectives of the present study were to determine if lameness, a model for a natural chronic stressor, affects hormone concentrations in milk prior to estrus and/or the subsequent expression of estrus in the postpartum period. Dairy cows > 20 days postpartum were scored for lameness and observed for estrus intensity using a weighted scoring system (> 100 points = estrus = Day 0). Increasing lameness score was not associated with daily profiles of milk progesterone (throughout Days − 18 to 0), estradiol (Days − 6 to 2) or cortisol (Days − 18 to 2) around estrus, maximum estradiol values or estradiol concentrations on Day 0. However, post hoc pair wise comparisons revealed that prior to estrus, severely lame cows had lower maximum progesterone concentrations compared to nonlame cows (1.3 ± 0.1, 1.2 ± 0.2, 0.7 ± 0.1 ng/ml milk; P = 0.042). Furthermore, severely lame cows expressed behavioral estrus with lower intensity (284 ± 128 points, n = 9) compared to moderately lame (662 ± 310 points, n = 9) or nonlame animals (583 ± 275 points, n = 18; P = 0.05 and P = 0.02, respectively). Resting concentrations of cortisol (Days 20–80 postpartum) did not vary between days postpartum or lameness score. The incidence of behavioral estrus was not affected by increasing lameness score, as 54.2%, 56.2% and 50.0% periods with low progesterone were associated with spontaneous estrus expression, respectively. Concluding, in this biological model of chronic stress, lameness did not affect the incidence of behavioral estrus but did reduce estrus intensity once ovarian cyclicity had resumed after calving. This reduced intensity of estrus was associated with lower maximum progesterone values prior to estrus but not abnormal daily cortisol or estradiol values in milk.