Effect of ovariectomy on milk yield and mammary gland activity in lactating cow

Milk yield (MY) in dairy cows is highly dependent on both mammary tissue remodeling and mammary epithelial cell activity during lactation. Previous studies on lactating cows suggested that ovarian steroids (estradiol and progesterone) have a negative effect on MY after the peak of lactation. However, little is known about the effects of ovarian steroids on mammary tissue. The aims of this study were to investigate the effect of ovariectomy on MY and on mammary gland remodeling and molecular activity in lactating cows. Fourteen multiparous Holstein cows were either ovariectomized (Ovx, n = 7) or sham-operated (Sham, n = 7) around 60 d in milk. Cows were milked twice daily during 12 mo; MY was recorded daily throughout the study and milk samples were harvested weekly to follow the milk composition. Mammary tissue samples were collected at 1.5 mo in milk (2 weeks before ovariectomy) and at 4, 8 and 12 mo of lactation to perform molecular analysis on the mammary tissue. Ovariectomy at the time of lactation peak improved MY in the end of lactation (+ 17.1% on mo 8 (P < 0.1) and + 60.8% on mo 12 of lactation (P < 0.1) for Ovx cows in comparison with Sham cows) without significantly modifying milk composition. Ovariectomy also reduced somatic cell release in milk as of mo 8 of lactation. Moreover, zymography performed on milk samples revealed that mammary tissue remodeling (MMP2 activity) was reduced in Ovx cows. Molecular analysis performed on mammary tissue samples showed that mammary cell apoptosis (Bax transcript level) was reduced by 22.2% (P < 0.05) in the Ovx group in an advanced stage of lactation. The Western blot quantification of lactoferrin protein showed that lactoferrin increased over time in both Ovx and Sham cows, but it was reduced by ovariectomy on mo 8 (- 27.4%, P < 0.1) and 12 (- 53.3%, P < 0.1) of lactation. Taken together these suggest that ovariectomy could delay mammary gland involution.