Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 15, 2001
Publication Date: N/A
Interpretive Summary: At the time of calving, the dairy cow also begins producing large quantities of milk. There are a great number of hormonal changes affecting the cow as she ends her pregnancy. She also is metabolically challenged to obtain the components needed to synthesize milk. It is common to observe a severe decrease in blood calcium at calving which can cause milk fever, a disorder affecting nearly 6% of dairy cows and costing $340/case. Calving is also accompanied by a great rise in body fat mobilization with subsequent deposition into the liver. This sometimes causes a condition known as ketosis. This energy deficiency condition affects 6-10% of dairy cows, costing approximately $140/case. The act of delivering a calf can stress the cow, resulting in some of the observed hormone changes. The onset of lactation also challenges the metabolism of the cow. Which is the more important factor? To answer this question, we removed the mammary gland from several pregnant dairy cows and compared their blood metabolic profile to that of intact cows (which would experience both the act of calving and the metabolic demands of lactation). We found that elimination of milk production prevented changes in blood calcium normally observed in dairy cows. It greatly reduced body fat mobilization and changes in antioxidant vitamin status, but did not completely eliminate them. These data suggest that lactation is the greater stressor of the dairy cows. The results of this research will greatly benefit dairy producers worldwide.
Technical Abstract: The objective of this study was to compare blood profiles of intact and mastectomized periparturient cows to discriminate those metabolic changes associated with the act of parturition from the metabolic changes caused by lactation. Mastectomized and intact cows had similar increases in plasma estrogens and cortisol concentrations around the time of calving. Mastectomy eliminated hypocalcemia and the rise in 9,13-di-cis retinoic acid observed in intact cows. Mastectomy reduced, but did not eliminate, decreases in plasma phosphorus, alpha-tocopherol, and beta-carotene associated with parturition in intact cows, suggesting the mammary gland is not the sole factor affecting plasma concentrations of these compounds. Dry matter intake was similar in both groups prior to calving. The day of calving, dry matter intake was lower in intact cows than in mastectomized cows, but after calving the mastectomized cows exhibited a pronounced decline in feed intake. Plasma NEFA concentration rose dramatically in intact cows at calving and did not return to baseline level for more than 10 days. In contrast, NEFA concentration in mastectomized cow plasma rose moderately at calving and returned to baseline level 1-2 days after calving. This study provides evidence that hypocalcemia in the cow is solely a result of the calcium drain of lactation. The act of parturition affects blood phosphorus, dry matter intake, and NEFA concentration independent of the effect of lactation.