The start of lactation greatly raises the need for calcium in a dairy cow. On the day a cow freshens, she will produce 2-1/2 gallons or more of colostrum containing at least 23 grams of calcium. The continued loss of calcium through milk can exceed 50 grams per day in early lactation. This sudden shift challenges the cow's ability to maintain blood calcium within an adequate range, potentially leading to the development of periparturient hypocalcemia, or milk fever.
Current milk fever prevention strategies focus on dietary changes near the end of the dry period. This includes manipulating dietary anion-cation difference (DCAD) and feeding lower calcium diets. Unfortunately, these ration changes can be expensive and make the diet less palatable, leading to reduced dry matter intake and subsequently more fresh cow health issues.
The dairy cow has some natural pathways to protect itself against hypocalcemia at calving, explained assistant professor Laura Hernandez at the University of Wisconsin-Madison Dairy Science Showcase. The intestines take in more calcium from the diet, and renal mechanisms reduce calcium elimination. However, to deal with most of the calcium demand, the normal reaction of a dairy cow is to mobilize calcium from the bone, where 99 percent of calcium body reserves are located. The dam secretes parathyroid hormone related protein (PTHrP) to mobilize this calcium.
Research has shown that serotonin (5-HT) is involved in regulating bone mass, maintaining bone density and bone remodeling. Hernandez and others have discovered that 5-HT is also very involved in mammary gland PTHrP induction and bone turnover for calcium mobilization during lactation in rodents. Initial studies in dairy cattle found that 5-HT concentrations on Day 1 of lactation were positively correlated with ionized calcium concentrations and plasma PTHrP concentrations. There was also a negative correlation with milk fever incidence and severe ketosis grades during the first 10 days after calving.
These interesting correlations open the door for potential new ways to prevent hypocalcemia, which in the subclinical form can affect 25 to 50 percent of fresh cows. Adding 5-HT to the diet before calving may help mobilize blood calcium sooner, warding off some of the negative affects of calcium imbalance. Hernandez and her group are in the process of studying the relationship between 5-HT and PTHrP and the possible opportunity to supplement cows with 5-HT in late gestation and early lactation.
The author is an associate editor and covers animal health, dairy housing and equipment, and nutrient management. She grew up on a dairy farm near Plymouth, Wis., and previously served as a University of Wisconsin agricultural extension agent. She received a master's degree from North Carolina State University and a bachelor's from University of Wisconsin-Madison.