Novel Concepts Regarding Calcium Homeostasis during the Transition Period

Adequate circulating calcium (Ca) concentrations throughout the transition period are necessary for a productive lactation, but large quantities of Ca are lost from maternal Ca pools into milk and colostrum. A rapid, substantial drop in maternal blood Ca causes 5-10% of cows to be afflicted with clinical hypocalcemia (CH) and an additional 50% to suffer from subclinical hypocalcemia (SCH). Subclinical hypocalcemia and CH are significant risk factors of early lactation culling/premature removal from the herd (DeGaris and Lean, 2008; Reinhardt et al., 2011; Roberts et al., 2012). Furthermore, SCH increases risks of developing ketosis; displaced abomasum; and metritis; SCH depresses immune function; prolongs the interval until pregnancy is achieved; decreases pregnancy rate; and reduces overall productivity (Figure 1; Kimura et al., 2006; Goff, 2008; DeGaris and Lean, 2008; Chapinal et al., 2011; Reinhardt et al., 2011; Chapinal et al., 2012; Martinez et al., 2012). During lactation, dietary Ca is not sufficient to maintain maternal Ca concentrations while supporting milk formation. Therefore, activation of maternal bone Ca mobilization during the dry period is critical for the prevention of post-partum SCH and CH. Using the estimates of Dr. Garrett Oetzel ($300 loss for each treatment of CH, and $125 loss for each treatment of SCH; Guard, 1996) the annual cost to the U.S. dairy industry, which has approximately 9,200,000 cows, is approximately $575,000,000 for SCH and approximately $325,000,000 for CH (NAHMS, 2007; Oetzel, 2013). SCH and CH are detrimental to animal health and welfare, and a formidable economic burden to U.S. farmers (Oetzel, 2013). Currently accepted practices for treatment and prevention of SCH and CH include oral Ca supplementation post-calving and anionic salt supplementation (DCAD) pre-calving (Oetzel, 2004; Oetzel, 2013). However, post-calving Ca supplementations, while critical for treatment, are not sufficient to prevent maternal hypocalcemia and its associated peripartum disorders. Use of anionic salts in the pre-partum period has reduced the incidence of SCH and CH. However, approximately 25% of cows will still be afflicted with SCH (Oetzel, 2004). Additional limitations in the use of anionic salts as a prevention strategy include cost, reduced feed palatability, and the difficulty of finding low potassium forages to include in the diet (Oetzel, 2004; Goff, 2004; Goff, 2008). The lack of adequate therapies targeted towards preventing hypocalcemia leaves a large percentage of the U.S. dairy cow population unprotected and new therapeutic strategies are lacking because the physiological mechanisms of SCH are not fully understood.