The transition period, spanning the three weeks before and after calving, is a critical time for dairy cows as their whole body adapts to the upcoming lactation. During this time, cows face increased energy and protein requirements and reduced feed intake, which consequently puts them in a negative nutrient balance, and an increased risk for metabolic and reproductive disorder development (Figure 1). Ensuring a successful transition for the cows is crucial, as a poor transition can lead to reduced milk yield and an increased risk of culling.
One factor that plays a role in the success of the transition period is the cow’s body composition, more specifically the fat and muscle of the cow. Both fat and muscle tissues are mobilized during the transition period to compensate for the negative nutrient balances and provide precursors for milk and milk components.
Within the dairy cow, there are multiple types of fat reserves, including subcutaneous, muscular and visceral fat. Subcutaneous fat, located just below the skin, is visually assessed when body condition scoring cows and accounts for approximately 25% of total fat reserves. Muscular fat accounts for approximately 50% of the fat in the body and is located within and around muscle tissue. Visceral fat, which surrounds the organs, makes up approximately the remaining 25% of fat reserves in the body.
Current management strategies aim to avoid overconditioning cattle before the dry period. Feed diets in the dry period that are lower in energy but maintain gut fill also aim to avoid excessive fat accumulation during that time. The more fat a cow has going into lactation, the more fat she will mobilize. Fat that is mobilized then goes to the liver and reduces the liver’s ability to produce glucose. The cow relies on the liver to make a majority of her glucose, which is required for milk synthesis. Excessive fat mobilization also increases the risk for metabolic disorder development and reduces feed intake, further reducing milk production.
In addition to fat mobilization, a significant amount of muscle tissue is mobilized during the transition period to provide for increased amino acid requirements. Less is known about how muscle tissue amount and mobilization impact our dairy cows, and current feeding strategies do not evaluate or manage muscle amount in dairy cows.
This is in part due to the feasibility of on-farm muscle assessment. Body condition scores are highly related to whole body fat reserves but only marginally related to muscle reserves. Because of this, we cannot use body condition scores to visually assess muscle amount of dairy cattle as animals with high subcutaneous fat, and thus a high score may have little muscle reserves or vice versa. Instead, assessment of muscle reserves requires ultrasound imaging and is thus not typically evaluated commercially (Figure 2).
Recent research efforts have explored how muscle amount and mobilization impacts transition success and early lactation performance. Similar to fat, it has been found that the more muscle a cow has, the more she will mobilize in early lactation. However, unlike fat, no negative consequences have been identified as being associated with high levels of muscle mobilization.
Interestingly, we have reported that dairy cows with more muscle during the dry period begin to mobilize their muscle prior to calving, while cows with less muscle reserves gain muscle in the weeks approaching calving. The cows with more prepartum muscle mobilization subsequently give birth to larger calves. Additionally, dairy cows with more muscle prepartum produce significantly more milk in the first 30 to 60 days in milk, and more milk protein and fat yields. As muscle is mobilized, protein is broken down into amino acids and taken up by the mammary gland for milk protein synthesis. However, during muscle mobilization, muscular fat, which again accounts for about 50% of the cow’s fat reserves, is also mobilized. Thus, it makes sense that cows with more muscle produce more milk protein and fat as they mobilize more muscle and subsequently muscular fat than cows with less muscle. Current efforts have not explored long-term implications of muscle amount on production responses, so it is unclear if benefits persist past early lactation.
In addition to these positive production responses, larger muscle reserves could prove to be beneficial in other areas for dairy cattle. As mentioned, the transition period is a critical time for cows during which they face increased odds for negative health events to occur. Having more muscle during the transition period means the cow has a larger pool of amino acids to pull upon for energetically demanding responses such as milk production of immune function.
Mounting an immune response has a high energy cost; however, larger muscle reserves may help to overcome immune challenges due to the increased availability of protein. Recent research efforts have begun to explore this idea, with one study noting that larger muscle reserves resulted in reduced odds for metritis development postpartum and reduced odds for overall clinical disorder development in the first 28 days of lactation. These findings suggest that muscle amount and extent of mobilization play a key role in not only production responses, but that it may also impact factors relating to animal health.
We know that dairy cows utilize muscle in early lactation to make up for gaps between intake and requirements; however, we are unclear on the implications of that. Because of this, current research aims to better understand the impact that muscle amount and the extent of mobilization has on production and health of our dairy cows, both short- and long-term.
As research evolves, we can refine and shape our transition dairy cow management strategies to maximize profitability for producers and enhance welfare of our dairy cows. Some of these strategies may be to feed cows and heifers to optimize muscle, which they can draw on during times of increased amino acid requirement.
