Skeletal muscle is a critical tissue for dairy cattle, required for movement and posture and also serving as a protein and energy storage location. During the stages of gestation and lactation, when cows are supporting growth of the fetus or large amounts of milk, they are able to pull from their skeletal muscle reserves to meet their requirements. Muscle reserves can influence animal health and milk production. Recently, a series of studies have been conducted at Purdue University to evaluate skeletal muscle changes throughout lactation.
Mobilizing muscle for milk production
At the onset of lactation, dairy cows experience a significant increase in nutritional demands. These requirements outpace the cow’s ability to consume enough feed intake. To meet the extra energy and protein requirements for milk production, the cow starts mobilizing body stores from primarily fat and muscle. Dairy cows can lose a significant portion of their bodyweight in the first few weeks after calving, varying from 3% to 17% of their bodyweight. Skeletal muscle is where dairy cows can access additional stored protein for both amino acid and energy requirements.
During lactation, the body doesn't simply break down muscle for fuel; skeletal muscle also helps the cow regulate her glucose levels. Skeletal muscle plays a big role in glucose metabolism, especially when cows become insulin-resistant after calving. The muscle helps maintain insulin sensitivity, which is critical for overall health and milk production.
Measuring muscle reserves
A method to measure muscle reserves is to ultrasound locations of the animal across time. By measuring muscle depth of the longissimus dorsi, researchers can estimate how much muscle the cow has at any given point. For example, ultrasound images taken from cows before and after calving show that muscle depth can decrease by 30% to 40% during early lactation. Taking small samples of the muscle before and after calving, we observe a reduction in muscle fiber size that coincides with this reduction in muscle depth.
In addition to ultrasound, metabolites like creatinine (a waste product of muscle metabolism) and 3-methylhistidine (an amino acid marker for muscle breakdown) can be measured to estimate the extent of muscle mobilization. These markers help researchers understand how much muscle is being lost and when the cow is rebuilding muscle tissue to result in a net gain of muscle. When evaluating cows at the beginning, middle and end of their lactation, markers of muscle mass were lowest in mid-lactation, indicating lower muscle mass in mid-lactation compared to early and late lactation. Whereas markers of muscle breakdown were elevated in both early and mid-lactation and lower in late lactation. This shows that at the end of lactation, cows are breaking down less muscle.
When does muscle depletion occur?
Muscle depletion is a natural part of the cow’s adaptation to the onset of lactation. How much muscle is lost and when it happens can vary from cow to cow. Some cows may mobilize muscle prior to lactation while others begin to deplete muscle reserves after parturition. The extent of muscle depletion is influenced by the cow’s initial muscle reserves, metabolic efficiency and overall health. The reduction in skeletal muscle mass can be observed through ultrasounds of muscle tissue where we observe less skeletal muscle depth as well as a reduction in metabolic markers for muscle mass from parturition to peak lactation.
Using ultrasound images, researchers have observed that cows can lose up to 40% of their muscle depth, as measured at the muscle that equates to ribeye, in the first 60 days of lactation. This muscle tissue is not all protein, as it also contains intramuscular fat as well. In research studies measuring body composition, dairy cows have lost up to 25% of their skeletal muscle reserves in order to support lactation. In some cows, muscle loss starts even before calving as their bodies prepare for the energy demands of lactation and the amino acid requirements of the growing fetus. However, muscle depletion slows down after the first couple of months and significant muscle recovery typically doesn’t occur until much later in lactation, sometimes not until after 240 to 270 days in milk (Figure 1).
Muscle mass and accretion
Dairy cows are capable of gaining muscle mass during late gestation, as indicated by ultrasound scans and muscle mass metabolites. In several research studies, dairy cows with greater muscle reserves during the dry period give birth to higher-birthweight and muscle mass calves. Cows with greater muscle reserves before calving tend to lose more muscle in early lactation but also produce more milk. On the other hand, cows with smaller muscle reserves may gain muscle during the dry period leading up to calving but may produce less milk in early lactation.
Dairy cattle are born with a certain number of skeletal muscle fibers. These muscle fibers can grow larger in size during periods of time when there are abundant amino acids. When cows are gaining muscle they are reducing the amount of amino acids available to the mammary gland, resulting in less milk and milk protein yields.
The bigger picture
Skeletal muscle plays a crucial role in dairy cows’ health and productivity, but it’s also a very dynamic tissue. It can be mobilized to support milk production during periods of negative energy balance, but recovery of lost muscle is much slower. Understanding how and when muscle depletion occurs – and how nutrition can impact this process – is important for improving cow welfare and maximizing milk yields.
Research is ongoing to better understand these processes and to find ways to optimize nutrition and management strategies for dairy cows. The goal is to support the cow’s health through all stages of lactation, helping her to produce the highest-quality milk while maintaining muscle mass and overall well-being.
