Dairy cows are heavily dependent upon their skeletal muscle tissue as they go through their production cycles. But skeletal muscle can be depleted as cows prepare physiologically for lactation. Jacquelyn Boerman, a professor at Purdue University, explained how skeletal muscle may be affected at the recent Cornell Nutrition Conference held in Syracuse, N.Y.
It has been estimated that dairy cattle lose 3% to 17% of their body weight in the first five weeks postpartum. Some cows add body weight back as they near peak milk yield. Research has shown that animals who gained weight postpartum had better conception rates, but they also experienced a drop in milk yield during the first 30 days of lactation. “A dairy cow that weighs 1,500 pounds postpartum may lose an excess of 220 pounds through peak milk production,” cited Boerman. It is important to note that body weight can fluctuate based upon changes in both muscle and adipose tissue.
What makes up skeletal muscle? Bovine skeletal muscle consists of three major muscle fiber types: I, IIA, and IIX, which are named in correlation to each specific contractile myosin heavy chain protein present. Skeletal muscle is the largest internal organ in dairy cattle, functioning as a pool of amino acids and playing a role in movement, posture, and protection to help carry out bodily functions.
Early in lactation, amino acids are used by the cow for milk protein synthesis. Other amino acids, such as alanine and glutamine, which are derived from skeletal muscle, are used by the liver to produce glucose. “Amino acids are stored in skeletal muscle which can be drawn on during times when requirements exceed provided metabolizable protein. Skeletal muscle serves as both an input and output for the fate of amino acids,” Boerman said.
Dependent upon lactation stage, some tissues may start to deplete. The main tissues depleted in early lactation are adipose and protein, with protein coming primarily from skeletal muscle tissue. For the transition dairy cow, depending on body condition score, adipose tissue may contribute to over 25% of the empty body weight of the cow, with protein representing approximately 13% of empty body weight, Boerman noted.
Ultrasound measurements
To determine muscle depth, ultrasound measurements of the longissimus dorsi muscle are observed because of its correlation to whole body protein. Boerman said that while using ultrasonography, it has been observed that muscle mobilization begins prior to calving and provides amino acids to the fetus and for colostrogenesis. Muscle depth stabilizes approximately one month postpartum. During the dry period, dairy cattle saw variations in muscle depth anywhere from 2 centimeters to 6.5 centimeters. “Dairy cows with greater muscle reserves prepartum mobilized a greater amount of their longissimus dorsi depth by 60 days in milk (DIM) and subsequently produced greater milk yields in early and mid-lactation,” cited the professor.
Another study compared ultrasound images collected monthly from parturition to 300 DIM from 40 multiparous cows. The study measured time points and longissimus dorsi depth. The greatest longissimus dorsi depth was recorded at zero DIM. After this, cows consistently lost muscle depth until 60 DIM and did not gain muscle until 270 DIM. Even at 300 DIM, cows were not back to their original recorded muscle depths. “Interestingly, even when cows were post-peak and gaining body weight and adipose tissues after 90 DIM, they did not appreciably gain muscle until much later in lactation,” Boerman noted. This finding suggests that as cows near the dry period, they may have less muscle than the previous lactation.
Although further research is needed to understand skeletal muscle depletion, we do know that muscle depletion in early lactation is dependent on the amount of muscle present. Dairy cattle skeletal muscle estimates are not well documented, so additional attention is needed in this area.
