ARE YOUR DRY COWS GETTING ENOUGH METABOLIZABLE PROTEIN?
Nutrition and management during the dry period are critical factors for the metabolic status and lactation performance of the dam and health of calves. In particular, the notion that protein nutrition of the dry cow influences lactation performance and health is widespread. Many studies have focused on crude protein needs of dry cows but the results have been mixed, with several studies finding little response in milk yield, milk protein content, or milk protein yield (Bell et al., 2000). Part of the inconsistency in results is because of the poor relationship between crude protein intake and metabolizable protein supply in dry cows. The relationship is dependent on the quality and quantity of dietary protein and the availability of fermentable carbohydrates for microbial growth in the rumen. Improvements in models for diet formulation (i.e. CPM Dairy and CNCPS) have made it possible to estimate metabolizable protein supply and needs of dry cows. Although models provide estimates of metabolizable protein and amino acid supply, the use of crude protein for diet formulation is still prevalent. Currently, formulating diets for dry cows in regards to metabolizable protein and amino acids in an area of opportunity.
Recommendations for metabolizable protein for mature dry cows are typically in the 1000 to 1200 g/d range (Block, 2010; Drackley, 2008). Diets that are lower in fermentable carbohydrates, in particular starch, may need to be supplemented with rumen undergradable protein (e.g. protected soy products) to provide the appropriate amount of metabolizable protein and the proper amino acid profile. French (2012) conducted a literature review of published studies with dry cows and protein nutrition and generated a database. The database contained 12 published studies, 30 treatments, and 382 animals. Descriptions of rations and cows were modeled in CNCPS to generate metabolizable protein values. Metabolizable protein fed in the close-up period was positively related to milk protein yield in early lactation as long as cows were fed >75% of their metabolizable protein requirement in early lactation. Based on the database, French suggested the following guidelines for feeding close-up cows: 1300 g/d of metabolizable protein, 30 g/d of metabolizable protein-methionine, and 90 g/d of metabolizable protein-lysine.
Protein mobilization is a concern for both dry and early lactation cows. If dry cows are deficient in metabolizable protein they will breakdown muscle and other protein sources in the body. Early lactation cows have an inability to consume sufficient protein to meet the mammary and non-mammary amino acid requirements. Cornell researchers (Bell et al., 2000; Burhans and Bell, 1998) calculated the metabolizable protein balance of 80 Holstein cows during the first 28 days postpartum and found that the nadir was -600 g/day at 7 days postpartum. Cows returned to positive protein balance during week 3 postpartum. Bell et al. (2000) suggested that during the first 7 to 10 days postpartum, high producing cows may need to mobilize up to 1000 g of tissue protein per day to satisfy the mammary gland’s demand for amino acids and glucose.
Mobilization of labile protein reserves before calving reduces the amount available after calving. In a recent study (van der Drift et al., 2012) a large variation was observed among cows in the onset and duration of protein and energy mobilization during the transition period. Based on plasma 3-methylhistidine concentrations and muscle thickness profiles, protein mobilization started before calving and continued until week 4 of lactation. Interestingly, protein mobilization occurred before lipid mobilization in most cows. The authors speculated that it might be due to a prepartum amino acid deficiency in the absences of negative energy balance. In addition, it appears that the timing of protein mobilization is related to hyperketonemia (high ketones). Cows with lower 3-methlyhistidine concentrations (indicating less muscle breakdown) had higher serum β-hydroxybutyrate concentrations. Greater protein mobilization to a certain extent after calving may provide amino acids for gluconeogenesis (glucose production) and limit ketone body production. However, this hypothesis needs to be confirmed with additional research.
The Miner Institute Farm Report is written primarily for farmers and other agricultural professionals in the Northeastern U.S. and Eastern Canada. Most articles deal with dairy and crops topics, but also included are articles dealing with environmental issues and global agriculture as well as editorial commentary.
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