Protein Nutrition for Transition Cows Vital
The transition period — three weeks prepartum until three weeks postpartum — is an extremely challenging time to feed dairy cows. During this period, nutrient requirements for maintenance, pregnancy and milk production are changing dramatically, as is the level of feed intake. For many reasons, it is not practical or advisable to make numerous diet and pen changes during the transition period. However, it is possible to have a pre-fresh group from approximately three weeks prepartum until calving and a post-fresh group from calving until three weeks after calving. By implementing pre- and post fresh groups, nutritionists can help accommodate the changing nutritional needs of cows during this time. While there has been considerable research and discussion about how to meet the changing energy requirements during the transition period, much less attention has been paid to protein nutrition of transition cows. Prepartum The National Research Council’s (NRC) 2001 dairy nutrient recommendations estimated the dry matter intakes and crude protein (CP) requirements — metabolizable protein (MP) requirement/0.7 — for pre-fresh transition heifers and cows (Figure 1). The CP percentage needed in the diet to meet the cow’s requirement can be calculated — (CP requirement/predicted dry matter intake) x 100 — for any day prior to calving. The NRC committee did not have sufficient data for determining the requirement for mammary growth, so estimates from VandeHaar et al. (1999) have been included for calculating a more liberal estimate of CP needs. Several important points from Figure 1 include:
1. Heifers require more CP in the diet than cows do because heifers have lower feed intakes.
2. Heifers also have higher protein requirements than cows to accommodate growth. Although it appears that cows could be fed diets with less than 12% CP for the majority of the pre-fresh transition period, this is not recommended since it is believed that 12% CP is the minimum needed to maximize digestion and microbial protein synthesis in the rumen.
3. Because the drop in feed intake accelerates as calving approaches, the need for a greater percentage of CP in the diet also accelerates.
4. CP requirements for mammary development are not trivial and increase CP needed in diets by 1-2%. The large increase in the percentage of CP needed as calving approaches poses an interesting question: What is the appropriate density when formulating a diet for the entire three-week pre-fresh period? Should the diet be high in CP to minimize the likelihood of the cow or heifer ever experiencing a negative balance for MP Or, should a lower level of CP be formulated to meet the needs of the cow or heifer for the majority of the pre-fresh transition period and to minimize the period of time in which she would be overfed protein? Most, but not all (Chapter 9 of NRC, 2001), studies have indicated that the milk and protein yield of cows is not in? uenced by the prepartum protein content of the diet. In some studies, increasing the CP content of the diet above 12-13% has decreased postpartum feed intake. The reason for this is not known, but it may be related to the reduced capacity of the liver to detoxify ammonia during the transition period (Strang et al., 1998). Pennsylvania State University researchers (Putnam and Varga, 1998) indicated that cows fed 10.5%, 12.6% or 14.5% CP during the pre-fresh transition period were all in positive nitrogen balance. If cows and heifers are intermingled and fed the same diet prepartum, the diet should be formulated to meet the needs of the heifers. If they are housed separately, then separate diets could be formulated for each group. Considering additional requirements for mammary growth, heifers need approximately 1,000 g of MP per day (or 1,400 g of CP), while cows need approximately 860 g of MP per day (or 1,230g of CP). It is best to take measurements of 2. Estimates (NRC, 2001) of potential deficiencies in MP when feeding 15.7% or 17.0% CP diet to cows producing 60 lb. or 100 lb. of milk dry matter intake on the farm so the percentage of CP in the diet can be calculated, but remember to never go below 12% CP. Typically, cows will need 12% CP in the diet and heifers will need 14% CP. There are recommendations for greater requirements for MP or CP and possibly for inclusion of protein sources that have high levels of rumen undegradable protein (RUP). These higher recommendations are usually made as “safety nets,” but research supporting them is lacking. Supplementing protein sources high in RUP may also come into play when pre-fresh diets are high in very poor-quality forage, e.g., straw.
1. Estimated percentage CP needed in diet to meet protein requirements of cows and heifers at 21, 7 and 1 day prior to calving (NRC, 2001)
As is the case with energy, protein intake during the first three weeks post calving may be insufficient for meeting requirements for milk production due to low feed intake. The cow responds to this by mobilizing reserves. However, in contrast to energy, the density of protein in the diet can be increased to reduce metabolic stress associated with mobilizing reserves. Unfortunately, there is far less research on which to base recommendations for post-fresh transition cows. Most studies examining protein or amino acid supplementation of early lactation cows started the treatments beyond the post-fresh transition period. In several studies, treatments were started prepartum and continued postpartum. Due to the potential negative effects of overfeeding protein prepartum, interpreting results from these studies is different cult because any positive effects of increasing protein postpartum may have been negated by feeding additional protein prepartum. For example, Socha and co-workers (2005) did not observe a response to increasing dietary CP from 16.0% to 18.5% immediately after calving;however, the prepartum diet contained 15.6% CP. In contrast, Wu and co-workers (1997) observed a 10 lb.-per-day increase in milk production, but only when cows came off a prepartum diet that was low in RUP (14% CP diets with 33.6% versus 41.4% of CP as RUP). Despite the paucity of research data examining protein feeding during the first three weeks postpartum, a strong case can be made for not shortchanging cows on protein or amino acids during this period. Figure 2 shows the potential for a negative MP balance for cows producing 60 lb. or 100 lb. of milk per day at seven or 21 days postpartum. Two different postpartum diets were evaluated using NRC (2001) recommendations: one with 15.7% CP and the other with 17.0% CP. The rumen degradable protein (RDP) content of the diets was adequate. NRC allowed estimation of dry matter intakes and MP balance. There are a few important points to note: Assuming that the NRC predictions are correct, diets will not provide sufficient MP. As the length of time postpartum increases, cows will gradually achieve an MP balance due to increased feed and protein intake relative to protein requirements. However, prior to that, cows are likely to be in a negative MP balance, and the likelihood is greater with higher levels of production. Consequently, the cow will either mobilize protein to support lactation, or milk production will be limited and below the inputs (60 lb. or 100 lb.) used for this simulation using NRC data. Amino acid feeding The concept of supplementing ruminally protected amino acids to improve MP balance and quality and reduce dietary CP should be as applicable to transition cows as those later in lactation. Beginning 21 days prior to the expected calving date, Ordway et al. (2009) fed heifers and mature cows a basal prepartum diet containing 13.8% CP (diets averaged 1,200 g per day of MP with an average MP balance of 313 g per day, according to NRC predictions) with either no additional rumen-protected methionine supplementation (control) or with additional MP-methionine in amounts required to generate a 3:1 ratio of MP-lysine to MP-methionine. These same dietary treatments were continued through 140 days postpartum, with the basal diet containing 16.4% CP; diets averaged 2,400 g per day of MP with an average MP balance of -145 g per day, according to NRC predictions. These researchers observed a linear response in milk protein concentration with the additional MP-methionine, suggesting that cows did benefit from an improvement in amino acid supply as the ratio of MP-lysine to MP-methionine was improved to 3:1, even on a relatively lowCP ration. Socha et al. (2005) observed that it was beneficial to supplement rumen-protected methionine and rumen- protected methionine plus lysine to cows receiving a basal diet containing 15.6% CP beginning 14 days prepartum and continuing on their respective amino acid treatments for 105 days postpartum when receiving either 16.0% and 18.5% CP diets.These researchers concluded that there was no difference between rumen-protected amino acid-supplemented cows receiving a 16.0% or 18.5% CP diet, and numerically, the cows on the 16.0% CP diet consumed more dry matter, produced greater amounts of energy corrected milk and were more efficient at converting dietary nitrogen into milk nitrogen than cows on the 18.5% CP diet. This may indicate that the 16.0% CP diet was similar — and perhaps superior — in nutritive content to the 18.5% CP diet. Interestingly, Socha et al. increased the CP content from 16.0% to 18.5% by increasing the RDP fraction of the ration rather than the RUP fraction and concluded that this may have been the reason for the lack of difference between the diets. Indeed, the researchers have routinely observed that these dietary differences are quite common on commercial dairy farms, i.e., diets containing higher levels (greater than 17.5%) of CP contain higher levels of RDP than lower-CP diets (less than 17.5% ) probably because RDP sources have historically been less expensive than RUP sources. Given the current and, most likely, future high economic and environmental costs associated with all protein sources — both RDP and RUP — the results of Ordway et al. (2009) and Socha et al. (2005) support the concept of supplementing both rumen-protected lysine and rumen-protected methionine in transition cow diets to lower CP levels without sacrificing production or metabolic health.
2. Estimates (NRC, 2001) of potential deficiencies in MP when feeding 15.7% or 17.0% CP diet to cows producing 60 lb. or 100 lb. of milk
Due to the cost of protein supplements and environmental concerns with overfeeding protein, there is increasing pressure to scale back the percentage of CP in dairy diets. More research is needed, but nutritionists should carefully consider formulating diets for the post-fresh transition pens that are of higher amino acid quality relative to other stage-of lactation diets. They should concentrate on providing sufficient amounts of RDP and fermentable carbohydrates to stimulate microbial protein production and improve the quality of RUP by providing highly digestible sources of RUP and supplementing rumen-protected amino acids such as lysine and methionine. The concept of providing limiting amino acids is probably most applicable
to the cow immediately postpartum, particularly if there is any temptation to feed lower-protein diets.
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