Not the crude protein but the amino acids are required nutrients for dairy cows. Balancing for amino acids is important as it is a contributing
factor to higher milk yields, higher milk component levels and greater dairy herd profitability.
Balancing for amino acids (AA) is increasingly accepted in dairy nutrition. This is due to the desire to feed lower protein diets, high prices for protein supplements, an overall trend of higher milk protein prices, continued refinement and improvement of nutrition models, and increased availability of rumen-protected amino acids (RP-AA).
In 2001, the Dairy NRC model was released which allowed for evaluation of diets for rumen degradable protein (RDP), rumen undegradable protein (RUP) and for lysine (Lys) and methionine (Met) in metabolisable protein (MeP). Metabolisable protein is defined as the true protein that is digested post-ruminally and the component AA is absorbed by the intestine. For the dairy cow, AA and not protein are the required nutrients. Milk protein content and yield can be increased by improving the profile of AA in MeP, by reducing the amount of surplus protein in the diet, and by increasing the amount of fermentable carbohydrate in the diet. However successful balancing for AA requires acceptance of several basic aspects of AA nutrition and 'letting go' of balancing for crude protein (CP).
Sources of absorbed amino acids
In ruminants, AAs are provided by ruminally synthesised microbial protein (MP) and, to a lesser extent by endogenous protein. Microbial protein typically supplies a majority of the AA and may supply more than 50% of the absorbed AA in high-producing cows fed a high-concentrate diet that is balanced to meet requirements for rumen degradable protein (RDP) and RUP. The quantity of AA provided by endogenous protein secretions is smaller, assumed to account for less than 10% of total absorbed AA. Methionine and Lys have been identified as the two most limiting AA for lactating dairy cows fed maize-based rations. This is not surprising given the low concentrations of Met and Lys in most feed proteins relative to concentrations in rumen bacteria and in milk and tissue protein (Table 1).
Importance of histidine
Other essential AA have also been evaluated for their possible limitation after Lys and Met supplementation. Particular attention has been given to histidine (Hist) and the branched-chain AA (BCAA; isoleucine, leucine and valine in part because some models predict them as more limiting than other AA. However, results with Hist are much more positive than with BCAA. Hist maybe important when lower RUP diets are fed. Hist is the first limiting AA for milk and milk protein yields when high forage, grass silage diets, supplemented with barley and oats, with or without feather meal as primary source of supplemental RUP are fed. Further evidence for the significance of Hist as a limiting AA come from work by Lee et al., (2012) and Hadrova et al., (2012) where cows fed maize-containing diets, already supplied with supplemental Lys and Met, responded to Hist supplementation. In both sets of experiments there were low dietary concentrations of RUP (4.5 and 3.6% of diet DM, respectively). Dry matter intake and milk and milk component yields were restored back to positive control levels with supplemental Lys, Met and Hist.
Balancing dairy cow diets for amino acids
Considerable progress has been made in recent years to build nutritional models to predict supplies of absorbed AA for dairy cows. Further progress in balancing diets for AA could be made by establishing the optimal concentrations in MeP of the most limiting AA for the model of choice, formulating diets to come as close as possible to meeting those optimal concentrations, and reducing the RUP content of the diet as much as possible without sacrificing any production benefits realised by balancing for AA. The first step to balancing for Lys and Met (i.e., establishing the optimal concentrations in MeP) was accepted by NRC (2001). This approach not only eliminates the need for validated AA requirements, but it has the decided advantage of allowing for the establishment of optimal concentrations of the most limiting AA in MeP.
Enhanced milk and milk component yields
The benefits of balancing for AA, with the focus being almost entirely on Lys and Met thus far, are well known (Schwab, 2012). These include reducing the risk of an AA deficiency, optimising transition cow health, increasing milk and milk component yields, and feeding less RUP to post-transition cows. Feeding less RUP not only decreases feed costs but also allows for increased carbohydrate feeding. The consequence is increased synthesis of MP, a protein of high quality, and increased synthesis of volatile fatty acids, important substrates for lactose and fat synthesis. The benefits of balancing for AA are most noticeable in transition and early lactation cows but benefits exist throughout lactation. Enriching rations with metabolisable Lys and Met in seven trials commencing immediately post-calving or within the first 2 or 3 weeks of lactation and continuing to at least 120 days in lactation was summarised by Garthwaite et al., (1998). They showed that daily milk yield was increased by an average of 0.7 kg, milk protein by 80 g and milk protein increased by 0.16%. In five similar studies where the rations were enriched in Lys and Met in the close-up (pre-partum) ration as well as for the first third of lactation, daily milk yield was increased by an average of 2.3 kg, milk protein by 112 g and milk protein increased by 0.09%. In all cases, the AA-balanced diets had either the same or lower levels of dietary CP than the basal diets. This summary of experiments not only shows the importance of enriching diets with Lys and Met on milk performance, but it also shows that the principles of balancing rations for Met and Lys should be applied in the close-up rations to extract maximum benefit during lactation. Responses to enriched Met and Lys have sometimes been variable but in most cases, economic benefits were obtained. The poor responses almost always appeared to have been the result of less than optimum carbohydrate nutrition.