ARTICLE FEED & ADDITIVE MAGAZINE October 2025 69 tal lysine was very impactful for the production response. When cows were provided additional RPL (total lysine at 7.6% MP) during early lactation (<90 DIM), milk yield increased 1.5 kg/day. Interestingly, the response to RPL was only 0.8 kg more milk when it was fed in mid lactation (>90 DIM). This could be due to decreased intake during early lactation causing not only decreased lysine intake but would also decrease microbial protein flow. Combining decreased lysine intake and microbial flow could be why providing additional RPL was more impactful. Dairy cows may also have a greater requirement for lysine during early lactation. In addition to feeding lysine in early lactation, the duration of feeding was also quite important. When cows consumed RPL for longer than 70 days, milk increased 1.5 kg compared to 0.8 kg if they consumed RPL for less than 70 days. Based on this, feeding RPL early in lactation and for a longer duration provided the greatest milk yield response. But what about milk components? Both milk fat and protein content and yield increased linearly when RPL was provided, meaning that the more RPL the cow consumed, the more her components increased. Interestingly, milk fat content was greatest when the metabolizable methionine was also higher (2.91% MP), alongside the elevated metabolizable lysine provided by RPL. At this same level of metabolizable methionine and lysine, feed efficiency was also improved. The interaction between lysine and methionine in increasing milk fat and feed efficiency is a perfect example of why balancing for all essential amino acids is so important. In an ideal world, we could provide exactly the required amount of essential amino acids, but this is difficult with only rumen protected forms of lysine and methionine currently available in the market. In this case, if we only balance for lysine or methionine instead of both, we could be missing out on milk fat and making the cows less efficient. Further evidence of this is a decrease in plasma methionine when RPL was consumed, meaning with more lysine, the cow can utilize more methionine. Overall, this would improve nitrogen and feed efficiency. With ever growing environmental concerns, improvements in nitrogen efficiency can be very impactful. FEEDING RUMEN PROTECTED LYSINE TO DAIRY COWS All this begs the question of how much lysine do we really need to be adding to dairy rations? Though milk, protein, and fat yields all had a linear response to increasing metabolizable lysine, just adding endless RPL is not economically feasible. When lysine as a percent of MP increased from 6.5 to 8.5%, milk, FCM, and ECM increased 1.8, 2.5, and 2.4 kg/day, respectively. This type of increase is a great starting point for dairy producers and nutritionists that want to try balancing for lysine. Though it is also important to note that in this meta-analysis, the average amount of lysine was 7.6% of MP in the groups consuming RPL and modest milk and protein yield results were still achieved at this level. But how do we get to the recommended level of lysine in an actual ration? Although the meta-analysis represents the amount of lysine and methionine as a % of MP, recent modeling and ration software programs such as Cornell Net Carbohydrate and Protein System (CNCPS) recommend representing amino acid requirements per unit of energy. Because the cow requires energy in order to make protein from amino acids, it is logical to represent amino acids requirements on a unit of energy (specifically metabolizable energy or ME) basis. To relate back to the meta-analysis, when lysine is 8.5% of MP, that equates to 3.2 g of metabolizable lysine/mcal of ME. Coincidentally, this is also the level of lysine recommended by CNCPS. For methionine, the recommended value is 1.14 to 1.17 g/mcal of ME. RUMEN PROTECTED LYSINE AND TMR STABILITY As mentioned previously, lysine is low in traditional corn-based dairy rations. By estimating the amount of lysine that bypasses the rumen, we can get an idea of how much we are supplying to the
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