Industry Summary: Modern swine production systems require highly productive animals. Thus, continued development of nutritional strategies to improve milk production and litter weight gain is critical. However, very little is known about the physiological mechanisms that control milk protein synthesis in pigs. Furthermore, with increasing concerns over environmental nitrogen losses, research has focused on strategies to improve efficiency of dietary protein utilization in growing pigs, with the breeding herd lagging behind. Indeed, the focus of amino acid nutrition for the lactating sow has been on litter growth maximization rather than on minimizing nitrogen excretion. However, lactating sows consume approximately 20 kg of crude protein (CP) over a 21-d lactation period, with substantial nitrogen losses in the urine. Hence, there is significant contribution of dietary nitrogen losses to the environment from the breeding herd as well. Understanding of the factors governing the efficiency of utilization of dietary protein and amino acid for milk synthesis is critical in order to minimize nitrogen excretion from the breeding herd while maintaining or even improving lactation performances. The objectives of this study were to 1) quantify the expression of genes that encode for milk and Lys transporter proteins and assess whether these genes are correlated; 2) measure the efficiency of Lys utilization for milk production in response to feeding an optimum amino acid balance in a reduced crude protein diet; 3) quantify the expression of genes that encode for milk and Lys transporter proteins in response to feeding an optimum amino acid balance in a reduced CP diet; 4) assess whether change in Lys utilization efficiency for milk production is related to changes in the expression of genes encoding for Lys transporter proteins.

For objective 1, the expression of 5 genes that encode for amino acid transporter proteins and of 2 that encode for mammary synthesized milk proteins were measured in sow mammary tissue across different stages of mammary physiological activity and milk demand. Results of objective 1 indicated that there was a high correlation between transcript abundance for 2 of the 5 amino acid transporter genes selected and the transcript abundance of genes encoding for milk proteins. Therefore, a select number of amino acid transporter genes were identified as potential molecular targets for improvement of sow milk production during lactation.

For objectives 2 to 4, 24 sows were used. The rate of uptake of amino acids by the mammary gland (that is, the percentage of AA that the mammary gland uses per blood pass) was quantified, along with the expression of genes that encode for transporter proteins responsible for amino acid uptake. Sows were fed 1 of 3 diets that contained 9.5 (Deficient), 13.5 (Ideal), and 17.5 % (Standard) CP but had similar indispensable and dispensable amino acid profile formulated to target an optimum profile. The results of objectives 2 to 4 demonstrated that decreasing the dietary CP from 19.4 (Standard diet) to 15.1 % (Ideal diet) with inclusion of crystalline amino acids did not affect piglet ADG, but increased mammary transport efficiency and A-V of Lys and Arg. The increase in Lys and Arg transport efficiency was associated with a decrease in plasma concentration of branched chain amino acids to Lys ratio but was unrelated to the expression of genes encoding for amino acid transporter and milk proteins. These results indicate that CP reduction with crystalline amino acid inclusion improves the efficiency of dietary AA utilization for litter growth, and that the mechanisms behind this response are independent of AA transporter or milk protein gene transcription.

The study demonstrated that feeding an optimum balance of amino acids achieved through crystalline amino acid inclusion improves the efficiency of Lys utilization (mammary extraction efficiency) and maintains lactation performance if provided in a reduced CP diet. These findings will allow the development of effective nutritional tools that can be immediately implemented to positively impact both lactation performance and the environment. Second, the study demonstrated that there are two Lys transporter genes that are related to genes encoding for mammary synthesized milk proteins, which in the longer term may be targeted to further understand how amino acid transporters regulate milk production. Finally, the results of the study demonstrated that changes in expression of genes encoding for Lys transporter proteins were not related to Lys extraction efficiency, indicating that other or additional mechanisms regulate Lys transport; for instance, in this study, increased Lys extraction efficiency by the mammary gland was related to lower circulating levels of the branched-chain amino acids, pointing to competitive inhibition mechanism between amino acids for uptake into the mammary gland. The later finding is highly significant to the swine industry: for the first time, it is shown that improvement in Lys utilization for milk production may be achieved by decreasing the levels of non-limiting amino acids, such as the branched-chain amino acids, in order to decrease the competition between Lys and those amino acids for mammary uptake. This latter finding does not only provide an important initial understanding of the biological basis behind the so called ‘ideal protein’ or ‘optimum amino acid balance’, but provides an alley for future technology development at a mechanistic level, including, but not limited to, modulators of nutrient repartitioning.