<p>Methionine (Met) is an essential amino acid (EAA) and is often considered one of the first limiting amino acids (AA) for milk production in dairy cattle. In diets of lactating dairy cattle, in the United States (U.S.), Met is considered limiting due to its low abundance in commonly fed ingredients like soybean and corn and its high degradability in the rumen. To circumvent this problem, rumen-protected Met (RPM) products were produced and are currently supplemented to dairy cattle. Supplemental RPM products protect Met from degradation in the rumen and deliver Met to the intestine to be absorbed into circulation, increasing metabolizable Met supply. Due to Met important roles in protein synthesis and as a methyl donor, dairy producers have supplemented RPM for decades. The primary benefit for supplementing RPM is improved milk protein concentration, which is important as a majority of dairy farms in the U.S. are paid based on milk components. </p> <p>Research is focused on finding a more economically viable RPM product that provides similar or better bioavailability of Met compared to commercially available products. This has led to the development of a novel rumen-protected Met product (EMT 3.4) in which Met is added to the soluble portion of distillers and incorporated with distillers grains. The result is distillers grains with solubles (DDGS) with elevated Met concentration. Incorporating Met into the solubles portion of DDGS has shown to protect Met from degradation when analyzed in vitro but has not been assessed in vivo. Thus, the primary objective of this study was to evaluate the bioavailability of EMT 3.4 via dilution of selenomethionine (SeMet) in milk protein and secondarily its impact on milk production and body composition. Dietary selenium (Se) yeast will deliver SeMet to the intestines and subsequently be absorbed. After adequate time feeding Se yeast, SeMet will label the different pools of Met similar to feeding radiolabeled nitrogen (N). Once pools of Met are labeled with SeMet, milk Se, a proxy for SeMet, can be measured in the milk. Concentrations of milk Se will fluctuate based on entry of Met, and as Met increases in the milk, the concentration of Se will decrease. To asses bioavailability milk Se is divided by milk N (Se:N) and the ratios of the treatments can be compared.</p> <p>The objective of this study was to evaluate the bioavailability of a novel rumen protected Met product and its effect on milk production and body composition compared to positive and negative controls. Dietary treatments consisted of a negative control treatment (NCT), not supplemented with RPM, a positive control treatment (PCT), supplemented Met via Smartamine M, and the experimental Met treatment (EMT), supplemented Met via EMT 3.4. Cattle fed PCT and EMT exhibited improved bioavailability indicated by a reduction in milk Se to milk N ratio when compared to NCT cows. Using milk Se concentration as an indicator of entry rate and amount of Met in the milk pool, PCT cows had a significantly lower milk Se concentration compared to NCT. Lower milk Se concentration of PCT cows indicates greater amount of Met in milk and subsequently a significantly greater milk protein yield was observed in PCT. Although EMT had similar bioavailability to PCT, the EMT cows had an intermediate response in milk Se but no production responses. The lack of production responses in EMT was observed alongside an elevated milk urea nitrogen (MUN) and a negative change in longissimus dorsi depth (LDD). A negative change in body weight (BW) was observed in NCT cows. Changes in body composition, as the ones observed in this study, are not usually observed unless cattle are energy or protein deficient. Given that diets of this study were formulated to meet requirements for producing 40 kg/day of milk and on average cattle were producing less, it is unlikely that cattle were energy or protein deficient. Results demonstrated that EMT provided similar Met bioavailability as Smartamine M in PCT demonstrated by responses in milk Se:N. The PCT tended to increase milk yield and increased milk protein, whereas EMT provided no observable production responses in this study.</p> <p><br></p>