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Simple Summary: The purpose was to assess the effect of exchanging crude protein of soybean meal with red clover silage (RCS) in total mixed rations (TMR) on ruminal degradation and intestinal digestibility (ID) of essential amino acids (EAA). Increasing proportions of RCS in the TMR increased the ruminal degradation and reduced the ID of EAA. The ruminal degradation of EAA of the TMR followed that of CP, but the ID of EAA differed from that of CP at higher levels of RCS in the diet. In conclusion, increasing proportions of RCS in TMR reduced the extent of EAA flow into the small intestine, reduced the ID of EAA, and consequently the intestinal absorbable EAA. The purpose was to assess the effect of exchanging crude protein (CP) of soybean meal (SBM) with red clover silage (RCS) in total mixed rations (TMR) on ruminal degradation and intestinal digestibility (ID) of essential amino acids (EAA). Four TMR and their individual feed components were studied. The TMR were composed of forage and concentrates (75:25), with proportions of RCS in TMR of 0.15, 0.30, 0.45, and 0.60 on a dry matter basis, resulting in diet groups RCS15, RCS30, RCS45, and RCS60, respectively. The ruminal degradation of EAA was determined using the nylon bag technique. For this, samples of TMR and their individual feed components were ruminally incubated for 16 h. The feed residues of TMR obtained after 16 h of incubation were used for the determination of ID of EAA using the mobile-bag technique. Increasing RCS and reducing SBM proportions linearly increased (p < 0.01) the in situ ruminal degradation of individual EAA from 75.5% to 83.5%. The degradation of EAA followed the trend of CP degradation among TMR, except for Cys that was lower (p < 0.05) than that of CP in RCS60 (79.7% vs. 86.3%). The degradation of EAA in individual feed ingredients not always corresponded to the degradation of CP and was feed dependent. Increasing the proportions of RCS in the TMR linearly reduced (p < 0.001) the ID of EAA (except for Ile) from 78.2% to 67.3%. However, the ID of EAA did not always reflect the ID of CP, and in general, the differences between the ID of CP and EAA increased as RCS increased in the TMR. The ID values of most of the EAA were similar (p > 0.05) to ID of CP in RCS15 and RCS30, while they mostly differed (p < 0.05) in RCS45 and RCS60, and being higher for EAA than CP (except for Cys that was lower than CP, p < 0.05). Similar trends were observed for intestinal absorbable AA, resulting in higher values (p < 0.05) of intestinal absorbable for all EAA than of CP in diet RCS60. In conclusion, increasing levels of RCS in TMR reduced the extent of EAA flow into the small intestine, the ID of EAA, and consequently the intestinal absorbable EAA. Therefore, accurate determination of metabolizable AA must be considered for optimal diet formulation when including high proportions of RCS in diets of high-producing dairy cows. [ABSTRACT FROM AUTHOR]

Simple Summary: The purpose of this study is to assess the relationship of milk odd- and branched-chain fatty acids (OBCFA) with urinary purine derivates (PD) and estimated ruminal microbial crude protein (MCP). The correlations and regressions demonstrate that yields and concentrations of individual or total OBCFA are weakly related to urinary PD and are low to moderate markers of MCP synthesis. Nevertheless, milk OBCFA can still be seen as a promising method for predicting rumen function and microbial protein supply to the duodenum in dairy cows because MCP flow was not directly measured in this study but instead indirectly estimated probably comprising considerable deviations of the assumed values from the true ones. The aim of this study was to evaluate the relationship of milk odd- and branched-chain fatty acids (OBCFA) with urinary purine derivates and estimated ruminal microbial crude protein (MCP) synthesis. Forty-four lactating Holstein cows were used in a 4 × 4 Latin square design with 21-day periods comprised of a 13-day adaptation phase to diet followed by an 8-day sampling phase. Differences in estimated MCP yield and milk OBCFA composition were found by feeding total mixed rations containing forage (maize silage, MS; red clover silage, RCS) and concentrates (0.75:0.25) with targeted proportions of RCS to MS of 0.15:0.60, 0.30:0.45, 0.45:0.30, and 0.60:0.15 on a dry matter basis. The MCP was estimated from the total urinary purine derivate (PD) excretion (MCPPD) and intakes of metabolizable energy (MCPME) or digestible organic matter (MCPdOM). The Pearson correlations of individual OBCFA with urinary parameters (uric acid, allantoin, PD and nitrogen) were generally weak (r = −0.37 to 0.55). Yields of individual OBCFA correlated positively with MCPME and MCPdOM (r = 0.21 to 0.55). The prediction of urinary PD concentration was moderate (R2 = 0.64) when including the proportion of iso-C17:0. The prediction of total PD excretion was low (R2 = 0.21) with yields of iso-C15:0, anteiso-C17:0, and iso-C16:0. The prediction of MCPPD was high (R2 = 0.99) when including the iso-C16:0 and cis-9 C17:1 concentrations, while those of MCPME and MCPdOM were low (R2 = 0.37 and 0.36, respectively) when including yields of iso-C15:0, cis-9 C17:1, and iso-C18:0. The correlations and regression analyses demonstrate that the estimated MCP synthesis and urinary PD excretion can be only moderately predicted by yields and concentrations of individual or total OBCFA in cow's milk. However, milk OBCFA can still be seen as a promising, non-invasive method for predicting rumen function and microbial protein supply in dairy cows because MCP flow was not directly measured in this study but instead indirectly estimated probably comprising considerable deviations of the assumed values from the true ones. [ABSTRACT FROM AUTHOR]