Objectives of this study were to quantify production responses of lactating dairy cows to supplying absorbable Met as isopropyl-2-hydroxy-4-(methylthio)-butanoic acid (HMBi), or rumen-protected Met (RPM, Smartamine M; Adisseo, Alpharetta, Ga.) fed with or without 2-hydroxy-4-(methylthio)-butanoic acid (HMB), and to determine whether Met supplementation will allow the feeding of reduced dietary crude protein (CP).

Seventy cows were blocked by parity and days in milk into 14 blocks and randomly assigned within blocks to one of the five dietary treatments based on alfalfa and corn silages plus high-moisture corn: one diet with 15.6 percent CP and no Met source (negative control); three diets with 15.6 percent CP plus 0.17 percent HMBi, 0.06 percent RPM + 0.10 percent HMB, or 0.06 percent RPM alone; and one diet with 16.8 percentCP and no Met supplement (positive control).

Assuming that 50 percent of ingested HMBi was absorbed from the gastrointestinal tract and 80 percent of the Met in RPM was absorbed at intestine, the HMBi and RPM supplements increased metabolizable Met supply by 9 g/d and improved the Lys:Met ratio from 3.6 to 3.0. After a two-week covariate period during which all cows received the same diet, cows were fed test diets continuously for 12 weeks. Diet did not affect dry matter intake (mean ± SD, 25.0±0.3kg/d), body weight gain (0.59±0.2kg/d), or milk yield (41.7±0.6kg/d). However, feeding HMBi increased yield of energy-corrected milk and milk content of protein and solids-not-fat. Moreover, trends were observed for increased milk fat content and yield of fat and true protein on all three diets containing supplemental Met. Apparent N efficiency (milk N/N intake) was highest on the RPM treatment. Feeding 16.8 percent CP without a Met source elevated milk urea N and urinary excretion of urea N and total N and reduced apparent N efficiency from 34.5 percent to 30.2 percent, without improving production. Overall results suggested that feeding HMBi or RPM would give similar improvements in milk production and N utilization.

Source: Journal of Dairy Science