Optimal concentrations of lysine, methionine, and threonine in milk replacers for calves less than five weeks of age.

The AA requirements of herd-replacement calves less than 5 wk old and fed milk replacers are not clearly defined and have been estimated in a limited number of studies using milk-fed calves ranging from 5 to 20 wk of age. The objective of these 4 studies was to investigate the effect of supplementing milk replacers containing 24 to 28% crude protein (CP; from milk sources) and 17% fat with Lys, Met, and Thr to estimate the optimum requirements for calves less than 5 wk of age. Holstein bull calves (initially 3 and 4 d old, 43 +/- 1 kg of body weight, BW) were fed an 18% CP (as-fed) starter ad libitum and weaned at 31 to 32 d of age (28-d studies). Calves were housed in an unheated, curtain-sided nursery. In study 1, 6 milk replacer treatments were fed based on the combination of 3 CP concentrations (24, 26, and 28% CP) each with or without added Lys and Met. In studies 2 and 3, 26% CP and 2.34% Lys milk replacer treatments were fed to test the concentration of Met (0.64, 0.68, and 0.72% Met in study 2 and 0.64, 0.72, and 0.80% Met in study 3). In study 4, 26% CP, 2.34% Lys, and 0.72% Met milk replacer treatments were fed to test the concentration of Thr (1.06, 1.43, and 1.80%). There was a 17% improvement in average daily gain (ADG) in study 1 from adding Lys and Met that was maximized with 2.34% Lys. The ADG response to added Met in studies 2 (linear) and 3 (quadratic) were 13 and 7%, respectively, with a plateau at 0.72% Met. There was no ADG or efficiency response to added Thr in study 4. Formulating 17% fat, whey-based milk replacers fed at 0.68 kg/d to 26% CP, 2.34% Lys, and 0.72% Met appeared optimum based on responses of body weight gain, feed efficiency, and serum concentrations of urea nitrogen, while feeding calves more CP and essential AA did not improved ADG and efficiency. Requirements for calves less than 5 wk old, averaging 48 kg of BW, consuming 204 g of CP/d, and gaining 0.46 kg of BW/d, appeared to be met with 17 g of Lys, 0.31 Met:Lys ratio, 0.54 Met+Cys:Lys ratio, and a Thr:Lys ratio less than 0.60.