The effect of dietary protein source and protected methionine (Lactet) on wool growth and microbial protein synthesis in Merino wethers

With increasing awareness of the importance of controlling tender wool, there is a need to describe supplementary feeds in terms of their effects on wool growth. Since wool growth is largely a function of the quantity and quality of absorbed protein, the aim of this experiment was to compare the protein quality of different diets in relation to their ability to promote wool growth. An additional aim was to compare observed responses with those predicted using feeding models based on metabolisable protein. Sixty-four Merino weaner wethers were allocated to 8 treatment groups in a factorial design consisting of 4 diets each at 2 levels of protected methionine. The 4 roughage-based diets consisted of different protein sources: Rumentek®-protected canola meal (at 33% of the diet), oats plus urea (oats at 46% and urea at 2.6%), Lupinus angustifolius (at 36%), and L. albus (at 36%). The 2 levels of protected methionine were 0 or 3 g/day as Lactet. The diets were formulated to be isonitrogenous and isoenergetic and were fed at 1.5 × maintenance metabolisable energy. The experiment lasted 13 weeks, which included a 3-week balance collection period in which faeces and urine were collected. Wool growth in sheep fed the protected canola meal diet was 37% greater than in sheep fed oats (1.37 v. 1.0 mg/cm2 per day), and 73% greater than in sheep fed L. angustifolius (P < 0.001). Wool growth from the oat diet was 26% greater than from the L. angustifolius diet (P < 0.05). There was no effect of diet on rumen volatile fatty acid concentrations. Lactet increased wool growth by 18% across all diets (P < 0.001), representing an additional 0.17 mg/cm2 per day or 1.7 g/head per day. There were no significant differences in liveweight gain between the diets unless Lactet was added. When Lactet was added, lupins produced a significantly higher liveweight gain than oats or protected canola meal. Lactet increased mean liveweight gain by 22% across all diets (P < 0.05). Lactet also increased the concentration of plasma albumin, decreased plasma α-amino nitrogen, and increased urinary excretion of creatinine and purine derivatives (P < 0.05). The findings highlight the need to evaluate feed sources in terms of protein degradability and sulfur amino acid composition, particularly when assessing effects on wool growth. The inability of current feeding models to incorporate differences in amino acid flows, particularly sulfur amino acids, is an impediment to evaluating supplement quality in terms of wool growth.