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, the re is a need to describe supplementary feeds in terms of their effects on w ool growth. Since wool growth is largely a function of the quantity and qua lity of absorbed protein, the aim of this experiment was to compare the pro tein quality of different diets in relation to their ability to promote woo l growth. An additional aim was to compare observed responses with those pr edicted using feeding models based on metabolisable protein. Sixty-four Mer ino weaner wethers were allocated to 8 treatment groups in a factorial desi gn consisting of 4 diets each at 2 levels of protected methionine. The 4 ro ughage-based diets consisted of different protein sources: Rumente(R)-prote cted 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 lev els of protected methionine were 0 or 3 g/day as Lactet. The diets were for mulated to be isonitrogenous and isoenergetic and were fed at 1.5 x mainten ance 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 th an in sheep fed oats (1.37 v. 1.0 mg/cm(2) per day), and 73% greater than i n 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 e ffect of diet on rumen volatile fatty acid concentrations. Lactet increased wool growth by 18% across all diets (P < 0.001), representing an additiona l 0.17 mg/cm(2) per day or 1.7 g/head per day. There were no significant di fferences in liveweight gain between the diets unless Lactet was added. Whe n Lactet was added, lupins produced a significantly higher liveweight gain than oats or protected canola meal. Lactet increased mean liveweight gain b y 22% across all diets (P < 0.05). Lactet also increased the concentration of plasma albumin, decreased plasma alpha-amino nitrogen, and increased uri nary excretion of creatinine and purine derivatives (P < 0.05). The finding s highlight the need to evaluate feed sources in terms of protein degradabi lity and sulfur amino acid composition, particularly when assessing effects on wool growth. The inability of current feeding models to incorporate dif ferences in amino acid flows, particularly sulfur amino acids, is an impedi ment to evaluating supplement quality in terms of wool growth.