Effectiveness of intravenous and abomasal doses of mimosine for defleecing sheep and effects on subsequent wool growth

Abstract

Four Merino sheep were given intravenous infusions of mimosine for 2 days at a rate of 110–120 mg/kg/day. Wool fibre growth stopped by about 1 day after the start of the infusions, and the sheep were subsequently manually defleeced. It was estimated that, on average, fibre growth stopped for about 12 days. Wool growth rates were above the pre-infusion rates in the early regrowth (3–5 weeks after dosing), and the mean fibre diameter was still about 2 µm above pre-infusion values at 11 weeks after dosing. Groups of Merino sheep were given intravenous infusions of mimosine for 2 days at rates ranging over 40–320 mg/kg/day. The minimal rate of infusion required to produce consistent defleecing was 80 mg/kg/day; infusions at a rate of 60 mg/kg/day were sometimes effective for defleecing, but at 40 mg/kg/day produced no discernible effects on the strength of wool fibres. No adverse effects were observed at a mimosine infusion rate of 80 mg/kg/day, but one out of four sheep died when dosed at a rate of 160 mg/kg/day, and higher rates (240 and 320 mg/kg/day) were lethal. The concentration of mimosine in plasma was related to the rate of infusion of mimosine. Consistent defleecing was associated with a concentration of mimosine in plasma approaching 100 µmoles/l; lethal doses resulted in plasma mimosine concentrations above 300 µmoles/l. Abomasal infusions of mimosine at a rate of 80 mg/kg/day were equivalent to intravenous infusions. Single injections of mimosine into the abomasum did not influence the strength of wool fibres; pIasma concentrations foIlowing injections indicated rapid absorption of mimosine and rapid removal from the body.