Growth response to increasing doses of microencapsulated vitamin B12 and related changes in tissue vitamin B12 concentrations in cobalt-deficient lambs

AIM: To investigate growth response of cobalt deficient lambs to increasing doses of microencapsulated vitamin B12, and to measure associated changes in serum and liver vitamin B12 concentrations over 243 days. METHODS: From a flock grazing pastures that had low cobalt (Co) levels (about 0.06 mg Co/kg dry matter), 4-6-week-old lambs (n=137) were assigned to four groups and received either no treatment or a subcutaneous injection of 3.0, 4.5 or 6.0 mg of microencapsulated vitamin B12 on Day 1. At approximately monthly intervals, all lambs were weighed and blood samples were collected from a selection (n=10) of monitor animals, up to Day 243. Liver biopsies were also carried out on the monitor lambs (n=8) on Days 1, 124 and 215. RESULTS: The vitamin B12-treated lambs grew significantly faster (p<0.001) than untreated animals. Liveweights after 243 days were 28, 45, 45 and 47 kg for the untreated, 3.0, 4.5 and 6.0 mg vitamin B12-treated lambs, respectively. Of the initial group of untreated lambs, 68% had to be removed before the end of the trial because of substantial weight loss, but none of the treated animals were similarly afflicted. Serum vitamin B12 concentrations increased in all vitamin B12-treated lambs, reaching a peak at Day 25, and those of the 4.5 and 6.0 mg vitamin B12-treated lambs remained significantly higher (except at Day 124) than the untreated lambs to Day 187. However, at Day 124, but not Day 215, the liver vitamin B12 concentrations of treated lambs were two to three times higher than those of controls. CONCLUSIONS: The growth rates of Co deficient lambs were markedly improved by injection of 3.0, 4.5 or 6.0 mg of microencapsulated vitamin B12, and liveweights were maintained for at least 243 days. Serum vitamin B12 concentrations were related to this growth response; concentrations of <220 pmol vitamin B12/l were associated with a 95% probability that lambs were Co deficient and would thus respond to Co/vitamin B12 supplementation. Based on these data, the current New Zealand reference criteria for Co deficiency should be reviewed. CLINICAL SIGNIFICANCE: An injection of 3 mg microencapsulated vitamin B12 given to lambs at tailing will treat Co deficiency and will increase and maintain liveweights in a flock for up to 8 months.