Live sheep trade initiatives

Summary

Treatment with vitamin B12 did not reduce death rates during shipping. The voyage mortality rate was 2.95% and 2.62% for the control and vitamin B12 treated groups (P > 0.05). The ivermectin group had 43% fewer deaths (2.84%) than the controls (4.99%; P = 0.09). Results of this trial suggested that ivermectin was the likely cause of 43% fewer deaths in treated sheep in the 1989 research voyage, in which several treatments were combined, as the same reduction in deaths was achieved in both studies. During simulated shipping, macrocyclic lactone treatment (ivermectin or moxidectin) was not associated with a significant increase in chaff intake during the first 7 days after treatment. Nor was there any difference in the proportion of feeders in any group. In trial 2, we chose to restrict the comparison to ivermectin and controls using shipper pellets instead of chaff. Again, there was no response to the treatment. Mean feed intake data indicated that internal parasite burdens, at the moderate levels in our sheep, did not adversely affect appetite during simulated shipping. There was no evidence that ivermectin stimulated appetite. There was no difference in mortalities in the research voyage.

We concluded that there was no evidence that macrocyclic lactones increased feed intake in adult Merino wethers, either by an effect dependent or independent of anthelmintic activity. We considered that macrocyclic lactones could not be recommended as a blanket treatment to reduce mortalities in sheep exported live. Sheep from two farms, with different mortality rates during actual export, responded differently to both metabolic and stress challenges. Insulin resistance was indicated by the increased basal and glucose stimulated secretion rates of insulin observed in the sheep from the high mortality farm. Obese ruminants have tissue resistance to the glucoregulatory effects of insulin.

In our study the insulin resistance did not relate well to condition score or liveweight but this may reflect limitations in using condition score as an indicator of body fat. There was a 30% greater cortisol release in the sheep from the high mortality farm. This may reflect the sheep over responding to a challenge which in turn could lead to adrenal exhaustion. We concluded that mortality was repeatable and that the likely mortality category (high or low) of a line of sheep can be predicted with considerable confidence. In addition to the evidence for repeatability, there was a strong association between regional location and mortality rate. The association was supported by the spatial analyses, the comparison of average death rates by zone and the comparison of the proportions of high mortality farms by zone.

The results showed that there were more high death rate farms and higher death rates in the longer growing season zones of the southwest of Western Australia. We considered that this was consistent with earlier published studies on the factors leading to mortalities. Follow-up studies are indicated to further investigate the reasons for differences in mortality rates with a view to determining practical strategies to reduce the problem.