Sc. Davies et al., SUBLETHAL EXPOSURE TO CORYNETOXINS AFFECTS PRODUCTION OF GRAZING SHEEP, Australian journal of experimental agriculture, 36(6), 1996, pp. 649-655
This paper reports the production losses resulting from treating sheep
with sublethal doses of corynetoxins, the causal agent of annual ryeg
rass toxicity. Merino ewes were given 3 levels of corynetoxins twice w
eekly for 11 weeks from 26 days before the start of joining to day 51
of pregnancy, giving a cumulative dose of 0, 0.4 or 0.8 mg/kg liveweig
ht. The effect on ovulation, pregnancy, lamb birth weight, lamb surviv
al and weaning weight were measured along with the liveweight and wool
growth of the ewes. Corynetoxin treatment depressed (P<0.005) the act
ivity of uridine diphospho-N-acetylglucosamine: dolichol-phosphate N-a
cetylglucosamine-1-phosphate transferase and liver damage was detected
in ewes on the high toxin treatment (P<0.05). Ewe liveweight was unaf
fected by treatment but wool growth and wool fibre diameter were reduc
ed by 10 and 7%, respectively (P<0.05). Corynetoxin treatment appeared
to increase ovulation (P<0.05) and, as there was no effect on concept
ion and embryo survival, this resulted in a 30-35% increase in the num
ber of lambs weaned. Lamb birth weight and survival were not affected
by treatment but weaning weight of the low toxin group was depressed (
P<0.05). While reproduction was not adversely affected, the sheep in t
his trial were exposed to extremely low levels of corynetoxins which d
id not accumulate to levels capable of causing detectable liver damage
until 23 days after joining. Even at these levels of intake, well bel
ow those required to show clinical signs (3-5 mg/kg liveweight), wool
growth was reduced, suggesting that sheep grazed on mildly toxic pastu
re experience wool production losses in the absence of signs of the cl
inical disease.