Data collected by Assure Quality since 2005 highlights a marked elevation in spore counts during March and April, even though air temperatures start to decline. Although cooler nights have traditionally signalled the end of zinc dosing for many farmers, what these graphs (below) show us is that there is still a significant risk of disease as late in the season as May.
If we look at the incidence of facial eczema in the second graph then we can clearly see that liver damage occurs in stock that consume high spore levels in the previous weeks and that this peak is generally reached around mid April.
Industry expert Andrew Oakley believes 2014 is shaping up to be a repeat of 2008, where “we experienced frosts in March and everyone thought the season was over – clearly it wasn’t”. He believes that ground temperature, rather than air temperature signals the end of FE risk, citing the latest reports from FAR which shows that soil temperatures have, in the last fortnight, risen above the long term average in all areas except Morrinsville.
He reminds us that supplementation can only be protective against FE if it maintains the cow’s blood serum zinc level at 20 to 35 micromoles per litre. Whilst this can be difficult to achieve using zinc sulphate via drinking water alone, feeding zinc oxide in grass or maize silage (as a balancer ) or in grain/concentrate in the bail at milking can be very effective for FE prevention.
However, the amount of zinc oxide included in each tonne of grain/concentrate for “FE prevention dosing must be carefully calculated to achieve the required dose of 20 mg elemental zinc/kg liveweight/day and minimise the risk of zinc toxicity.
If you are unsure whether you are successfully maintaining your cow’s blood serum zinc level within the target range of 20 to 35 micromoles per litre, the best thing to do is to get your vet to blood test at least 10 cows. Remember that by the time you see some cows in the herd affected by the photosensitisation caused by FE, a large proportion of the herd is likely to have already suffered liver damage, for which there is no specific treatment. Zinc can only prevent FE. It cannot reverse liver damage already done from exposure to the toxin.
Early intervention is critical – but equally knowing when to stop zinc dosing is vital. Using ground temperatures in combination with pasture spore counts could help avoid late incidences of liver disease.
As featured in NZ Dairy Farmer