Regular readers of the newsletter (assuming there are some!) may remember our issue a few months ago, reporting on a bizarre incidence of two spring calving herds with a high proportion of their calves being born with dwarfism. Fortunately, no one else came forward after that article came out with further reports of cases. However, there have been some further developments that came to light after more research that may be of interest to those interested.

No one knows the cause of this strange condition, but the two primary theories boil down to either a) a secondary Manganese deficiency in mid-pregnancy or b) the influence of a yet unknown fungal mycotoxin which may be more prevalent in clamp silage made during particularly wet summers.

On the subject of the manganese deficiency, it is believed that it must be a secondary deficiency due to interference with gut absorption from some other element or mineral such as iron, calcium or phosphorous – though none of these have been convincingly proven. It is rare for soil and grasses to be genuinely low in manganese (which would lead to a primary deficiency), especially as the body has relatively low requirements normally. The official recommendations for manganese supplementation to try and prevent the recurrence of dwarfism is to supplement at around three times the body’s requirement (1g per head per day) in a bid to try and overcome whatever antagonism may be occurring in the gut.

Although there are some boluses and lick buckets which provide a small amount of manganese (closer to the normal requirements), the only way to achieve this is with a custom dry cow mineral made to this specification, fed from month 3-6 of pregnancy. Basically, immediately from housing onwards. Given that you’re going to be feeding this mineral for at least 3-4 months, make sure you are not over-supplementing copper here, especially if boluses are used as well. Fortunately, manganese is a relatively cheap addition and can be fed quite safely at even this high dose rate.

 Should I be blood testing cows for manganese levels during mid pregnancy? An excellent question, one that it turns out is not simply answered! It turns out that blood levels of manganese can often be found to be low in sampled cows, even though their liver stores in liver, bone and elsewhere may be sufficient. This makes the blood test a little less reliable and raises the question of whether this is why manganese has been implicated as a possible cause. If you sample a general cross section of cattle, you’ll often find a decent number of low blood manganese results, even where no clinical signs or dwarfism history has been seen on farm.

We recommended, last time, that feeding a minimum of 25% of the DMI as straw would also help to reduce the risk. As it turns out, the idea behind this is to try and dilute the amount of potential fungal mycotoxin that might be in the silage. Farms feeding out 80-100% of the DMI as silage, if affected with the mycotoxins, would therefore be more likely to have cases as they would be exposed to more. Make sure that the straw and silage are always available in this ratio every day, rather than, for example, feeding out two days’ worth of silage and straw with the silage layered on top of the straw. Another way of reducing this risk would be to feed hay or substitute a portion of the silage in the ration for hay. It would appear that silage made in particularly wet summers or containing high red clover content are more prone to these issues. Mycotoxin binders may potentially be of some non-specific use though there is no real evidence out there of their efficacy in this area.

The most reassuring thing we have learned is that, for the most part, farms that experience these big outbreaks one year may never see it again, even having made no major management changes. A real head scratcher!