At this time of the year maize is being planted for silage, primarily for supplementing the dairy cow’s diet in late summer-autumn, winter and early spring.
Harvesting is still far away but this column will deal with the unsuspected occurrence of dangerous mycotoxins when hot, humid and rainy conditions happen during harvest.
Management during and after silage making is critical with whole plant maize silage as aerobic spoilage can easily happen. Levelling off, packing the edges and sealing the top of silage pits or bunkers with plastic and tyres will reduce air penetration into the harvested crop.
It is not uncommon to have pit and stack life problems with harvested forages, both maize, pasture and other crops that have been rained on before being chopped and ensiled.
This creates an environment where bacteria and fungi (mould) start to grow on the harvested plants, especially the cobs with maize.
A bit of minor mould will generally create no major problems at all with pasture silage. You get rid of this by cutting it off and discarding it. However, with maize silage there can still be a problem. And this I found out in Northland on Evan Smeath’s farm near Hikurangi a few years ago.
On his farm he was getting high percentages of empties in the heifers and cows. A number of factors were responsible.
A factor in the low conception rates was mineral deficiencies when you feed a combination of pasture and maize silage. Another was bovine viral diarrhoea (BDV), but this was still not the whole story.
Pastures in New Zealand contain a number of Fusarium species (fungi) in their microflora capable of producing the oestrogenic mycotoxin, zearalenone.
Fungal growth and zearalenone production is highest in autumn before and during the sheep mating season. During this period it is well-known that zearalenone levels in pasture can be sufficient to adversely affect ewe fertility.
Dairy cows also ingest pastures contaminated with zearalenone but they are usually mated in spring when zearalenone levels are lower. With split calving and winter milking the risks may also be present in pasture. However, no evidence of high levels of this mycotoxin in pasture was found on Smeath's farm.
But an unexpected finding was that mycotoxin levels in his maize silage were far too high and were causing serious problems.
Analyses by Ruakura found high zearalenone levels and also the presence of a second mycotoxin (nivalenol), both at potentially toxic levels.
The quantity of maize silage in the diet exceeded 50% over winter-early spring in his winter milking operation. This meant that intake of mycotoxins was at toxic levels. Blood and urine samples confirmed toxic levels.
Surprisingly the stacks did not show signs of visual mould spoilage but we still found high levels of mycotoxins produced by fungi.
The effects of mycotoxins on the dairy cow are still to some extent a "gray area". It is even more difficult as a large number of mycotoxins have now been documented in the scientific literature.
However, with split calving and winter milking, where a large part of the diet is often maize silage, the level of mycotoxins in dairy rations should be a concern.
Decreased feed intake, production losses of 5-10% and reduced reproductive performance are the most typical symptoms of a mould and mycotoxin problem.
There is also a general consensus that these mycotoxins will cause significant suppression of the immune system. It would appear that mature cows might be more resistant to zearalenone toxicosis than heifers.
What is the cause of a build-up of these high mycotoxin levels?
It relates to the humid hot conditions in Northland at the time of harvesting. Rain will create even better conditions for the growth of fungi on the cob. Such conditions are of course not infrequent in Northland. Hence you have to be aware of the dangers in Northland, less so with spring calving. But of course humid and hot conditions can occur occasionally in other areas too.
So what can you do if you have to harvest in warm humid and/or rainy conditions?
Admittedly sometimes what is thought to be a toxin can be something else (and mineral intake will interact with the mycotoxins in influencing dairy cow performance as happened on the Smeath farm).
But all moulds should be avoided in maize and pasture silage. Avoid feeding mouldy hay or silage to dairy cows and heifers, and preferably to all animals.
Maize silage, without a mould inhibitor, or any silage if left uncovered at the feeding face, can grow mould and cause foetus losses. Bad parts should be fed to empty animals or discarded. However, the hidden danger can be that mycotoxins are present without clear visual signs.
What is a dairy farmer to do if he suspects that he is feeding mycotoxin contaminated maize silage?
The first step would be to try to remove the offending feedstuff. However, often, either because the silage is contaminated or because supplies have been purchased in large quantities, complete removal of the feed is impossible.
If there are alternative feeds available, the amount of the suspect feed should be reduced. On Smeath's farm pasture silage and haylage did not contain significant levels of mycotoxins. Hence the amount of haylage and pasture silage in the diet was increased.
Even feed not obviously contaminated by moulds may still contain some mycotoxin. In Smeath's maize silage we did not notice any sign of mould apart from tiny patches. To suppress the effect of the mycotoxins a mycotoxin binder was added to the ration.
Whenever mycotoxin is suspected or as a safeguard this can and should be done.
A number of toxin binders are on the market. Toxin binders vary in their ability to bind toxins, and some can bind one type of toxin and not bind other types of toxin. In general, bentonite products have a disadvantage in that although they may bind the target mycotoxin they will also bind other minerals and vitamins.
Levels of vitamin A and E should be increased both because toxin binders may bind some of these vitamins, but also because they will help to protect the immune system.
Identifying the factors involved has brought a dramatic improvement in the percentages of empties in heifers and cows. Tackling only the mycotoxin issue definitely did not bring this about. It should not be forgotten that there are many interacting factors influencing dairy cow performance and reproduction.
However, the presence of mycotoxins in maize silage was a significant and unexpected factor in influencing reproductive performance.
