Ergot-affected and mouldy sorghum
Marketing and use of mould-affected grain often presents a dilemma for growers and livestock producers, and usually comes down to how much the grain should be discounted.
The key questions that need to be answered are whether the nutritional content has been affected and whether fungal toxins (mycotoxins) are present in high enough concentrations to adversely affect livestock.
Two groups of toxins
We need to be aware of two main groups of toxins in sorghum, which are controlled under Queensland stock food regulations. The first group is ergot alkaloids, produced by sorghum ergot growing prior to harvest, and the second is aflatoxins, which are mainly produced if sorghum is stored with high moisture content. Other types of mould associated with ´weathering´ generally produce only slight affects on the feeding value of the grain.
Ergot in sorghum is caused by a fungus, Claviceps africana. Infection occurs during flowering when spores of the fungus land on the feathery stigmas of flowers in sorghum heads. If the flower has been fertilised by pollen it resists infection and normal seed will develop. If the flower has not been fertilised, the spore germinates and grows into the unfertilised seed (ovary) and the ovary is rapidly replaced by a fungal mass. About seven days after infection, sticky honeydew oozes out of the flowers and drips onto leaves and the ground. When the weather is wet or humid, the honeydew turns white due to the production of the infective spores just above the surface of the honeydew.
Ultimately (near grain maturation), the fungal mass develops into a hard fungal body - the sclerote. Occasionally the developing sclerote can be overgrown by the black fruiting body of another fungus called cerebella.
The fungus can survive year-round in honeydew on other sorghum hosts, such as Johnson grass (halepense) and columbus grass (almum). The ergot which affects sorghum does not infect paspalum or winter cereals such as wheat and barley. Although ergot spores can survive in honeydew on sorghum seed, the fungicide thiram that is used routinely on sorghum planting seed will kill the spores. Sorghum ergot does not survive from season to season in sorghum stubble or as sclerotes in the soil.
Variable levels of toxin
Sclerotes of C. africana contain toxic chemicals, in particular the ergot alkaloid dihydro-ergosine. Feeding trials have established that sorghum contaminated with sclerotes can affect milk production in cows and sows, and weight gain in cattle. There is a large variation in the levels of alkaloids (and toxicity) between ergot-contaminated grain samples, that is caused by differences in the maturity of the sclerotes and perhaps other factors such as weather and variety.
Weathering refers to any deterioration in the appearance of grain caused by climatic influences on a crop. This results in:
- shrunken and pinched grain, following drought or from soil fertility constraints
- discoloured grain, often covered with black, gray, pink or orange fungal growth (mould), after extensive warm, humid periods during grain maturation
- premature germination of the grain caused by wet or humid weather conditions at harvest, resulting in sprung, shot or sprouted grain.
A range of fungi is associated with weathered grain, mostly requiring moisture contents of >20% to grow, so they are rarely a problem after harvest. The most important of these are Alternaria alternata and species.
These moulds can produce mycotoxins, but even in very badly affected grain these rarely reach concentrations that can adversely affect livestock. Pigs and poultry are far more likely to be affected than cattle and other ruminants. There have been instances of false oestrus in young pigs from zearalenone from Fusarium species, and of slightly impaired feed conversion in pigs, and altered feathering in chickens from very badly weathered sorghum in CQ, possibly from Alternaria mycotoxins. Dilution of damaged grain and not feeding it to young livestock will minimise this risk.
Mould growth in storage, particularly of Aspergillus species, will occur when grain either is stored having a high moisture content or suffers water damage during storage. Lightweight material and weed seeds increase the risk since they block aeration channels in the grain. If grain is stored with high moisture contents (14 - 20%), resultant heating promotes the growth of Aspergillus fungi. These produce aflatoxins which can cause severe liver damage and reduced growth rates in pigs and other livestock. There have been several instances of this occurring in Queensland grain-growing areas.
Nutritional changes can result from weathering of grain and this varies depending on the cause. Grain that is shrunken and pinched due to drought or impaired soil fertility will have reduced starch content and increased fibre. Protein and lysine are usually higher, but less available for pigs and poultry. Such grain will be lightweight and have reduced value as feed. Grain that has is in the early stages of sprouting (shot or sprung) can have a slightly increased digestibility for pigs.
Mould growth in sorghum can also reduce its value for pigs and poultry feed mixes (there is little effect on ruminants). In severe instances, reduction of the nutritional value of grain occurs by the removal of fat and starch and the hydrolysis of protein. The amount of fibre in mouldy grain will increase relative to the decline in starch, protein components and lipids and, in some instances, lead to reductions in digestible and metabolizable energy. However, in the most instances, the nutritional changes in weathered grain due to mould growth are slight. Badly damaged grain often exhibits ´off´ aromas and flavours which pigs may find unpalatable, so some caution is warranted when feeding young pigs where maximum feed intakes are sought.
Stockfeed intended for feedlot cattle has been further limited to 0.1% sclerotes by weight since 2004.
Deliveries of sorghum with sclerote levels higher than 0.3% will be rejected by grain merchants, and higher than 0.1% will be rejected by cattle feedlotters. Most commonly, a sorghum sample containing 0.3% sclerote will contain about 1 mg alkaloid/kg (1 ppm), but because the alkaloid concentration can vary, it will be advisable to minimise ergot wherever possible.
Although there is a 0.3% sclerote contamination limit for sorghum intended for livestock, some end-users will not accept ergot-contaminated grain at all. Grower pigs, chickens and laying hens are most tolerant of the alkaloids in sclerotes, and so are a potential market for sorghum that contains 0.3% sclerotes. Sorghum with levels higher than the animal feed limit can be mixed with clean grain to reduce the sclerote levels. Fortunately, the incidence of ergot contamination of bulk grain has been extremely low over the past few years.
Weather damaged or mouldy grains
There is a legislated limit of 0.2 mg/kg for aflatoxin B1 in grain sold for stockfood in Queensland. Testing over many years has indicated that it is extremely unlikely that sorghum will contain this limit without a history of high-moisture storage. Aflatoxins are not a significant risk in weathered sorghum. No other mycotoxins are legislated against in sorghum in Australia.