Producersgrowing maize for the first time to take advantage of the biogas opportunitymust be very careful over the next few weeks of harvest if they are to maximisethe value of their crop, warns maize specialists Grainseed Ltd.
Withmany crops being paid for on a dry matter basis, harvesting at the wrong timeor when cobs are immature could have a major impact on profitability, says thecompanys maize specialist Wilson Hendry. Growerswho are by and large used to cereal crops, could be caught out by notrecognising the signs of when maize is ready to harvest.
Thisis a particular problem now that the recent hot weather has accelerateddevelopment of crops making predicted harvest date very much a moveable feast.
Methaneproduction is driven by dry matter so many buyers are using a pricing modelbased on around a 1 for each per cent of dry matter, he says. Ifthe maize is harvested at 30% dry matter, the grower would get 30/tonnewhereas if it were only 25% dry matter it would realise only 25/tonne. Butunlike cereals, the final dry matter is made up of an average of both stem andcobs so judging the precise time for harvest is far from easy.
Thecob can contribute 50 – 60% to total dry matter yield so assessing plant andcobs carefully before harvest is key, Wilson Hendry explains. Cobscan be ripe when stems are still green which can throw newcomers, but you needto be aiming for a time when the cob is firm and only a small amount ofmoisture can be squeezed out of the grain whilst the plant itself can stillhave some green leaf present.
Ifgrains are still clear or milky, dry matter is likely to be less than 20% andharvest probably 3-4 weeks away. If starch is gritty, dry matter will be around25 28% and harvest probably around a week away. Mostcrops will increase in dry matter by about 2 3% per week in the 5 weeksleading up to harvest but this is affected by the weather and also the varietygrown and its cob ripeness score, he says.
Growersshould be aiming for varieties with a good cob ripeness score so that the plantwill mature properly even when heat units are lower than is ideal.
Theworst thing you can do when growing maize for biogas is select a potentiallyhigher yielding but later maturing variety that never reaches the correctripeness and you are forced into harvesting at 25% dry matter or less.
Ifthere is any doubt about dry matter, growers should carry out the MGA microwaveor AGA test where crops are dried out and a calculation is done between thefresh weight going in and the resulting dry weight. Wherelarge volumes of maize are being grown to feed a digester, a range of maturitydates should be chosen to enable a suitable spread of harvest, he says.
Formaximum methane production, particle size is important and ideally is 6 – 10mm.Chop length and maceration method should reflect this. Oxygenbarrier sheets and secure covers should be used to prevent clamp losses and iflong periods of storage are anticipated, a additive should be considered tokeep energy levels high.
Inthe new NIAB list of maize varieties for anerobic digestion in favourablegrowing conditions, only the two varieties Marco and Dualto topped 60t/hafreshweight yield and furthermore they do so with dry matter contents at thecritical 30% harvest target.