Harvest date critical for maximising value of biogas maize

Producers growing maize for the first time to take advantage of the biogas opportunity must be very careful over the next few weeks of harvest if they are to maximise the value of their crop, warn maize specialists Grainseed Ltd.

With many crops being paid for on a dry matter basis, harvesting at the wrong time or when cobs are immature could have a major impact on profitability, says the company’s maize specialist Wilson Hendry.

“Growers who are by and large used to cereal crops, could be caught out by not recognising the signs of when maize is ready to harvest.

“This is a particular problem now that the recent hot weather has accelerated development of crops making predicted harvest date very much a moveable feast.”

Methane production is driven by dry matter so many buyers are using a pricing model based on around a £1 for each percent of dry matter, he says.

If the maize is harvested at 30% dry matter, the grower would get £30/tonne whereas if it were only 25% dry matter it would realise only £25/tonne.

But unlike cereals, the final dry matter is made up of an average of both stem and cobs so judging the precise time for harvest is far from easy.

“The cob can contribute 50 - 60% to total dry matter yield so assessing plant and cobs carefully before harvest is key,” Wilson Hendry explains.

“Cobs can be ripe when stems are still green which can throw newcomers, but you need to be aiming for a time when the cob is firm and only a small amount of moisture can be squeezed out of the grain whilst the plant itself can still have some green leaf present.”

If grains are still clear or milky, dry matter is likely to be less than 20% and harvest probably 3-4 weeks away. If starch is gritty, dry matter will be around 25 – 28% and harvest probably around a week away.

Most crops will increase in dry matter by about 2 – 3% per week in the 5 weeks leading up to harvest but this is affected by the weather and also the variety grown and its cob ripeness score, he says.

“Growers should be aiming for varieties with a good cob ripeness score so that the plant will mature properly even when heat units are lower than is ideal.

“The worst thing you can do when growing maize for biogas is select a potentially higher yielding but later maturing variety that never reaches the correct ripeness and you are forced into harvesting at 25% dry matter or less.”

If there is any doubt about dry matter, growers should carry out the MGA microwave or AGA test where crops are dried out and a calculation is done between the fresh weight going in and the resulting dry weight.

Where large volumes of maize are being grown to feed a digester, a range of maturity dates should be chosen to enable a suitable spread of harvest, he says.

“For maximum methane production, particle size is important and ideally is 6 - 10mm. Chop length and maceration method should reflect this.

“Oxygen barrier sheets and secure covers should be used to prevent clamp losses and if long periods of storage are anticipated, a additive should be considered to keep energy levels high.”

In the new NIAB list of maize varieties for anerobic digestion in favourable growing conditions, only the two varieties Marco and Dualto topped 60t/ha freshweight yield and furthermore they do so with dry matter contents at the critical 30% harvest target.