“Maize should be harvested when the crop combines maximum starch content with only limited leaf die back so maintaining high digestibility in the vegetative part of the plant,” he comments.
“At the same time the crop must have sufficient moisture to allow effective consolidation in the clamp. Harvesting a crop too soon will result in sub-optimal starch content, as sugars will not have been converted into starch.”
Mr Copestake stresses the factor determining when to harvest is not the date or contractor availability, but the crop itself. He says the move to earlier varieties is generally bringing harvesting dates forward, but that increasingly the season is having a marked impact.
There is considerable variation in crop development regionally this year, reflecting drilling conditions and this will affect when crops will mature. Mr Copestake recommends starting to walk the crop from late August to assess maturity and fitness to harvest, to prevent crops going over. He advises walking well into the crop and looking at plants in several locations. Never evaluate plants on the field margins. 
He says it is a misunderstanding that maize needs to be dead before harvesting. The target range for an optimum crop is 32-35% dry matter. At dry matter levels higher than this, palatability and intakes can be reduced, digestibility will be compromised, and the crop may prove difficult to consolidate, increasing the risk of aerobic spoilage.
“Crops typically dry down at 2% per week but last year it was considerably higher, perhaps as high as 4%. So, it is important to start measuring dry matter and assessing maturity, sooner rather than later.
“In more mature crops, lignin will increase, reducing digestibility and intakes still further.
To assess the maturity of the vegetative material he advises looking at the flow of juice from the stem. “Look to harvest when no juice emerges as the stem is twisted, and when the leaves level with the cob are just beginning to turn brown.
Then assess the grains using the thumb nail test. The grains at the top of the cob should be like soft cheese, the ones at the bottom should be like hard cheese and the ones in the middle should be soft enough to leave the imprint of a thumbnail on.
“By walking the crop and assessing the state of maturity and the rate at which dry matter is increasing, you increase the prospects of harvesting the crop at the optimum stage to maximise the production of high quality forage,” Mr Copestake points out.