14 September 2001

Variety and classification key to harvest decision

Maize crops are fast

approaching harvest, but all

the good work growing the

crop can easily be wasted

when its cut too early or too

late. Jessica Buss reports

DECIDING when to harvest maize must be based on the variety grown, with the optimum time differing depending on type classification.

According to Sarah Harrison of CEDAR, part of the University of Readings Dept of Agriculture, research shows different ideal harvest dates when varieties are classed as one of four types. These types are conventional first early, conventional second early, staygreen second early and compact staygreen second early.

In a Dalgety-sponsored trial conducted at CEDAR, these crops were harvested on four dates to assess the optimum timing in terms of whole-plant dry matter (DM) yield, DM content and the DM yield of the ear component.

The first harvest on July 28 confirmed the early yield advantage of the first early varieties. But at the second harvest date of Aug 31, this yield advantage was marginal compared with the conventional second early varieties.

Four weeks later, at a more typical harvest date, the second early variety had a higher yield. First early varieties continued to rise in yield until mid-October with ear DM yield following a similar trend to crop DM yield.

"This shows that first early varieties have a yield advantage early in the season, but harvesting too early will compromise yield. When you plan to delay harvesting first early crops until later in the season, keep a close eye on DM content to make sure the crop does not become too dry," says Dr Harrison.

She adds that when deciding on optimum harvest date, always consider DM content as well as yield to help ensure good fermentation and minimise aerobic spoilage. On Sept 27 first early types had a whole plant DM of 32.6%, but by mid-October it was 40.5%, which is too dry and would risk spoilage in the clamp.

In this study, the staygreen trait did not alter either whole-plant DM yield or DM content until the final harvest date. Then, the retarded stover development reduced DM content compared with conventional varieties. This was also evident for compact staygreen varieties.

At the most likely on-farm harvest dates in late September and mid-October, these varieties had reached approximately 30% and 35%, respectively, similar to the second early varieties.

"The dry matter data indicates a wider optimum harvest window for the staygreen and compact staygreen varieties.

"This could be beneficial in reducing the risk of enforced harvesting of a very high DM crop and associated risks of aerobic spoilage and clamp DM loss," says Dr Harrison.

CEDARs David Beever says many maize growers harvest crops too early to achieve their potential, particularly in marginal areas. "Last year there were only a few dry days suitable for harvesting. That makes it even more tempting to harvest crops early."

Deciding on that optimum time requires careful crop assessment. "The appearance of the crop from the headland is not a guide to cob maturity."

Ideally the crop should be 32-35% DM, but cob maturity must drive the decision to harvest. &#42

Table 1: Whole-plant DM

yield (t/ha)

Harvest date

July Aug Sept Oct

28 31 27 13

First early 10.4 18.9 20.4 21.3

Second early 9.4 18.4 21.6 21.0

Staygreen 9.8 18.9 21.4 20.6

Compact sg 9.3 17.6 20.4 21.5

Table 2: Ear DM yield (t/ha)

Harvest date

Aug Sept Oct

31 27 13

First early 10.2 14.0 14.1

Second early 9.0 13.9 13.2

Staygreen 7.2 11.9 12.0

Compact sg 8.3 12.6 13.6