23 February 1996


As hot and dry weather caused many maize crops to wilt last July and August, comparative heat unit totals for the maize growing season from Wye College, London University, make useful reading. Wyes

David Scarisbrick and Alex ODell report

VARIOUS systems of accumulating temperature in the form of "day degrees" or "heat units" have been devised to map production sites, plan drilling dates and fertiliser application for various crops.

For maize, the "Ontario Heat Unit" system of heat accumulation is used.

Values greater than 2500 OHUs, which are required for forage maize maturity, were exceeded in all seasons during 1989-95 at Wye. Indeed 1989, and not 1995, was the warmest (see Fig 1).

It is important to discuss these totals in relation to heat units accumulated during the critical establishment period. Forage maize is sown when soil temperatures reach nine to 10C in late April to early May, but the number of days from sowing to emergence can be increased by late May frosts.

For example, it took 28 days for the rows of maize to become clearly visible during 1991 when only 552 OHUs accumulated in the two months after drilling (Table 2). However, an acceptable plot yield of 11.8t/DM/ha was still achieved during this particular season because 511 OHUs accumulated during the final stages of development and maturity during an unusually warm September (Table 1).

In 1995, May and June were actually cooler than the previous six-year monthly means, and crop emergence was not complete until 18 days after drilling. The subsequent outstanding "sub-tropical" temperatures recorded in July and August would have resulted in above average yields of dry matter if rainfall had been closer to average throughout the growing season. Fig 2 (p29) shows that rainfall was below the longer term mean during May to August. As temperatures climbed above 30C on two days at the end of July and four days in early August, maize leaves curled in order to reduce water loss through transpiration. At harvest in mid-September maize plants were stunted and grain "set" impaired, hence plot yields ranged from 7.5 to 10.5 t/DM/ha.

Ontario heat units are not entirely foolproof as a maize management tool, because establishment and crop development depend on a wider range of environmental and rotational factors. For example, at marginal maize sites where 2300-2500 OHUs are achieved in most years, problems associated with weed control are increased when cold and wet conditions persist during May and June. For these locations the longer term pattern of heat accumulation in the two months following drilling is more important than overall OHU totals. The variable 1995 yields of forage maize also emphasise that it is important to also take into account monthly rainfall data when predicting growing. &#42

Table 2: Forage maize:Days from drilling to emergence

Number of OHUsNumber of daysPlot yields

accumulated infrom drillingt/DM/ha*

May and Juneto emergence







199574818 7.5-10.5

*Experimental studies on forage maize at the Wye College Farm

Table 1: Monthly accumulated Ontario Heat Units (OC) for Wye College (1989-1995)

Year 89 90 91 92 93 94 95 Ave







Maize harvest last autumn taught producers the dangers of over-maturity.