Management of canopy expansion and senescence in winter wheat

Canopy size determines the proportion of sunlight intercepted, which drives the accumulation of dry matter. Learn how to manage canopy growth and lifespan to help maximise winter wheat yields.

The crop’s green area index (GAI) is the ratio of green leaf and stem area to the area of ground on which the crop is growing. Use measurements of GAI during the growing season, alongside targets, to guide canopy management.

The crop canopy comprises all green surfaces – mainly leaf blades. However, husbandry has little effect on leaf number or size, so canopy management focuses on shoot number.

As shoot number and GAI increase, the extra light intercepted decreases. For example, an increase from GAI 2 to GAI 3 captures 15% more light, whereas an increase from GAI 6 to GAI 7 only captures 2% more light.

Upper leaves become more important for light interception as GAI increases.

Benchmark: GAI = 2.0 by GS31

Varietal influence: Low

Other influences: Shoot count, N supply

From emergence to early April, ground cover increases. Cover increases as leaves and tillers emerge during early autumn and winter but GAI rarely exceeds 1 before March. During this phase, photosynthesis and growth are slow, because ground cover is incomplete, and because light levels and temperatures are relatively low.

Benchmark: 1 GAI unit increase in every nine days, GAI at GS61 = 6.3

Varietal influence: Medium

Other influences: N supply

Canopy expansion accelerates in late April as temperatures rise and the largest leaves emerge. As stems and leaf sheaths extend, they contribute to GAI.

N availability controls canopy expansion quite closely because crop N for each unit of green area remains constant at 36 kg/ha – the canopy N requirement.

Light interception is sufficient for rapid growth when GAI ≥3.

The benchmark rate of canopy expansion is 0.1 GAI/day, hence crops expand by 3 GAI units during May.

At flag leaf emergence, leaf blades comprise about 85% of total GAI.

The benchmark date for maximum canopy size, which occurs between flag leaf emergence and ear emergence, is 26 May.  

Maximum canopy size occurs earlier in N-starved crops, as lower leaves begin to die.

The benchmark canopy size at flowering is GAI of 6.3 (fertile shoots 6.0, infertile shoots 0.3).

The benchmark maximum green area for each fertile shoot is 130cm².

Small canopies (GAI <4), which can result from inadequate shoots or N deficiency, waste sunlight.

Large canopies (GAI >7), which can result from high seed rates and high N supplies, cost more than is necessary to intercept all available sunlight. They are also at a higher risk from foliar disease and lodging.

The benchmark maximum GAI is 6.9.

Optimum canopy size for varieties with erect leaves or low canopy N requirement only differ slightly from varieties with lax leaves. Differences in leaf greenness have little effect on photosynthesis.

Full light capture is hastened by early sowing, warm winters and springs, and adequate moisture and N.

Third and fourth leaves from the ear significantly increase GAI, but contribute little to grain filling.

Benchmark: GAI falls to <2 by GS87

Varietal influence: Medium

Other influences: N supply, disease

The canopy senesces from June onwards. Lowest leaves die first, unless disease intervenes. Leaf sheaths usually die last. GAI drops below 2 at the end of July, causing the end of rapid crop and grain growth. The canopy continues to respire, losing greenness and weight.

Pre-sowing, the following factors tend to lead to larger canopies:

• Early sowing
• High seed rate
• Plentiful soil N
• Adequate P and K
• Optimum pH

During the growing season, the canopy can be managed by:

• The amount and timing of fertiliser N applied
• Disease control measures

Often, canopy growth needs to be kept in check to avoid exceeding target GAI.