Ear formation, grain development and crop ripening in wheat

Capacity for grain filling is set by grain number per unit area and the storage capacity of each grain. Satisfying the capacity depends on photosynthesis, as well as the redistribution of stem reserves.

Ear development

Benchmark: 18 days from flag leaf to ear emergence

Varietal influence: High

• Ears are initiated during the foundation phase
• Spikelet initiation is completed as stem extension starts (GS31)
• Floret initiation and development proceed until flowering

The number of potentially fertile florets depends on assimilate supplies to the ear, particularly during booting, and is affected by shoot numbers. At shoot numbers of over 400/m², mutual shading results in fewer grains in each ear.

Grain number

Benchmark: 48 grains/ear – 22,000 grains/m²

Varietal influence: High (grains/ear)

Other influences: Shoot numbers, pests

The benchmark for grain number per ear is 48. With 460 surviving fertile shoots/m², this gives 22,000 grains/m².

The number of grains on each ear depends on the number of fertile spikelets on the rachis (the central ‘stem’ of the ear). Compared to barley, wheat is relatively flexible, with spikelets containing up to nine fertile florets.

Cool, bright conditions – especially, one or two weeks before flowering – can prolong or enhance the ear formation period and increase grain number per ear. However, inclement weather at flowering, such as heavy rain, heat or drought, can impair pollination and reduce the number of fertilised florets.

Varieties with smaller culm leaves tend to have more shoots/m² and fewer grains/ear. Breadmaking varieties tend to have fewer grains/ear.

Some insects, such as orange blossom midge, can reduce grain numbers and feed on developing grains.

Ear weight at flowering

Benchmark: 420 mg/ear (dry weight)

Varietal influence: High

Other influences: Photosynthesis during the construction phase

Ear weight increases rapidly during booting. By flowering, the benchmark dry ear weight is 420 mg/ear, comprising glumes, rachis and florets – weights remain similar throughout grain filling and become the chaff at harvest. As a result, ear weight at flowering multiplied by fertile ear number per unit area can indicate yield potential.

Photosynthesis in the ear contributes significantly to grain growth. The benchmark final ear weight at harvest is 2.8 g, 15% of which is chaff.

In wheat, yield variation due to region, soil type and early crop management is more strongly related to grain number rather than the weight of each grain.

Benchmark: 43 mg dry matter per grain in 45 days from flowering to 26 July

Varietal influence: Medium

Other influences: Water availability, disease

Grain filling starts when flowering is complete and continues until grain reaches about 45% moisture.

Grains accumulate more water than dry matter for about four weeks after flowering, when water content is at its maximum. Water enables cells first to divide, then expand. Severe drought or disease can significantly reduce grain size.

As water uptake stops, dry matter accumulation accelerates. Rapid dry weight growth continues with starch and protein deposition in expanded grain cells – these are supplied by both photosynthesis and redistribution of reserves.

Suboptimal photosynthesis during the first two or three weeks of grain growth reduces the cell number and potential weight of each grain.

The benchmark period from flowering until maximum/final dry weight (the grain filling period) is 45 days. However, this varies considerably – it can be 28 days in severe drought conditions.

The benchmark weight/grain is 43 mg dry matter, which equates to a ‘thousand grain weight’ of 50 g at 15% moisture.

Varietal differences in average grain weight are shown in the AHDB Recommended List.

Canopy survival during grain filling

Benchmark: GAI <2 after maximum grain weight achieved

Varietal influence: Medium

Other influences: N supply, foliar disease

Canopies start to senesce rapidly from GS71, about nine days after flowering. Most remaining greenness is lost just after grain weight reaches its maximum.

High nitrogen uptake, fungicide use or cool, moist weather all tend to delay senescence. Crops at northerly latitudes generally have high grain weight, because cooler temperatures prolong grain filling. Hot weather reduces grain weight by shortening the period of grain growth, even in bright and moist conditions. Grain weight is also reduced by leaf or root disease, pest infestation or early lodging.

Note: No further grain filling occurs once grain moisture is below 45%, at around GS87, so yield cannot be affected by any treatment applied at or after this stage.

Ripening

Benchmark: 45 to 20% moisture content in two weeks

Varietal influence: Low

Other influences: Lodging

After grain filling, moisture content provides the best indication of ripening until grains are dry enough to harvest. On average, grain takes about two weeks to dry, from 45 to 20% moisture. Frequent rain re-wets grain and slows moisture loss, especially at low moisture contents. Lodged crops dry slowly.

Glyphosate products may be used pre-harvest on a range of crops to control weeds, aid harvesting and help protect grain quality (particularly in wet harvests) and food safety.

Grain filling period*: The number of days between GS65 and when the gain reaches 45% moisture content.

Grain moisture: Thresh grain from around 20 representative ears, weigh the sample, dry the sample (at 80–100°C) until no further weight loss and weigh again.

Moisture content: 100 - (grain dry weight ÷ sampled grain weight) x 100.

Grain ripening period: The number of days from 45 to 20% moisture content.

*If a grain sample is taken at the end of the grain filling period, the dried and weighed grain can then be counted and divided by the number of grains to determine the dry matter per grain.