Wheat grain size is determined by the plants genetics i.e. variety, and the length of the grain filling period. As soon as pollination has occurred the embryo and endosperm begin to develop with the plant redirecting photosynthates and also previously stored starch and protein (in leaves and stems) to these developing grains. The longer this period of grain fill is, the larger wheat grain size is likely to be.
Nitrogen management will have the most impact on final wheat grain size and weight. Applications made early will ensure a canopy that is large, containing high levels of stem carbohydrate that is translocated to the developing grain during maturation.
This is particularly important in drought areas where 60% of the grain yield could come from this store. It is important to monitor plant nitrogen levels ensuring the canopy does not senesce early and curtail the grain filling period.
Phosphate has a major role in the supply of energy for plant processes. Redistribution of stored carbohydrate requires energy making phosphate nutrition important in achieving good wheat grain size.
Both solid fertiliser phosphate and foliar phosphate can be used to improve final grain size.
Potassium aids movement of nutrients around the plant as it regulates the transpiration flows as the wheat plant redistributes minerals to the ear potassium levels need to be maintained.
Potassium will also help avoid early senescence often brought on by drought during grain fill. Potassium deficient plants are poor at regulating transpiration through the leaf leading to heat stress and wilting. Adequate potassium supplies will extend the grain filling period leading to improvements in wheat grain weight.
Both manganese and zinc with their roles in nitrogen metabolism by the plant will improve wheat grain weight.
Besides nutrient management, the grain size can be influenced by water management (irrigation to avoid drought stress), as well as disease management – use fungicides and nutrients to maintain green leaf area and reduce disease incidence improving plant health.