The objective of crop production is to convert Photosynthetically Active Radiation (PAR) into dry matter that can then be utilised in the form of nutritious food, animal feed or feedstock for energy production.
There is a classic linear relationship between light interception and total dry matter produced with wheat. It is also known that in wheat varieties currently grown, there is an Harvest Index of 50-55% i.e. 50-55% of total dry matter will be converted to grain yield.
These simple relationships form the basis from which we develop our agronomic principles in growing wheat. In order to intercept the radiation we need to develop a sufficiently large canopy (not too big) of healthy green leaves to capture 90% of the incoming PAR. This canopy must remain green and healthy during the grain filling period. There are therefore two distinct objectives:
The construction of the canopy starts by drilling the seed at the correct rate to achieve the target spring plant population. The seed rate will be determined by the germination percentage, seedbed conditions, date and expected field losses. Having determined this then the soil pH and nutrient status needs to be checked and remedial action taken where needed. When soils are not at the optimal pH of 6 – 6.5 then interactions in the soil render some nutrients less available. Any nutrients showing deficiency need to be replaced using fertilisers and foliar micronutrients in the crop management program.
As the plant develops it will be relying on soil nutrients after two/three leaves, especially phosphate and nitrogen. Of the micronutrients, manganese and zinc are important and often deficient. The first 50 – 60 days of active growth is when the yield potential is established, with all the main yield components developing i.e. leaf numbers, ear numbers and grain sites. Early establishment is affected by temperature and moisture.
As the sowing of winter wheat is delayed, the length of time to produce a leaf (a phyllochron) increases, thus slowing the canopy development. Nutrients, especially nitrogen and phosphate can be used at this stage to speed up the growth rate, and increase the size of leaves.
Following the initial establishment phase, the individual plants move into tiller production and further leaf production. These all contribute to the Green Area Index (Green area : soil area) that is the light capturing canopy being constructed. It is during this phase that they experience the ‘vernalisation’ that is required in combination with the day length (photoperiod) to move the crop development into the next stage of construction known as the Grand Growth phase.
The individual plant stems begin to extend eventually giving a canopy which has six – seven times as much leaf area as soil area (GAI 6-7). During this important period nutrient uptake is at its peak and thus this demand by the plant must be met through applied fertiliser and micronutrients.
The Nutrient Management Plan will need to include multiple applications of fertiliser and foliar micronutrients to give the greatest Nutrient Use Efficiency per unit of production. When the last leaf, or Flag Leaf, is fully unfolded canopy construction is complete and then all agronomic consideration needs to be focused on ensuring this canopy survives to be the ‘source’ of assimilate to fill the grains (or sinks as sometimes referred to) delivering the final yield and quality. At this stage the number of ear bearing shoots should be between 400 and 600 / m2.
Final yield is influenced by many factors which determine the number of grains / m2 including weather, disease and essential plant nutrients. Two of the latter, namely Copper and Boron have distinct roles to play in producing spikes (ears) with around 50 grains / spike (ear). However these grains will only be filled if the plant has supplies of stem carbohydrate produced during canopy construction and green leaves that are photosynthetically efficient during grain filling.
Chlorophyll content is a very important component of green leaves (and hence photosynthesis), with both nitrogen and magnesium being an essential ingredient of it. Sub-optimal photosynthesis during the first two or three weeks of grain growth will reduce cell number and potential weight of each grain.
Deficient levels of nitrogen in the flag leaf leads to floret (grain sites) death. Grain filling depends on the capacity of both ‘sink’ (i.e. all grains in the ear) and the ‘source’ (i.e. materials from photosynthesis and reserves). Where ‘source’ does not satisfy ‘sink’, e.g. due to late drought or disease, grains will be inadequately filled and, after ripening, may appear shrivelled. One component of grain fill involves the translocation of stem reserves into the grain which requires energy that comes from ATP, a phosphate rich molecule.