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 barley. It is also known that in barley varieties currently grown, there is an Harvest Index of approximately 50% i.e. 50 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 barley. 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:
Before setting out a detailed agronomy strategy it is important to understand what the key targets for achieving high yield and quality are. The main components of a strategy should be as listed below:
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 fertilizers and foliar micronutrients in the crop management programme.
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 barley 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. Tillering is very significant in agronomic terms as it is a mechanism by which the barley plant can compensate for low plant numbers.
The maximum number of tillers formed is genetically determined, however the actual number will be influenced by seasonal conditions, day length, sowing date, plant population and nutrition. Under conditions of moisture or nutrient deficiency the tiller buds remain dormant and plants produce few tillers. Under optimum conditions it is competition for light as the canopy closes that causes tillering to stop.
The growth stage reached at the end of tillering varies by variety but in low plant population crops it can continue till the later growth stages. Not all the tillers produce fertile heads. Senescence of tillers will start after floral initiation in the main shoot as the developing spikelets and florets compete for the available assimilates. The tiller survival rate will be a function of the amount of available nutrients, light and water to the plant.
This is how yield is influenced by available precipitation.
Tillers that survive but have no ear / spike act as a source of nutrient and carbohydrate for the developing grain. The number of tillers that survive to form grain varies with variety, seasonal conditions and crop management.
It is during this phase that the plants 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 times as much leaf area as soil area (GAI 6). During this important period nutrient uptake is at its peak and thus this demand by the plant must be met through applied fertilizer and micronutrients.
The Nutrient Management Plan will need to include multiple applications of fertilizer 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.
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.