The most important macro nutrients for early shoot, leaf growth, and shoot survival are nitrogen, phosphate, and sulphur and of the micronutrients, manganese and zinc are the most important for increasing leaf and shoot numbers.
Nitrogen is the single most important nutrient for achieving high yields. Adequate supplies will give bigger leaves and therefore more shoots, with each leaf developing faster. Nitrogen deficiency on the other hand reduces the rate of primordia initiation, which reduces the number of potential spikelets (grain sites).
The survival of the shoots / tillers that are produced can be influenced by interactions between nitrogen application rates and timing.
Where shoot numbers are below the optimum, the applications of higher nitrogen rates early in the season (Zadoks 25 – 30) will increase shoots and thereby final ear numbers.
This crop manipulation should not be used where plant populations are on target, as it can lead to excessive leaf and shoot growth and lodging. Lower application rates early should be used where shoot numbers are on target.
Where spring wheat is being grown, the time for leaf and total shoot production is very limited so early supplies and high rates of nitrogen are critical for achieving the required early growth rate and shoot numbers.
All of these factors contribute to the increased yield that is observed from nitrogen applications in wheat.
The influence of nitrogen on yield can clearly be seen from this nitrogen response curve with the application of nitrogen almost doubling the crop yield. The response to applied nitrogen levels off and eventually declines once very high rates are applied due to excessive leaf growth and crop lodging. The optimum nitrogen rate (N opt) is the rate at which the maximum economic benefit is achieved.
Phosphorus is considered to be the second most important nutrient after nitrogen in terms of its influence on plant growth and development. After the crop has developed two to three leaves it will begin to rely on soil available phosphorus for continued leaf and shoot numbers growth. Phosphate availability in the soil is influenced by many factors including pH, other nutrients such as aluminum, iron and calcium, soil moisture and temperature.
It is important therefore to ensure that freshly available phosphate is applied to avoid this limiting early shoot growth. In winter wheat there are two timings that are important to consider. Firstly during early crop establishment (Zadoks GS 13-25) when it is growing rapidly with shoots and roots developing, and secondly as spring growth commences (Zadoks GS 25 -30). In the period from March to May 70% of phosphate will be taken up, so applications of phosphate fertiliser should be targeted to meet this demand.
Applications of spring phosphate have been shown to give yield increases.
Foliar phosphate applications can be included into the phosphate crop nutrition program to again improve the crops phosphate status when soil availability is limited. This will improve autumn and early spring growth and development.
Sulphur nutrition is another critical nutrient required by the plant to ensure continued growth of the developing shoots. The building blocks for plant growth are the sulphur based amino acids from which many of the plant proteins develop.
Sulphur is poorly translocated around the plant with new growth suffering first. To overcome this a continual supply with multiple applications of sulphur should be made to satisfy the peak demand period. Foliar sulphur can also be included to overcome transient deficiencies.
Manganese and zinc are two important micro nutrients that will affect the developing components of yield, such as increasing the ear number and grain size, resulting in increased final harvested yield.
Manganese and zinc have also been associated with improving the uptake of other nutrients, both macro and micro nutrients.
The number of shoots that develop will be affected by the following:
In general as the number of seeds is reduced, and the earlier the drilling, the more each individual plant will tiller, with the final shoot number being dependent on the number of tillers that survive through to producing ears. The tiller number and survival can be influenced by: