Improve water use of wheat
By Chris Burbidge: Agronomist - Western Cape Business Unit
The water use efficiency (WUE) of a grain crop is a measure of how much grain is produced by a crop for each millimetre of effective rainfall and is usually expressed as kg of grain produced per hectare per mm of rainfall (kg/ha/mm). Effective rainfall is the total rainfall during the time the crop was in the ground less the evaporation, runoff and drainage past the root zone.
An Omnia client recently spread a single application of one lane of 100 kg/ha MAXI PHOS™ (36) over and above his normal GREENSULF™ 35 top dress practice at a relatively late development stage of the crop, namely at the end of tillering BBCH (29). This was then analysed as a strip trial, where the GREENSULF™ 35 was the control with the MAXI PHOS™ (36) served as the treatment with an additional 18 kg/ha phosphate (P), 8 kg/ha nitrogen (N), 5 kg/ha calcium (Ca), 4 kg/ha sulphur (S), 0.5 kg/ha magnesium (Mg) and 0.5 kg/ha zinc (Zn) applied over and above that of the control. Grain mass per m² and ears or spikelets per m² were the most significant yield parameters measured (Figure 1). It can also be mentioned that the treatment was tested for grain protein content versus the control. This was 13.2% and 11% respectively.
The specific soil type was a Cartref with a depth of between 15 mm and 25 mm. The crop rotation system that is followed is lupins/oats, barley, canola and wheat. The stubble serves to conserve moisture before planting (summer rain) and forms part of a continuous no-till cropping system. The lupins/oats are sprayed at a later stage with herbicide and act therefore as a cover crop. The effective cation exchange capacity (ECEC) ranged between 8 and 9. The pH (KCl) of both treatments was 6.2, the P (Bray1) of the control and treatment was 20 mg/kg with a 30% stone fraction. The S and Zn were on the lower side of the optimum spectrum.
As can be seen from Figure 2, the evapotranspiration (ET) and rainfall balance out early in the growing season, just after emergence. The specific field was planted between the 20 and 30 May 2013. As the crop grows and develops over time it is evident that the risk of a shortage of water increases as the ET starts exceeding the amount of rainfall. This is where stored soil moisture plays a vital role and has been enhanced by conservation agriculture (CA) practices. As these soils (Cartref) cannot hold enough plant available water, it is normally the amount of rainfall towards the end of the season in combination with energy, humidity and wind that determines yield.
During the 2013 planting season the rainfall recorded exceeded 500 mm from planting to physiological maturity. In terms of the water use efficiency (WUE) this means that the control produced 15.5 kg/ha/mm while the treatment produced 19 kg/ha/mm. This is a 22% improvement in WUE. It has also been reported in literature that a WUE Performance % can be calculated. This can be derived from Actual WUE/Potential x 100/1. The reported potential WUE for wheat is 5 – 20 and even as high as 22 kg/ha/mm. The WUE Performance % for the treatment strip at Moddervlei is therefore 95% or 86% performance.
It was evident that under the soil, plant and atmosphere conditions that prevailed in the previous season, the plant nutrients phosphate, and to a lesser degree nitrogen, together with secondary plant nutrients such as zinc, sulphur and magnesium increased yield and quality at a given amount of effective rainfall. This is especially important in a soil with an E-horison which is highly leached of plant nutrients. Other reasons for the positive yield response in relation to plant nutrients such as phosphate and magnesium are well documented in the literature.