Crop response to lower nitrogen applications in the long-term conservation agriculture trial at Langgewens

by Dr Johann Strauss, johannst@elsenburg.com

TThe long-term conservation agriculture trial was started in 1996 at the Langgewens research farm near Moorreesburg by Dr Mark Hardy as researcher and Samie Laubscher, the scientific technician at the time. The aim of the trials was to compare different crop rotation systems for the Swartland area. Over time, the trial developed into a complete conservation agriculture trial with full no-till implementation in 2002 and zero-till in 2016, with the aim of lowering production input.

The trial went through three phases. In the first phase (1996 to 2001), minimum-till was practiced. This included a loosening of the soil with a tine implement before seeding. From 2002 to 2015, no-till was applied with the help of an Ausplow tine seeder. The third phase started in 2016 with the addition of a Piket disc seeder. Over the 24-year period from 1996 to the end of 2020, there were significant changes in the overall performance of the trial in response to the drought. In 2003, during the first drought, the average wheat yield was only 500 kg/ha. The second severe drought year was in 2015, followed by a third in 2017. The wheat yield response in those two years (even with less rainfall in both years compared to 2003) was 2 100 kg/ha and 2 400 kg/ha, respectively. This can be attributed to the improvement in soil carbon levels and soil structure, which improves the water-holding capacity of the soil along with the lowering of evapotranspiration by residue retention. Over the past two seasons, no pesticides were applied to any crop on the farm. This is the culmination of several factors. The decision was made to stop spraying the medic pastures, followed by the introduction of multispecies cover crops playing host to natural predators.

System Sequence

WWWW

WWWC

WLWC

WWLC

MWMW

Wheat-wheat-wheatwheat

Wheat-wheat-wheatcanola

Wheat-lupine-wheatcanola

Wheat-wheat-lupinecanola

Medic-wheat-medicwheat

McWMcW Medic/clover-wheatmedic/clover-wheat

MWMC

MCWMcW+S

Medic-wheat-mediccanola

Medic-canola-wheatmedic/clover-wheat+ saltbush

One of the ideas of practising crop rotation is to include a legume in the rotation, which produces free nitrogen in the soil. This can lead to lowering applied nitrogen over time. As the soil health improves over time, nutrient recycling also improves, leading to the lowering of fertiliser input over time. Now, the question is: what has happened in the trials with the amount of applied nitrogen and what was the impact of this on the nitrogen use efficiency of wheat? Figure 1 represents the different rotation systems. The top four systems are cash-crop-only systems, whereas the bottom four are pasture- or cash-crop systems that include an animal factor. In these systems lupine, medic, and clover are included as legumes.

In Figure 2 the average amount of nitrogen applied in each system in each phase of the trial is shown. In most cases there was a considerable lowering of application between the minimum-till and the no-till phase, with another significant lowering since the implementation of zero- till. The decrease in the last phase was due to a decision to lower the total amount of nitrogen in each system. It was done to (a) study the effect on the lowering of the nitrogen input on production and (b) to evaluate the effect on the gross margins of these systems.

In this article we will only be focussing on the first, though, looking at the effectiveness of wheat production per kilogram of applied nitrogen. Although two very dry years were experienced in the zero-till phase (2017 and 2019), the 2018 season was also plagued with considerable losses. This was due to strong, gusty winds during harvesting. Yet, the effectiveness of the wheat production in each of the systems improved substantially, as can be seen in Table 1.

In all the systems the improvement in nitrogen use efficiency was between 74% and 145%, without affecting the average yield of the different systems over time.

The impact of these improvements goes further than only lowering the input costs of the systems, it also has a tremendous effect

Table 1: Representation of the production efficiency of wheat in kilogram produced per kilogram of applied nitrogen over the three phases of the trial, including the percentage improvement from the first to the third phase on the environment and the sustainability of the systems. These results only focus on the nitrogen part of the applied fertiliser and does not include the whole fertiliser application. By merely lowering the amount of fertiliser, the carbon footprint of these systems is lowered significantly. The production of synthetic nitrogen is energy-hungry. By lowering the nitrogen fertiliser, less energy is needed in the overall production of nitrogen, and less fuel burnt to ship and transport the fertiliser base components. Less nitrogen being applied also means a lower likelihood of leaching and contamination of ground water and rivers. The trials also show that it is possible to produce competitive yields with lower nitrogen input through good agricultural practices based on conservation agriculture principles.

Other conservation agriculture-related articles worth reading are:

Strauss, J. 2021. Klimaatsverandering – wat produsente moet weet. SA Grain , March 2021:65-67.

Strauss, J. 2020. Kan daar op bemesting gesny word? SA Grain, March 2020:65-66.

Strauss, J. 2019. Taking conservation agriculture best practices to the next level. AgriProbe , 16(4):44-46.

Strauss, J. 2019. Bewaringslandbou. Maak dit ’n verskil aan die produksie-effektiwiteit van koring? SA Grain , August 2019:8-11.