Agronomic and economic performance of arable crops as affected by controlled and non-controlled traffic of farm machinery

Agronomic and economic performance of arable crops as affected by controlled and non-controlled traffic of farm machinery

In mechanised agriculture, soil compaction occurs mainly as a result of traffic with heavy farm equipment. Compaction adversely affects the physical, chemical and biological properties of soils, and the ability of crops to efficiently use water (irrigation and rainfall) and nutrients, which therefore reduces crop yield and the amount of fertiliser recovered in grain.

This study was conducted to investigate the effect of traffic compaction on crop response to nitrogen (N) fertiliser and N use efficiency by replicating conditions representative of controlled (CTF) and non-controlled (non-CTF) traffic farming, respectively. The agronomic and economic performance of wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor (L.) Pioneer G22) were assessed in-field conditions over two consecutive seasons (2014-2015 and 2015-2016, respectively). The soil type at the experimental site is a well-drained Red Ferrosol (69% clay, 11% silt, and 20% sand). Three N fertiliser formulations, namely, urea (46% N), urea ammonium nitrate (UAN, 30% N, solution), and urea treated with 3,4-dimethyl pyrazole phosphate (ENTEC®, 46% N) were applied at rates between 0 (control) and 300 kg ha-1 N at regular increments of 100 kg ha-1 N. Soil conditions (bulk density and strength) representative of CTF and non-CTF systems were achieved by first removing historical compaction using a subsoiler fitted with vertical, winged, tines operated at a depth of approximately 300 mm. A surface leveller was attached behind the tillage unit to smooth the surface in the same operation. Subsequently, six passes with a Belarus 920 tractor (100 HP, gross mass: 4 Mg) driven at a speed of 5 km h-1, and fitted with 11.2-20 (front) and 15.5-38 (rear) tyres inflated to 0.24 and 0.18 MPa, respectively, were performed on the non-CTF soil. Given the vehicle available, this level of traffic ensured that soil compaction conditions representative of non-CTF systems were achieved.

Soil physical and hydraulic properties were determined and results used to guide parametrisation of the Agricultural Production Systems Simulator (APSIM) model to enable long-term (115 years) prediction of traffic impacts on crop productivity and water use efficiency (WUE), and to quantify likely yield gaps in non-CTF relative to the controlled traffic farming (CTF) system.

For wheat, results showed that grain yield, total aboveground biomass, and harvest index were 12%, 9%, and 4% higher, respectively, in the traffic treatment representing CTF relative to that of the non-CTF system. For sorghum, grain yield was approximately 40% higher in the traffic treatment representative of CTF compared with that of the non-CTF treatment, and consistent with differences (P<0.05) in all measurements of crop yield components (total aboveground biomass, harvest index, and thousand-grain weight). Overall, there was no fertiliser type effect on grain yield, which was observed in both crops (P>0.1). This observation therefore confirmed that traffic compaction was the main factor affecting crop performance and the amount of N fertiliser recovered in grain.

Overall, agronomic efficiency (AE) and nitrogen use efficiency (NUE) calculations for wheat were greater in CTF by up to 35% and 40%, respectively, compared with the non-CTF treatment. For sorghum, AE and NUE calculations were both approximately 60% higher in the CTF treatment compared with non-CTF.

On average across the three fertiliser types the most economic rates of nitrogen (MERN) applied to wheat were 122 and 108 kg ha-1 N for CTF and non-CTF, respectively. The corresponding grain yields at these levels of N were 3337 and 2887 kg ha -1 for CTF and non-CTF, respectively (P-values <0.05). These differences in yield equated to agronomic efficiencies of 28 and 27 kg (grain) per kg N for CTF and non-CTF, respectively. Average MERN calculations for sorghum across all fertiliser types were 145 and 100 kg ha-1 N for CTF and non-CTF, respectively. The corresponding grain yields at these levels of N were 3430 and 1795 kg ha-1 for CTF and non-CTF, respectively (P-values <0.05). These differences in yield equated to agronomic efficiencies of 24 and 18 kg (grain) per kg N for CTF and non-CTF, respectively. The results derived from the modelling work showed that in average rainfall years, yield reductions in non-CTF may be up to 13% and 38% for wheat and for sorghum, respectively, relative to the yields achieved in CTF. In below-average rainfall years, yield reductions in non-CTF can be up to 4% and 12% greater for wheat and sorghum, respectively, compared with the yield achieved in average rainfall years. In above-average rainfall years, differences in yield between CTF and non-CTF treatments were small, which showed that the effect of traffic compaction on crop yield is dependent on the seasonal effect of rainfall.

Modelled WUE and runoff were measured (sections 4.3.1 and 4.3.2, respectively) and were also significantly affected by compaction. For wheat, the simulated conditions of the CTF system reported up to 15% higher WUE compared with non-CTF (≈20.90 vs. 17.50 kg ha-1 mm-1 for CTF and non-CTF, respectively). For sorghum, WUE was 43% higher in CTF compared with the non-CTF treatment (≈8.40 vs. 4.80 kg ha-1 mm-1 for CTF and non-CTF, respectively). Modelled runoff increased proportionally with an increase in total rainfall, but it did to a significantly greater extent in non-CTF compared with CTF. Overall, modelled runoff volumes in wheat and sorghum were, respectively, 28% and 45% higher in non-CTF compared with CTF.

Given current price ratios (nitrogen-to-grain), and depending upon the fertiliser type used, gross margin penalties of approximately AUD50-70 and AUD110-190 per ha may be incurred in wheat and sorghum, respectively, when controlled traffic is not practised. This study also confirmed that N use efficiency cannot be significantly increased if the mechanisation system does not allow for avoidance of traffic compaction. Therefore, the agronomic, and possibly the environmental benefits associated with the use of enhanced efficiency fertiliser formulations may not be fully realised if soil compaction is not avoided. Improved soil structural conditions are, therefore, a pre-requisite for increased fertiliser use efficiency, crop productivity and sustainability.