Agronomy Library > Winter Annuals/Other

Direct Seeding Reduces Wheat Stress and Improves Yields
Author: Wang, H., Lemke, R., Goddard, T. and Sprout, C
Date Created: February 27, 2007
Last Reviewed: March 02, 2007

Source: Canadian Journal of Soil Science 87:3-10. 

Open the PDF for the complete report including figures and tables

Crop residue management is a key component of direct seeding systems. Crop residues enhance moisture absorption from rain or snow, reduce evaporation losses, reduce erosion and provide nutrients. The soil system moves from constrained cycling under a tillage system to a broader, dynamic nutrient cycling with direct seeding. Some recent research from central Alberta has documented another benefit to add to the list for crop residues, the reduction of root heat stress. Wheat under less heat stress gives better yields.

The Three Hills long-term crop rotation plots ran from 1992 to 2006. The plots were split by conventional versus no-till practices. In 2000 after eight years of no-till, the plots served another purpose of providing information on greenhouse gas emissions (nitrous oxide specifically). Along with measuring the nitrous oxide emissions from the soil, temperatures at 5 cm and 10 cm depths (2 and 4 inches) were continuously monitored. Grain and total above ground biomass (grain+straw to ground level) were sampled for wheat and the results for the four years from 2000 to 2003 were analyzed for this study. The four years encompassed both drought and “average” years. Only the continuous wheat rotation treatment was used.

Soil temperature
The daily fluctuations in soil temperature mimicked that of the air temperature (Fig. 1) with the soil temperature under conventional tillage (CT) often higher than the air temperature in the heat of the day while the soil under no-till (NT) was cooler. The temperature patterns at the 10 cm depth were similar to those at the 5 cm depth as depicted in Figure 1. The differences between the CT and NT temperatures were statistically different for much of the day (symbolized by the black triangles along the bottom axis of Fig. 1).

Soil moisture
Soil moisture measurements of the surface depths (0-15 cm or 0-6 cm) were taken each time that nitrous oxide gas samples were taken over the four-year period (Fig. 2). Even though the NT had more moisture than the CT, there were few times over the growing season that the differences were statistically different (two of the years were drought years with little moisture to measure).

The pre-seeding moisture content in the top two feet (0-60 cm) was higher under NT than under CT for each year of the study however, the differences were statistically significant only in some years unless the statistical test was relaxed from a 95 % confidence to 88 % (Table 1). The treatment differences were negligible when we looked at the top 1.2 m indicating NT differences are greater in the upper root zone.

Heat stress index
Scientists studying wheat physiology over the last two decades have found that heat stress of shallow roots can affect the whole plant by altering the balance of photosynthates (products of photosynthesis) partitioned to the roots and shoots. If only roots are heat stressed, the effect on the plant is worse than if only the shoots are heat stressed. A heat stress index (HSI) was calculated as, HSI = ∑ (Ti – Tc) , where Ti is the temperature for each hour and Tc is the critical temperature. HSI was cumulative where Ti was greater than Tc during the growing season. Based upon the best available data we used a critical value of 20 C which is likely a conservative value for roots.

The calculated HSI was higher for the CT treatments indicating more crop stress (Table 1). The cumulative HIS under NT was about half that under CT over the growing season (ranged from 0.33 to 0.70).

YieldsWe found that wheat yields were always higher under NT than CT and they were statistically higher in three of the four years of this study (Table 1). The advantage was greater in the two drier years (44-147 % greater in 2000, 2002) than in the two wetter years (3-18 % greater in 2001, 2003). Over the four years an extra 26 bu/ac of wheat was produced under the NT system.

There was statistically more biomass production in three of four years. The differences in biomass (grain+straw) were greater than the difference in grain yields. In fact, over the four years an additional 2400 lb/ac of straw was produced with no-till practise.

We then addressed the question of which has more of an effect on yield, soil moisture or heat stress index. Using a statistical procedure called pathway analysis we determined that HSI was responsible for 75 % of the direct and 8 % of the indirect effects on biomass production while for grain yield it was 66 % and 24 %, respectively. Pre-seeding soil moisture was responsible for 12 % of the direct effects and 53 % of the indirect effects on biomass production while for grain it was 34 % and 46 %, respectively. HSI had much more of a direct effect on both grain and biomass than soil moisture did.

Some implications/thoughts:
- Published Harvest Index values are determined on CT research or breeding plots. If most of our land is under NT are the HI values inaccurate?
- If wheat breeding is done on CT plots, should we expect the same relative performance of a new variety under NT?
- If more biomass is produced under NT then more straw is available for soil cover/protection or as a biofuel crop. Remember to keep enough residue…
- If our climate is changing to warmer, drier summers then the advantage to NT is more valuable. 
- Crop residue on the soil surface is a valuable component in a NT cropping system. Loss of crop residue by cultivation, fire, etc will result in increased root heat stress and reduced yields.

Funding for this research was provided by internal funding of Alberta Agriculture and Food and, Agriculture and Agri-Food Canada. Environment Canada funded the original research on greenhouse gas emissions which spawned this sub-study.