Is strategic tillage a step backward for zero-till agriculture?
Many would say yes, but others argue it’s the next logical step forward in the evolution of zero-till science.
What began in the 1970s as a seed drill experiment to address wind and water erosion has become the accepted way to grow crops on the Prairies.
The original benefits of soil and water conservation quickly expanded to encompass lower costs for machinery, fuel, fertilizer and pesticides.
As the first zero-till farmers watched their input costs fall and their yields increase, more of their neighbours adopted the practice solely for those economic reasons.
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The guiding mantra became “thou shalt not disturb the soil.” It was the driving force behind everything from research and development to on-farm field practices.
But what about those wet times when fields were impossible to farm because they were so badly rutted from seeding, spraying and harvesting?
Cultivation has sometimes been necessary to smooth a rutted field. Strict adherence to the mantra could result in broken equipment and downtime.
Tillage in zero-till fields has also been forced by wet or cold soil, problems with tall or excessive residue and tough residue such as Bt corn. As well, problems with insects and plant diseases can force cultivation.
Persistent weed problems such as foxtail barley have coerced zero-till producers to drag the cultivator out from time to time.
Any discussion of persistent weeds leads to the issue of herbicide resistance. Zero tillers say herbicides are the heart of their crop production system. Without them, the practice of zero-till farming would disappear.
Strategic tillage
Strategic tillage is a new term that describes those occasional planned tillage operations.
Because zero till does not require seedbed preparation, the term encompasses virtually any cultivation that happens on a zero-till field.
The most recent cultivation innovation on zero-till fields is called vertical tillage, conducted with rippled coulters, but strategic tillage can include any type of cultivation with any type of implement.
“The reasons for this management may be to reduce field roughness, control certain weeds, increase soil temperature or bury crop residues in an attempt to manage disease,” said Byron Irvine of Agriculture Canada in Brandon.
Irvine said many zero-till producers assume tillage will seriously hurt their system. The evidence for this is limited, he added, but he and his colleagues decided to put the assumption to the test.
In 1999, they set out to determine if strategic tillage could be used to lower disease in canola. They also wanted to determine the impact of a single tillage operation on crop yields, weed numbers, foliar disease and nutrient uptake in a long-term zero-till field.
The two rotations were canola-wheat-peas and canola-wheat-flax using different tillage implements.
The study concluded that strategic tillage did not have long-term negative impacts on crop yields or weed pressure.
“Thus tillage could be used in the sub humid black soil zone to control difficult weeds or level a rough field without long-term negative impacts on crop yields, disease or soil quality,” Irvine said.
Yields
In the trial, wheat yields were two percent greater with low disturbance zero-till seeding than with low disturbance after cultivation or heavy harrowing, but Irvine said this is not a significant difference.
Heavy harrowing and phoenix harrows had little or no impact on wheat yields.
Flax, peas and canola yields tended to be lower when planted after strategic tillage rather than low disturbance zero-till seeding, but this was significant only with dry peas.
In the year following strategic tillage, only pea yields were lower on land that had been tilled.
These differences may be caused by a larger number of pre-spray weeds in tilled ground. Data from other studies indicates that weeds generally emerge more rapidly and uniformly on tilled soil and thus may require earlier control.
Preharvest glyphosate was sprayed on the trial fields in the fall of 2002 to control perennial broadleafed weeds before Robust barley was seeded in 2003.
Under these test conditions barley yields were eight, 16 and 22 percent higher than continuous direct seeding when tillage had occurred one, two and three years previously.
Weeds
Pre-spray weed numbers were about 30 percent higher in the year the land was tilled and remained at this elevated level in the year following tillage. However, pre-spray weeds numbers were similar to continuous low disturbance zero-till plots two years after tillage.
In 2001 wild oat and wild buckwheat numbers were lower overall in crops on fields tilled the previous year. In 2002 wild oat and wild buckwheat numbers were equal or greater where tillage had occurred the previous year.
Dandelion numbers were variable. This indicates that a short-term tillage regime is not an effective dandelion management tool in zero tillage and that fall applications of chemicals to control dandelions are more effective.
Residual weed counts, which are weeds remaining after spraying, were lowest in wheat. Plots that were heavy harrowed had slightly lower residual weed numbers, which may indicate greater uniformity of emergence of weeds and thus improved control by the chemical application.
Low disturbance zero-till plots had similar or higher residual weed counts compared to plots where high disturbance had occurred in the current or previous season.
Diseases
Fusarium head blight was measured on wheat plots from 2000 to 2003, as were sclerotinia and blackleg.
Neither the previous crop or previous tillage operation had significant impacts on disease levels. The trial was established on land where canola was last grown in 1991 and peas in 1993 and 1997. The blackleg resistant canola variety Invigor 2573 was planted from 2000-02.
Blackleg levels in the soil were initially low partly because of the high level of genetic resistance as well as conditions not conducive to development of the disease.
Sclerotinia levels were low because of low levels of inoculum resulting from a limited number of susceptible crops in the rotation and the relatively dry conditions during flowering.
Soil quality
Bulk density at zero to five centimetres was not affected by tillage or crop choice. Penetration resistance was similar in high and low intensity tillage at zero to five cm and five to 10 cm. No significant differences were observed between types of crop stubble.
There were a greater proportion of large aggregates (38 to 68.8 millimetres) under the low disturbance system compared to soil that had been tilled one or two years previously. Soil disturbance resulted in a correspondingly higher proportion of aggregates smaller than 0.5 mm.
“The impact of these smaller aggregates may not be critical because a return to direct seeding means that standing stubble and roots will again prevent wind erosion,” Irvine said.
“Additionally, one year after disturbance, the proportion of water stable aggregates was equal to soils which had never been disturbed. These water stable aggregates are an indirect indicator of water erosion potential.”
No significant difference was observed in organic carbon or nitrogen of soil aggregates because of tillage or crop choice. High disturbance tillage and crop stubble had no significant effect on wet stable aggregates (1.3 to two cm).
Tillage and the preceding crop had no significant effect on soil organic carbon and total nitrogen at zero to 10 cm, 10 to 20 cm and 20 to 30 cm.
Neither high or low levels of tillage intensity or preceding crops of peas or canola significantly affected phosphate phosphorus at the zero to 10 cm depth.
There was more nitrate nitrogen at zero to 10 cm in high disturbance tillage and following canola. High levels of nitrate nitrogen and ammonium are attributed to residual nitrogen fertilizer.
Strategic tillage facts
The Agriculture Canada study into the effects of strategic tillage on zero till made several observations:
- Tillage just before planting flax or peas resulted in lower yields of that crop in that season. This may have been due to higher weed pressure and the inability to control weeds at an early stage with herbicides.
- Barley yields in 2003 were greater on plots that received tillage in any of the previous seasons. This suggests the potential for improved growth by mixing surface residue with lower levels of the soil, an idea farmers have debated since the birth of zero till.
Byron Irvine of Agriculture Canada said the study did not confirm this and cautioned that tillage for this reason should not be practiced until the mechanism, timing and frequency of response can be determined.
- Total weed numbers were up to 30 percent greater in the year of tillage and the next season. Weeds levels returned to the same level the second season after tillage. n Strategic tillage did not affect blackleg or sclerotinia levels in the years of the trial. As well, there were no significant yield differences between canola in a rotation with peas and canola in a rotation with flax. The low levels of disease in this trial made it difficult to determine whether occasional strategic tillage significantly affects canola and wheat diseases.
- While aggregate size was reduced by tillage, this may have only limited impact on wind erosion in years after disturbance.
- There was little or no impact on soil carbon, nitrogen, bulk density and potentially mineralizable nitrogen.
