Battle of the tills

A new experiment, using precision water control, gives hard data about the gains that can be made growing wheat under zero-tillage conditions.

This was a classic showdown. On the right one hundred wheat lines (from the 14th and 15th International Semi-Arid Wheat Yield Trials) planted in the conventional way on tilled soil. On the left an identical one hundred wheat lines, but this time planted without tillage into the residue of a zero-tilled sorghum crop (the field had previously been tilled normally). The objective? To determine which cropping method would give the best results under different water conditions. Biggest yield wins.

When the team at the CIMMYT experimental station near Obregón in northwest Mexico planted the two identical sets of seeds, they had high hopes that they would find significant differences. This relatively straightforward experiment was designed by CIMMYT rainfed wheat breeder Yann Manes. It took advantage of the fact that it rarely rains during the growing season at Obregón, so precision irrigation could be used to simulate various rainfall conditions. Manes expected the zero-tillage field would give higher yields when there was water stress but he needed to prove it. “The stubble from the sorghum should help the soil retain water,” says Manes. "But this was the real test. No one had actually done the zero-tillage face off under different but carefully-controlled water conditions on a large set of wheat varieties."

The two plots were divided into three strips, each one receiving a different, carefully-controlled amount of water. They used what the Obregón teams calls "the dinosaur", a fifty-meter-long, three-armed machine that can deliver water precisely to each growing row, simulating rainfall. One set of plants in each plot received a normal amount of water (320 mm). The middle strip was water-stressed, receiving a reduced amount of water (175 mm), and the last strip in each plot was grown under drought-like conditions, receiving only 105 mm of water during the whole growing season.

As the wheat approached maturity, some differences started to appear in the two plots. Manes was pretty excited. “You can see there is a difference in biomass,” he says. “Look here to the left, in the drought-stressed wheat on the zero-tillage side there is more than in the same strip on the right.”

But biomass and yield are not the same thing. What if the wheat plants under zero-tillage conditions just made bigger leaves and stalks but did not have larger or more grains in their spikes? The team had to wait until each strip was harvested and the results from all the lines, all the strips, and both plots were computed.

What the team found was that under normal rainfall conditions there were no appreciable differences in yield between the two plots. This reflects what has been seen in long-term trials of various tillage practices run in Obregón; that the advantage of zero over normal tillage starts to show only after four or five years. But under water stress conditions, it was a totally different story. Under both reduced-water conditions and simulated drought there was an average yield advantage of between 8 and 9% to the wheats on the zero-tillage side. Zero-tillage wins, plows down.

Samples taken during the crop cycle confirmed that zero-tilled soil held moisture better than conventionally-tilled soil in this experiment. The data also gave other interesting insights into how different wheats respond to drought conditions as well as to the cropping practice, and Manes says that opens the door to a whole new line of research—determining whether you get different results in breeding when you make your selections from zero-tillage rather than conventional plots.

The work was done in collaboration with CIMMYT’s agronomy team led by Ken Sayre, who analyzed the soil samples, and with Jose Crossa, from the Crop Research Informatics Laboratory (CRIL), who did the statistical analysis.

Manes cautions that this is just one season of data. He intends to repeat the experiment again next year, and in the meantime former CIMMYT breeder Richard Trethowan is doing a similar experiment in Australia.

“I think next season the results might be even better,” says an optimistic Manes. “The soil will have had another year of zero-tillage, with more organic residue available to hold water. At least that is what I would expect. Of course, I won’t know until I try it.”

For more information: Yann Manes, rainfed wheat breeder (y.manes@cgiar.org).