Features Agronomy Soil
Tall stubble affect on yield varies

Seeding into tall stubble is a management strategy that can be used to help
conserve soil moisture. But how does that practice work in wetter areas where
moisture is not as limiting? Separate research projects illustrate the differences
between the semi-arid climate at Swift Current and sub-humid climate at Brandon.

"I think one of the big differences is wind. The extra wind we get has
a big impact on soil water and evaporation," says Herb Cutforth, research
scientist with Agriculture and Agri-Food Canada (AAFC) at Swift Current, Saskatchewan.
"I think because water is usually our most limiting factor, tall stubble
has a greater effect on yield here."

At Brandon, AAFC research scientist Karl Volkmar also found microclimate differences
among stubble treatments, but did not observe differences in yield between cultivated,
short and tall stubble. "The results suggest that the practice of seeding
into tall stubble is not economically disadvantageous. The savings associated
with the reduced energy expenditure as a result of cutting the crop higher up
the stem could reflect savings to the producer without a yield penalty."

Similar microclimate effects
Both studies found microclimate alterations when moving from cultivated, to
short, to tall stubble. In the studies, stubble was left standing over the winter.
In the spring at Brandon just before seeding, the stubble was left at 20 inches
tall, cut to six inches in another treatment, and cut to six inches and then
cultivated with one pass at a three to six inch depth.

At Swift Current, two projects were conducted. The first project, in the late
1990s, used a similar methodology, except stubble treatments were cultivated,
six inch and 12 inch stubble. In the second project conducted from 2000 to 2003,
an additional extra tall treatment was added with stubble left at least 18 inches
tall. Canola, pea and wheat were included at both locations, and chickpea was
also part of the Swift Current evaluation.

At Swift Current, as stubble height increased, the reduction of wind speed
and solar energy near the soil surface reduced water evaporation. Compared to
cultivated stubble, tall stubble reduced potential evaporation by more than
25 percent. For tall stubble treatments, wind speed near the soil surface was
lower, soil temperature was lower, and evaporation was lower.

At Brandon, the stubble treatments also caused surface and subsurface temperatures
to be cooler than cultivated soils. Tall stubble resulted in reduced wind speed
and greater soil moisture, basically the same microclimate effects as Swift
Current.

In areas where cold spring soils may raise germination concerns, standing stubble
does not necessarily impair crop development. Yes, cooler soils can reduce root
growth which can decrease the rate of soil exploration by the root system. At
Brandon, on the other hand, tall stubble was found to increase day-time air
temperature which can accelerate crop growth. In the end, the cooler soils did
not negatively affect crop development at Brandon. "We always had better
crop emergence with tall stubble," says Volkmar.

In fact, during crop development, the taller stubble treatments at Brandon
had a consistent trend toward increased crop growth midway through the growing
season. While that did not result in a yield increase, it shows that crop growth
was not negatively influenced by the cooler soils.

Yield increases observed in drier areas
The impact of reduced evaporation and greater moisture availability under the
tall stubble treatments was reflected in higher yields at Swift Current, but
not at Brandon. In the first study at Swift Current using only cultivated, short
and tall stubble, the spring wheat, pulse and canola crops all had higher yields
as stubble height increased. That trend was also evident with the addition of
the extra-tall treatment in this most recent study. "I was surprised that
this linear relationship continued," says Cutforth.

In the most recent Swift Current study conducted from 2000 to 2003, the overall
trend was towards higher yields on taller stubble. Water use efficiency was
also higher, although the increase was not statistically significant. "Canola
was more consistently responsive to stubble height than spring wheat or chickpea,"
says Cutforth. "Chickpea was a fair bit more variable. There is something
else going on in the microclimate that has chickpea responding differently to
stubble height."

At Brandon, the better soil moisture, improved germination and greater mid-season
growth did not translate into higher yield. "Early improved crop performance
under tall stubble did not consistently translate to higher yield. In all three
years, tall stubble grain yield fell far short of the potential indicated by
mid-season biomass yields," says Volkmar. "One possible explanation
is the lower than average precipitation that occurred during grain formation
and filling in all three years of the study."

Volkmar also says that another possible reason is that the higher biomass associated
with taller stubble plots increased nutrient demand, resulting in nutrient deficiency
and unfulfilled yield.

Looking at the results from the two areas, figuring out how high to leave the
stubble at harvest becomes a function of the ability to seed through the crop
residue. In semi-arid areas, which encompasses much of the Dark Brown and Brown
soils if not some of the Black transition areas, leaving stubble as high as
practically possible at harvest can pay back in additional yield.

In wetter areas, the decision to leave stubble higher hinges on the ability
to seed through the crop residue in the spring since there are no negative yield
effects from tall stubble. Another consideration is that by cutting the crop
higher at harvest, combine capacity can be increased.

"Farmers in sub-humid areas need to balance the benefit of increased combine
capacity with their ability to seed through standing stubble in the spring and
any delays that might also be associated with wet spring soils," says Volkmar.
"But from a yield perspective, the data shows that there shouldn't be any
yield penalty for leaving the stubble higher." -30-