Physical and Cultural Weed Control Working Group of - European ...

9 th EWRS Workshop on Physical and Cultural Weed Control 43
Samsun, Turkey, 28 – 30 March 2011
Crop rotation as the ultimate cultural practice: 18 years of cereal yield stability
in three tillage systems
A. Légère 1 , F.C. Stevenson 2
1 Agriculture and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, SK, Canada S7N 0X2
Email: anne.legere@agr.gc.ca
2 Private Consultant, Saskatoon, SK, Canada S7N 3T6
Current agricultural systems are often based on crop monocultures or very short rotations, in
spite of little if any benefits from agronomic, environmental, and even economic perspectives
compared to well planned crop rotations. The benefits of diversified crop rotations have been
extensively documented. Our objective is to demonstrate that crop rotation can ensure greater and
more stable crop yield over time, even when using conservation tillage practices.
Crop yield is determined by its various components, each of which is being affected by agroenvironmental
conditions at different times during the growth season. For cereal crops, seed density
(number of crop plants x number of flowers per head x number of head per plant) is determined
mainly in the period prior to weed management operations whereas grain weight (1000-seed wt)
will be affected by conditions in the post weed management period, including the effects of
potential competition from residual weed communities during grain filling. Using a grouping
methodology based on means and coefficient of variation (c.v.), we examined crop rotation and
tillage effects on cereal yield and yield components from a long-term conservation tillage study,
conducted at La Pocatière Québec. Two time periods were examined: a first phase (1988-1994)
which compared a 2-yr cereal-forage rotation to a cereal monoculture, and a second phase (1998-
2005) which compared a 4-yr cereal-oilseed rotation to a cereal monoculture. Tillage treatments
included: moldboard plow (MP), chisel plow (CP), and no-till (NT).
On average, seed density and yields were greater and less variable in the second phase (c.v. =
28% than in the first phase (c.v. = 41%). Seed density and yields from rotation treatments were
greater and generally less variable than that from the monoculture. According to the groupings
defined by the average of means and c.v. over time, first-phase rotation-MP and rotation-NT were
optimum treatments, providing high yields with low variability (3732 kg ha-1, 24.4% and 3397 kg
ha-1, 33.4%, respectively), whereas the monoculture-CP provided the lowest yield with the highest
variability (2450 kg ha-1, 50.6%). In the second phase, rotation yields averaged 3946 kg ha-1, with
a c.v. of 19.3%, compared to 3155 kg ha-1, with a c.v. of 35.6% in the monoculture. In both phases,
variability of seed density was in the same range as that for yield, and treatment effects followed
very similar trends. Conversely, values for 1000-grain wt were much less variable, with an average
c.v. of 7% and 14%, in the first and second phase, respectively. Values for 1000-grain wt were
consistently greater in the crop rotation than in the monoculture. Overall, cereal yields were
influenced more by early (seed density) than late (1000-seed wt) season agro-environmental
conditions, which suggested minor effects of residual weed populations during the grain filling
period in crop rotation treatments.
Crop rotation ensured greater and more stable yields over time than the monoculture. These
benefits were slightly greater in the 4-yr crop rotation (second study phase), in spite of greater
environmental variability compared to conditions during the first study phase. Whether the crop
rotation effect in the second phase is attributable solely to the cereal-oilseed rotation or to the
cumulative effects of both cereal-oilseed and previous cereal-forage rotation could not be
determined. Diversified crop rotations proved beneficial, particularly in these aging conservation
tillage systems.