Crop Rotations For The Brown Soil Zones

Perry Miller and Brian McConkey

AAFC-SPARC, Box 1030, Swift Current, SK S9H 3X2

This paper discusses strategies for diversifying crop rotations in the water-limited Brown and Dark Brown soil zones of the Canadian Prairie. We will try to answer the questions of fallow vs. stubble and best cropping sequences based on water use and resulting grain yields. In practice many other field-specific factors must also be considered in selecting the best crop rotation such as herbicide carryover, price forecasts at seeding, competitive ability of different crops on weedy fields, and crop residue needed to control erosion.

Fallow vs. Wheat Stubble

The choice of fallow or wheat stubble for growing an oilseed or pulse crop is one of the most commonly asked questions by producers. Normally all crops will yield best on fallow. Spring soil water and N levels are typically much lower on wheat stubble than fallow but excellent crop yields on stubble are still expected when adequately fertilized, providing there is above-average growing season rainfall. So the fallow vs. stubble question is really a question of the probability of the crop on stubble returning a profit in years with average growing season rainfall and of at least covering production costs in drier years. Our research results from Swift Current show that pulse crops, such as chickpea, lentil and pea are good bets on wheat stubble, while mustard (and canola) is better suited to production on fallow (Table 1). Why did pulse crops perform better on stubble compared with wheat and mustard? There's at least two reasons.

1. Wheat and the oilseed crops received fertilizer N based on a yield target set for climatic conditions of 'normal' stored soil water and rainfall amounts. This experiment was conducted during the period of 1992-96 that included two wetter than normal, three near-normal, and one drier than normal growing seasons coupled with stored soil moisture levels that were generally much above normal. Under these conditions of generally favourable moisture, wheat and mustard yields were likely limited by insufficient N supply. However, the yields of the properly inoculated pulse crops were not limited by N because of their N-fixing ability.

2. The water use pattern for pulse crops, at least lentil and pea, provides a competitive advantage relative to other stubble-sown crops. The shallow water use pattern of pea and lentil (Table 2) indicates that less soil water recharge is required to produce a good pea or lentil crop, compared to the deeper rooted wheat and mustard that rely more on stored soil water. As such, even though pea yields were lower than wheat, because pea used less water, the water-use-efficiency was practically the same for both crops.

Values within a column followed by the same letter are not different (P=0.05).

Cropping Sequence

Once a crop rotation is extended beyond a 'crop-fallow' or '50-50' scenario, then knowing how the previous crop affects the next crop is important. Research conducted in the Parkland region has shown consistently that pea has a yield boosting effect on the next crop in the rotation. In our research, this same effect has also occurred under drier soil conditions of the Brown soil zone. A CWRS wheat crop, fertilized the same and receiving the same herbicides, yielded 16% more and had increased grain protein levels by 0.8%-units when seeded onto pulse crop stubbles (desi chickpea, dry pea, lentil) compared with wheat stubble (Table 3). Wheat grain protein, but not yield, was higher after the oilseed crops than after wheat.

We decided to look at these crop sequence benefits more closely in a follow-up experiment begun in 1995. Preliminary data from this experiment is presented in this paper. Soil water use was measured for the 1^st-year crops (see Table 2 above) and then grain yields were measured on three 2^nd-year crops (Table 4). In order to equalize N supply in all 1^st-year crop stubbles, we adjusted the rate of fertilizer N applied when seeding the 2^nd-year crops to account for the residual soil N and the calculated pulse crop N credits. Thus far, relative to wheat

stubble, pea stubble has required 20 lb N/ac less, and chickpea and lentil 7 lb N/ac less, for recropping to an oilseed or cereal. Based on preliminary data, the yields of the 2^nd year crops have consistently ranked highest on pea stubble (despite pea stubble receiving the lowest levels of fertilizer N) and often lowest on wheat stubble.

It appears that soil N availability was underestimated for the pulse crop stubbles and that the wheat recrop is responding according to soil N availability (this hypothesis will be tested by examining total crop N uptake values when data is available). Mustard, on the other hand, appears to be responding to residual soil water, yielding highest on pea and lentil stubbles where soil water use was the lowest. We do not yet have sufficient data to explain the lentil yield response to the previous crop.

Strategy

Water is the key to developing good crop rotations in the Brown soil zone. We have to minimize the risks for when conditions are dry but must still have the ability to capture the rewards when good rains fall. Thus, the best rotations to combine good agronomic and economic performance are those that make most efficient use of water over a wide range of moisture conditions. Because wheat has the ability to perform well in wet to dry conditions while producing adequate residue for controlling erosion, we believe wheat will continue to be the backbone of crop rotations in the Brown soil zone. Given all the factors that must be considered in deciding what to grow, if anything, on a particular field, we don't suggest there is one ideal crop rotation. However, understanding generally how different crop sequences perform can help you do a better job in selecting the rotation for any one year.

From the water use patterns and yield responses reported here, it's clear that pulse crop stubbles can support a relatively productive wheat recrop. Pulse crops have done well on stubble and because they neither need the soil N or deep soil moisture stored in fallow, fallow acres are probably more profitably put into wheat or mustard (or canola). Mustard (and canola) is a deep rooting crop which grows well on fallow or often following shallow rooting crops such as pea and lentil. However, a pulse-mustard sequence would be back-to-back low-residue crops producing generally unacceptable soil erosion risks in tilled cropping systems (note that, in other research we have found good residue conservation for erosion control with total chem fallow after canola). Volunteer seedlings after mustard or canola can be a major problem in non-competitive pulse crops so wheat may be the better choice to recrop after oilseeds. Thus far, we have effectively controlled volunteer mustard seedlings in the 2^nd-year lentil crop with a timely split application of metribuzin (Table 4). Finally, providing the price outlook for spring wheat (or durum) is reasonable, those crops are good choices to end a crop sequence before fallowing.

Values with a column within a site followed by the same letter do not differ (P = 0.05).

* very low lentil yield due to a light volunteer wheat infestation (~2 plants/m²)

Acknowledgements

Agriculture and Agri-Food Canada Semiarid Prairie Agriculture Research Centre

Canada-Saskatchewan Agriculture Green Plan Agreement

Saskatchewan Agriculture Development Fund

Saskatchewan Pulse Growers' Association