Brian McConkey and Perry Miller

AAFC-SPARC, Box 1030, Swift Current, SK S9H 3X1

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.

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 accounts. 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 favorable moisture, wheat and mustard yields were likely limited by insufficient N supply. However, the yields of the properly inoculated pulse crops are 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 peas used less water, the water-use-efficiency was practically the same for both crops.

Table 1. Alternative crop yields averaged for 6 site-years grown on fallow and wheat stubble in a tillage experiment in southwestern Saskatchewan.

Fallow Stubble stubble/fallow

---------------- lb/ac ------------------------- %

dry pea 2640a 2320a 88

CWRS wheat 2580a 1700b 66

Mustard 1640b 980c 60

Desi chickpea 1460b 1110c 76

Lentil 1380bc 1090c 79

Safflower 850c 790c 93

Dwarf safflower 840c 760c 91

____________________________________________________________________________

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

Table 2. Yield, water use, and water-use-efficiency (WUE) by 5 crops from three soil depths when grown on fully recharged fallow at Swift Current and Stewart Valley, 1996-97 (prelim.data).

___________________________________________________________________________________________________________

Soil Depth

Lb/ac ------------------- inches of water -------------- kg/ha/mm

CWRS wheat 2770a 1.4a 0.8a 4.9a 9.5a

Yellow Pea 2350a 0.6c 0.3b 3.5b 9.2a

Desi chickpea 1710c 1.2ab 0.7a 5.1a 6.2b

Laird Lentil 1300cd 0.8c 0.3b 3.9b 4.8bc

Oriental Mustard 1270d 1.1b 0.8a 4.7a 4.4c

____________________________________________________________________________________________________________

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

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

Table 3. Yield and protein (basis 13.5% grain moisture) values for uniformly managed CWRS wheat on 7 crop stubbles from 1993 to 1997.

_________________________________________________________

Stubble Yield Premium

Bu/ac %

CWRS wheat 32b 12.1c

Dry pea 38a 13.0a

Lentil 37a 12.9 ab

Desi chickpea 36a 12.9 ab

Dwarf sunflower 33b 12.7 ab

Mustard 32b 12.8 ab

Safflower 31b 12.6 b

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

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 wheres oil water use was the lowest. We do not yet, have sufficient data to explain the lentil yield response to the previous crop.

Water is the key to developing good crop rotations in the Brown soil zone. We have to mininimize 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 response reported here, it's clear that pulse crop stubbles can support a relatively productive wheat crop. 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 zero-tillage practices) 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"'-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.

Table 4. Crop yields (prelim. data) for CWRS wheat, Oriental mustard and lentil when grown on stubble from desi chickpea, lentil, yellow pea, Oriental mustard and CWRS wheat, at Swift Current 1996-97 and Stewart Valley 1997.

1^st Year Crops 2^nd Year Crops

Wheat Mustard Lentil

Bu/ac ----------------lb/ac------------------

Swift Current 1996

Grande yellow pea 49a 1110a 1530a

Cheston desi chickpea - - -

Laird Lentil 44ab 950ab 1080b

Cutless Oriental Mustard 40b 780b 1240b

Katepwa CWRS wheat 41b 910ab 460c*

Swift Current 1997

Grande yellow pea 40a 1070a 1470a

Cheston desi chickpea 40a 980a 1180b

Laird Lentil 39a 1100a 1240b

Cutlass Oriental Mustard 41a 1170a 1150b

Katepwa CWRS wheat 39a 1070a 1100b

Stewart Valley 1997

Grande yellow pea 57ab 1690a 1660a

Cheston desi chickpea 59a 1290bc 1360a

Laird Lentil 59a 1630ab 1630a

Cutlass Oriental Mustard 52b 1250cd 1570a

Katepwa CWRS wheat 38b 920d 1410a

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

* very low lentil ield due to a light volunteer wheat infestation (~2 plants/m^{2 )}

Agriculture and Agri-Food Canada Semiarid Prairie Agriculture Research Centre

Canada-Saskatchewan Agriculture Green Plan Agreement

Saskatchewan Agriculture Development Fund

Saskatchewan Pulse Growers' Association