Making the Most of Limited Water

S. A. Brandt¹, B. G. M^cConkey² and H. W. Cutforth²

INTRODUCTION

In dryland agriculture on the Canadian prairies, water is the factor that most limits grain yield. This is particularly true for the Brown and Dark Brown soil zones where precipitation is low and potential evaporation is high. Even in the Black and Gray soil zones where moisture deficits are lower, moisture stress at some point in the growing season frequently limits yield.

While little can be done to alter precipitation, there are a number of management practices that can increase how effectively crops use what is received. In fact, almost anything that increases yield will improve water use efficiency. It is not our intent to deal with all of them, but rather to concentrate on a few issues that relate to conservation tillage and the viability of such practices.

Since we can't influence precipitation, efficient water use must be based on increasing the amount available as soil water at seeding and ensuring that it and growing season rainfall are used efficiently by the crop.

DISCUSSION

Improving water storage on summerfallow:

Summerfalllow was one of the earliest strategies developed to increase soil water prior to the planting of a crop. Improvements have largely been based on reducing water use by weeds and restricting water losses due to evaporation.

More than 60 years of progress in wheat production on fallow was summarized by Greb et al (1979). He described four major changes that occurred from 1916 to 1975 and predicted the change from 1976-90 (Table 1). In each case, tillage operations decreased while water storage and wheat yield increased. They attributed 45% or 11 bu/ac of the increase to moisture conservation. While this summary is from Akron, Colorado, similar trends occurred here, although the size of the changes were likely different.

At several locations in the Brown and Dark Brown soil zones, chemical fallow increased water storage over tillage fallow (Table 2.) Differences tended to be larger on heavier textured clay soils than on loam or sandy loam soils, reflecting the greater capacity of clays to hold water. In many cases, conventional fallow filled the sandy loam or loam soils to capacity and little improvement in moisture storage could be expected. A major limitation to improved water storage on summerfallow is the water holding capacity of the soil. Where tillage is eliminated and the rooting profile filled to capacity, further progress with fallow systems is unlikely.

Table 1. Progress in wheat-fallow systems at Akron, Colorado.

Table 2. Influence of fallow methods on available soil water and percent of precipitation conserved on fallow on Brown and Dark Brown Soils.

Tall cereal stubble not only catches snow but decreases evaporation at the soil surface. As shown in Table 3, using tall stubble on fallow can dramatically improve the water conservation on a heavy clay. The greatest advantage of the tall stubble for increasing moisture conservation occurred from spring to fall of the fallow year. This advantage occurred on both tilled and untilled fallow. Since the stubble was tilled under in late June for the minimum-tillage fallow, the main benefit of the tall stubble must have been to decrease evaporation during the spring when small rain showers are common. Note that snow trapping over the second winter on the chem fallow did not improve water conservation and, in fact, all fallows lost water over the second winter.

Table 3. Mean available water (mm) on a Brown heavy clay for four fallow periods as affected by straw management.

Improving soil water recharge for stubble cropping.

Research in stubble management has demonstrated the value of snowtrapping practices for increasing soil water at planting (Table 4). Similarly, snow management increased soil water recharge by an average of 13 mm for zero till spring wheat on stubble at Swift Current, Sask. (Campbell et al, 1986) and increased yield by an average of 1.2 bu/ac. Snow management is most effective on self-cracking clays because of their good snowmelt infiltration characteristics. In one study conducted in west-central Saskatchewan, snow management with tall stubble increased 4-yr average durum yields on stubble by 5 bu/ac compared with conventional height stubble.

Table 4. Field comparisons of stubble management practices on snowtrapping in Saskatchewan.

On Brown, Dark Brown and Thin Black soils, snow management alone did not enhance yield of continuous wheat, unless herbicides and fertilizers were used to control weeds and correct nutrient deficiencies (Table 4). Obviously adoption of a single water management technology in the absence of other complimenting technologies may not be beneficial. In this case, there was little benefit in improving water recharge if it was used by weeds or if nutrient deficiencies were more limiting than water. Where the technologies were used in combination, the yield response was greater than the sum of the technologies used alone.

Table 5. Influence of snow management, herbicides and fertilizer on yield of wheat grown continuously at Kindersley, Scott and Lashburn, Sask.

Few if any measurements have been made to evaluate the impact of tillage on losses of seedbed moisture between spring snowmelt and seeding. However in a comparison of zero with conventional tillage at Scott during 1979-96, it was noted that seedbed moisture was more favorable with zero tillage in most years. Where small seeded oilseeds, which require shallow seeding are grown, this can be a critical factor in crop establishment. In the same study, emergence rates for oilseeds were often 10-15% higher with zero than conventional tillage.

Other factors that could influence recharge of soil water between harvest and seeding the next spring are use of water by weeds, maturity of the preceding crop and moisture level of the soil going into winter. Relatively little is known about water use by perennials and winter annuals after harvest and in spring prior to seeding. However, with the growing trend toward total reliance on preseed burnoff for control of winter and early spring annuals, there is renewed need to know their water use characteristics. Depletion of seedbed moisture, while not having a large impact on total water supplies, could affect establishment of some crops and have a disproportionately large effect on yield.

There is ample evidence that earlier maturing crops permit earlier recharge of soil water from late summer and early fall rains. However, the benefits from greater soil water recharge at this time are often not realized. Greater surface soil water at freeze up in late fall often reduces infiltration of snowmelt water the following spring, resulting in variable responses of soil water at seeding.

Improving water use efficiency during crop growth.

Fertility and weed control are typically the factors that have the greatest influence on water use efficiency during crop growth. Fertility influences the ability of the crop to produce the photosynthates required for seed development. When crop fertility requirements are not met, water use continues but grain yield is reduced.

Adequately fertilized crops not only use water more efficiently but also tend to be more effective in extracting water and nutrients from the soil (Table 5). In the process, crops adequately fertilized with N actually reduce the risk of leaching of nitrates into groundwater.

Table 6. Soil nitrates and available water to 1.2 m at harvest 1991 and soil nitrates from 1.2 to 2.4 m after a 10-yr N fertilizer rate trial on a Brown loam at Swift Current (adapted from Campbell et al. 1993).

Weeds compete with the crop for moisture that would otherwise be available for crop growth. It is critical that the fertility and weed control needs of the crop are met. Other management practices directed toward improving crop water use are unlikely to reach their full potential if these requirements are not met.

In a long term tillage study at Scott, Saskatchewan, water use efficiency of zero till wheat was only slightly higher than conventional till during the first 6 years of the study (Table 6). In subsequent cycles, water use efficiencies of zero till wheat improved steadily compared to conventional tillage. The change over time reflected improved weed control and nitrogen fertilizer rate and placement for the zero till crops over time. A similar trend occurred with the oilseeds except that water use efficiencies were initially lower for zero than conventional tillage. This reflected some of the serious weed control problems with zero till oilseeds in early years of the study, which have been largely overcome in recent years.

Seeding spring wheat into stubble left standing as tall as practical can increase water use efficiency and yield. At Swift Current, seeding wheat into tall (>30 cm) stubble increased grain yield and water use efficiency by about 12% compared to wheat seeded into cultivated stubble (Table 7). The yield and water use efficiency for wheat seeded into short (about 15 cm high) stubble were intermediate between cultivated and tall stubble. Total seeding-to-harvest evapotranspiration was not affected by stubble

height. Standing stubble reduced the evaporative demand for water at the soil surface throughout the growing season. The evaporative demand was 30% less for the tall stubble than cultivated stubble before the wheat four-leaf stage and was still 24% less when the wheat was headed. On average, the short stubble only reduced evaporative demand by 6% compared with cultivated stubble. The tall stubble increased the proportion of soil water that was used productively by the crop and thereby increased the water use efficiency and yield.

Table 7. Water use efficiencies (bu/ac/in) of Wheat and oilseeds (flax and canola) in a long term comparison of zero with conventional tillage at Scott, Sask.

Table 8. Grain yield, evapotranspiration and water use efficiency for spring wheat grown on cultivated, short and tall stubbleat Swift Current, Sask. 1992-95.

In the drier Brown soil zone, crop rotation has had relatively little impact on water use efficiency. Water use efficiency tends to be slightly higher for wheat grown on summerfallow than on stubble. For wheat on stubble, water use efficiency does not depend on the preceding crop. At the same time, precipitation use efficiency of wheat on fallow is quite low because precipitation storage on summerfallow is inefficient.

On the Dark Brown soils, a different picture emerges. Compared to results in the Brown soil zone, precipitation use efficiency for wheat on fallow tends to be even lower, because summer fallow is less efficient for precipitation storage (Table 8). Where wheat is grown on stubble, the preceding crop can have a much greater impact on precipitation and water use efficiencies. For example, yields, precipitation and water use efficiencies for continuous wheat were lower than for wheat grown on pea or canola stubble. They were even lower for wheat on wheat stubble two years after a pea crop. Also, yields of wheat on plots that were continuous wheat since 1963 were higher than yields from plots that were continuous wheat since 1987. Apparently, long term continuous cropping has enhanced soil quality. Similar observations have been made for Black and Gray soils of Saskatchewan and Manitoba were cereals have yielded more when grown on oilseed or pulse stubble compared to cereal stubble, and similarly, oilseeds and pulses have yielded more when grown on cereal stubble compared to oilseed or pulse stubble(Townley - Smith 1990, Bourgeois and Entz, 1996).

Table 9. Grain yield, precipitation and water use efficiencies of crops grown in rotations on a Dark Brown soil at Scott, Sask. 1992-1996.

There is evidence that early May planting of pulse crops (pea and lentil) increases yield and water use efficiency, particularly in the drier and hotter conditions of the Brown soil zone. In the Dark Brown soil zone, yield and water use efficiencies of pea, lentil and canola were higher when sown early to mid May than for late May. It would appear that earlier planting allows these crops to develop through the critical flowering and early seed development stages under generally cooler temperatures and more favorable soil water conditions although earlier canopy development may also restrict evaporation losses from the soil surface.

More rapid development of a crop canopy that shades the soil surface and reduces evaporation losses may also explain the observed 12% decrease in yield and water use efficiency of direct seeded crops in 12 inch compared with 9 inch row spacings at Swift Current.

SUMMARY

Making efficient use of limited water is based on using good agronomic practices appropriate for conservation tillage. Summerfallow does not make very efficient use of precipitation, but economic constraints may make it difficult to eliminate in the driest areas of the prairies. However we should all be aware that continued frequent use of this practice may ultimately degrade such soils to the point where it is no longer economic to use them for annual grain crop production. For this reason we should not be deterred in our resolve to develop and adopt practices that will reduce the need for summerfallow.

During the period between crops, leaving stubble standing to trap snow and residue cover to reduce evaporation typically increases soil water at planting of the next crop.

Adequate weed control and fertility must be emphasized as the basis for efficient crop water use. Other efficient water use practices such as direct seeding, rotations of cereals, oilseeds and pulses, selecting proper timing of seeding operations or plant spacings build on these technologies to enhance water use. The end result is a production system that is appropriate to the soil and climatic limitations of your farm and is based on sound agronomic practices.

References

Bourgeois, L. and Entz, M. H. 1996. Influence of previous crop type on yield of spring wheat. Analysis of commercial field data. Can. J. Plant Sci. 76: 457-459.

Brandt, S. A. 1992. Zero vs Conventional tillage and their effects on crop yield and soil moisture. Can. J. Plant Sci. 72: 679-688.

Brandt, S. A. and Kirkland, K. J. 1991. Yield of continuous spring wheat with several combinations of stubble, weed control and fertility management. P. 463-472. IN. Economics of Prairie Agriculture in the 1990's. Proc. 1991 Soils and Crops Workshop - University of Saskatchewan, Saskatoon, Sask.

Campbell, C. A., Read, D. W. L., Biederbeck, V. O. and Winkelman, G. E. 1993. The first 12 years of a long-term crop rotation study in southwestern Saskatchewan - nitrate N distribution in soil and N uptake by the plant. Can. J. Plant Sci. 63: 563-578.

Campbell, C. A., Nicholaichuk, W., Zentner, R. P. and Beaton, J. D. 1996. Snow and fertilizer management for continuous zero-till spring wheat. Can. J. Plant Sci. 66: 535-551.

Gray, D. M. 1991. Snowcover distribution, wind transport and snow management practices. IN: Handbook of Hydrology, Draft, Division of Hydrology, University of Saskatchewan, Saskatoon, Sask.

Greb, B. W., Smika, D. E. and Welsh, J. R. 1979. Technology and wheat yields in the central Great Plains: Experiment Station Advances. J. Soil and Water Conserv. 34: 264-268.

Lindwall, C. W. and Anderson, D. T. 1981. Agronomic evaluation of minimum tillage systems for summerfallow in southern Alberta. Can. J. Plant Sci. 61: 247-253.