Wind Speed, Stubble Heights, and Moisture Conservation

Brian McConkey, Herb Cutforth, Dan Ulrich, and Perry Miller

Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-food Canada

P.O. Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2

ph (306) 778-7200 fax (306) 773-9123

Abstract

We seeded spring wheat, lentil, field pea, and chickpea into cultivated, short (6 to 7 inch), and tall (10 to 16 inch) spring wheat stubble. All plots overwintered as tall stubble to equalize snow trapping effects so that we could determine the in-crop effects only. Standing stubble changed the microclimate near the soil surface by reducing soil temperatures, solar radiation, wind speed, and potential evapotranspiration. Standing stubble effects on microclimate continued well beyond the time when the taller crops grew above the stubble. These effects were more much pronounced for tall versus short stubble. Spring wheat, lentil, chickpea, and field pea all grew about 2 to 3 inches taller with tall stubble than with cultivated stubble. The lowest pod height was 0 to 1.5 inches higher with short stubble than with cultivated stubble and about an additional 1.5 inches taller than short stubble when grown in tall stubble. Crop water use efficiency generally increased as the stubble height increased. The exception was chickpea for which yield and crop water use efficiency appeared to decline when grown in tall as compared with short stubble. This may be due to shading effects on the relatively small chickpea canopy that grew entirely within the tall stubble. Compared with cultivated stubble, the average yield advantages for seeding into short stubble was 6%, 10%, 9%, and 12% for spring wheat, chickpea, field pea, and lentil, respectively, while the corresponding yield advantages for seeding into tall stubble were 12%, 5%, 9%, and 21%.

1. Introduction

The potential evaporative demand for water usually exceeds the water available to the crop representing the greatest limitation to crop production on the Canadian prairies. Low disturbance direct seeding into standing cereal stubble is the most effective practice to reduce that limitation for dryland agriculture. Lafond et al. (1992) have attributed much of the yield benefit of direct seeding to the extra water conserved by trapping snow with standing stubble once fall tillage is eliminated. The snow trapping potential of cereal stubble is directly proportional to its stubble height (Steppuhn 1994). Standing cereal stubble also reduces evaporation of soil moisture (Caprio et al. 1985) effectively making more water available to the crop. Further, standing stubble creates a more favourable microclimate for crops by reducing wind, solar radiation, and evaporative demand for water (Aase and Siddoway 1980).

Our objective was to determine how much standing cereal stubble changes the in-crop microclimate relative to seeding into a cultivated seedbed and, more importantly for those who are already direct seeding into standing cereal stubble, how stubble height affects the microclimate and crop growth.

2. Materials and Methods

We are reporting on two separate experiments, one conducted from 1992 to 1996 involving only spring wheat and the second conducted in 1996 and 1997 involving lentil, desi chickpea, and field pea (the latter experiment will continue in 1998). The spring wheat experiment is described in detail by Cutforth and McConkey (1997).

These experiments were conducted on a Swinton loam soil (Orthic Brown Chernozem) at the Agriculture and Agri-Food Canada Semiarid Prairie Agricultural Research Centre (SPARC), Swift Current. There were three treatments imposed on spring wheat stubble: tall, short, and cultivated. So as only to measure the in-crop effects of stubble treatments, all treatments overwintered as tall stubble to equalize snow trapping then the treatments were applied to in the spring before seeding. The cultivated treatment was worked slowly (<3 mph) with a tandem disc followed with a harrow-packer -- burying the about one-half of the stubble with the remaining residue lying flat on the soil surface (we judged the disturbance of these slow tillages to be about the same as produced with one pass at 6 mph with a heavy duty cultivator and mounted harrows). The short stubble treatment was cut with a forage harvester without windrowing to a height of 5.5 to 7 inches. The height of the tall stubble varied from year to year depending on the height and harvest condition of the preceding spring wheat crop and were 16.5, 12, 13.5, 16.5, 13, and 9.5 inches in 1992, 1993, 1994, 1995, 1996, and 1997, respectively. Stubble treatment plots were 40 by 40 m with measurements taken only near the plot centre.

Spring wheat ('Lancer') was seeded at 60 lb/ac in mid to late May with a prototype no-till offset disc drill (Dyck and Tessier 1986) with 7-inch row spacing. The pulse crops were seeded with the above drill in 1996 and with a Flexi-Coil 5000 with Stealth knives on a 9" row spacing in 1997. Seeding rates were 83 lb/ac for lentil ('Laird'), 115 lb/ac for desi chickpea ('Cheston'), and 157 lb/ac for field pea ('Grande'). Based on general fertilizer recommendations for our area, all spring wheat treatments were fertilized with 40 lb N (actual)/ac broadcast immediately before seeding and with 18 lb P (P₂O₅ equivalent)/ac seed placed. Pulse crops were fertilized with 15 lb P (P₂O₅ equivalent)/ac seed placed. Pulse crops were inoculated with appropriate peat-based inoculants applied to the seed. Tall and short stubble treatments were usually sprayed with glyphosate before seeding for initial weed control. Pulses were treated with granular Edge (spring applied for 1996, fall applied for 1997). All plots were sprayed with recommended post-emergent herbicides, as required. The desi chickpea and lentil were sprayed with a fungicide (Bravo 500) as required to control in-crop ascochyta.

Spring wheat yields were measured with a plot combine. Pulse yields were measured with a full-size combine (MF 550) after swathing (except chickpea in 1997 that was straight cut). Prior to swathing the pulse crops, the natural plant height, pulled plant height and lowest pod height were measured. Following combining, we counted seeds on the ground both on the swath and between the swath to determine the compound swathing, pick up, and threshing losses.

Soil water was measured just before seeding and after harvest to 1.2 m. From these measurements, evapotranspiration (ET), or water use, was calculated:

ET = (soil water at seeding - soil water at harvest) + growing season precipitation.

Water use efficiencies (WUE) were calculated:

WUE = Grain yield / ET.

For the spring wheat experiment, we measured water loss from four minilysimeters (white pvc tube 10.5 cm dia. x 12 cm deep) filled to 1 cm of the top with pea-sized gravel and water and placed in the soil with the pea gravel level with the soil surface. We measured solar radiation at 7.5 cm above the ground and radiation reflected back to the atmosphere. We also measured soil temperatures at depths of 5- and 30-cm and air temperatures and wind speed at 15- and 100-cm above the soil surface.

3. Results

3.1 Weather

During the growing season (May to August) temperatures ranged from near to slightly below long-term averages (Table 1). Precipitation during the months of May, June and July are particularly important to crop yield at Swift Current and in all years precipitation in these months were near to above long-term averages.

Table 1 . Average monthly temperature and monthly total precipitation for the spring and summer months of 1992 through 1997, and the long-term (109 yr) means.

3.2 Microclimate

The minilysimeters provide an index of potential evapotranspiration. From shortly after seeding to about mid-grain filling (the duration of data collection), the tall stubble significantly (P<0.05) reduced the rate of water loss from the minilysimeters compared with the cultivated and short stubble treatments (Table 2). There was no significant difference in rate of water loss between short and cultivated stubble treatments, although the rate of water loss tended to be lower for the short stubble.

Prior to and including the flag leaf emerged stage, the average daily wind speed was significantly less for the tall stubble compared to the short stubble and cultivated stubble (bare soil) treatments, irrespective of height at which measured. The short stubble and cultivated treatments were only significantly different (P<0.05) before the 3.5 leaf stage at the 15 cm height. At this height all three treatments were significantly different (P<0.05) with wind speeds highest for the cultivated treatment. Before the 3.5 leaf stage, compared to the cultivated treatment, tall stubble reduced wind speed by approximately 70% at 15 cm and 10% at 100 cm; short stubble reduced wind totals at 15 cm by about 15%. We are unable to explain why wind totals were lower for the cultivated treatment than for the short stubble treatment at 100 cm during flag leaf emerged, heading and maximum plant height growth periods. This observation was generally consistent across years (data not shown) and may be due to turbulence.

Before the 3.5-leaf stage, the daily total amount of incoming solar radiation measured at 7 cm above the soil surface was lowest for the tall stubble (Table 2); but thereafter, as plants grew all treatments became more similar. Reflected shortwave radiation was highest for the short stubble treatment and lowest for the cultivated treatment, regardless of measurement time. This is explained from reflection from residue lying on the soil surface; the short stubble treatment had the greatest amount of such residue, the cultivated treatment had the least.

Across measurement height and growth period, average daily air temperatures were not significantly different between stubble treatments, although values tended to be lower in tall stubble. However, average daily soil temperatures generally decreased (P<0.05) with increasing stubble height. Treatment differences for soil temperatures persisted well beyond the time when the wheat grew above the tall stubble.

3.3 Growth Characteristics

Plant populations and crop phenological development rates were not significantly affected by stubble treatment (data not shown).

All the crops grew taller as stubble height increased but only tall stubble provided practical increase in crop height above the cultivated treatment (Tables 3 and 4). The height of the lowest pod on the pulses also increased as stubble height increased but unlike total plant height, there was an apparent height advantage of short stubble compared with cultivated stubble (Table 4). We expected the increased pod height would reduce harvest losses significantly but measured losses were negligible (Table 5), probably because all the crop stands were uniform and the lowest pods were always sufficiently high for satisfactory mechanical harvesting. Nevertheless, the total harvest losses were lowest when seeded into tall cereal stubble.

Table 2. Daily average wind speed, air and soil temperature, daily total incoming and reflected solar radiation, and evaporation for spring wheat for 1992-95.

Values in the table are averaged over the years indicated in the title. Also, for a given growth period and a given height, letters indicate significant differences between stubble treatments at P=0.05.

Letters indicate significant differences between stubble treatments at P=0.05.

For the given year, letters indicate significant differences (LSD) between stubble treatments at P=0.05.

Table 5. Harvest losses for desi chickpea, field pea, and lentil grown in cultivated, short and tall stubble.

For the given year and species, letters indicate significant differences between stubble treatments at P=0.05.

For the spring wheat we collected more detailed measurements of plant growth. The

stubble treatment did not change the shape of the leaves since the specific leaf area (average 230 cm²/g) was not significantly affected by treatment. As would be expected for a taller plant, the wheat grown in tall stubble had a greater proportion of plant mass as stems and less as leaves. Nevertheless, the plants tended to have more total leaf area per unit ground area as the height of stubble they were seeded into increased. Although harvest index (i.e. grain/total plant mass) tended lower for the wheat grown in tall stubble, because there was more biomass, the grain yield for wheat grown in tall stubble was significantly higher (Tables 6 and 7).

3.4 Grain Yield, Evapotranspiration and Water Use Efficiency

In this semiarid climate, typically all available soil water and growing season precipitation is consumed as evapotranspiration (ET), or water use. Thus, since we purposely designed the experiments to equalize snow trapping and thereby available soil water at seeding, there were no consistent effects of stubble treatment on total ET (Tables 6 and 7). In 1995, there was a small but statistically significant stubble effect on ET reflecting a small difference in soil water at seeding (data not shown) that was, in turn, probably the result of small differences in snow capture and infiltration among plots in that year.

For all crops, there was a consistent yield benefit to direct seeding compared with seeding into the cultivated seedbed. For spring wheat and lentil, there was also a consistent yield benefit of seeding into tall as opposed to short wheat stubble. However, for the field pea and chickpea, there was no apparent advantage of seeding into tall stubble versus short stubble and, in fact, for chickpea, there was an apparent yield disadvantage to seeding into tall stubble versus short stubble.

The effects of stubble configuration on crop water use efficiencies (WUE) followed the same pattern as for grain yields. For all crops, the WUE was higher for direct seeding into standing wheat stubble than seeding into a cultivated seedbed. Except for chickpea, the WUE was also higher for seeding into tall versus short stubble. There are four main reasons that would explain the increased WUE for direct seeding in general and for seeding into tall stubble in particular. First, compared to cultivated stubble, standing stubble reduced evaporation from the soil surface by reducing incoming, wind speeds, and soil temperature. Thus, for the same total ET, this increased transpiration. Second, for a given amount of transpiration, the reduced potential evaporation within the stubble allow the crop stomates to remain more open for a longer period thereby increasing photosynthesis. Third, with moisture stress reduced, the tendency for the plants grown in stubble to have more photosynthetically active area would increase total photosynthesis. Fourth, and finally, reducing wind speed on the plant surfaces decreases transpiration and respiration more than photosynthesis, thus, increasing WUE (Waggoner 1969; Hagen and Skidmore 1974). The individual contribution to increased WUE for tall versus short stubble and for short versus cultivated stubble of each of the above four mechanisms would be relatively insignificant. However, when combined, the environmental alterations due to stubble height increased grain yield and WUE. Because the effect of standing stubble acted through several different mechanisms, the percentage increases in WUE and yields were remarkably similar among years -- including both "wet" years like 1993 and drier years like 1994.

Table 6. Evapotranspiration, grain yield and water use efficiency for spring wheat grown in cultivated, short and tall stubble.

For the given year, letters indicate significant differences between stubble treatments at

P =0.05.

For the four crop species studied, the proportionate benefit of direct seeding into tall versus short stubble increased as harvest index decreased (Figure 1). In particular, chickpea had lower average yield in tall compared with short stubble. As an adaptation to growing under water limited conditions, chickpea has minimized its plant canopy relative to its grain production as indicated by its high harvest index. Thus, there would not be the same growth benefit of sheltering the smaller chickpea canopy in tall stubble compared with a plant of similar height, like lentil, that has a more luxuriant canopy relative to its grain production. Further, the small leaf area of chickpea per unit of grain production combined with the fact that the chickpea grew entirely within the tall stubble would emphasize the reduction in photosynthesis due to shading by the tall stubble. Growing under well watered conditions, there would have been more than enough sunlight within standing cereal stubble for plants that use the C3 photosynthetic pathway (i.e. all the crops involved in this study) to reach their yield potential. However, when growing under water limited conditions, a disproportionate amount of the photosynthesis takes place under low sunlight conditions during cloudy days and in the morning hours just after sunrise. Thus, for chickpea grown in the Brown soil zone, the shading effect of tall stubble could limit photosynthesis and thereby WUE and grain yield relative to when grown in short stubble.

The general negative relationship between harvest index and the yield benefit of tall stubble versus short stubble suggests that canola growth could be particularly responsive to the enhanced microclimate produced by tall stubble (Figure 1). We are pursuing this research at SPARC.

Although production within tall cereal stubble was beneficial for increasing the yields of spring sown crops in a water limited environment, it could be detrimental for under other circumstances. The more humid environment within the standing stubble could increase leaf and stem diseases. This has been observed for winter wheat in Montana (Aase and Siddoway 1980). Under wet conditions, tall stubble could also slow drying of the soil sufficiently to seriously delay seeding. Finally, the shading effect of the stubble could also limit yields although the this effect should only be important for short crops that never grow above the stubble or winter crops in high latitudes that, as seedlings, grow under low sun angles. In a study involving winter wheat in Oregon, Wilkins et al. (1988) attributed shading as the cause of poorer early growth within tall stubble compared with the better early growth with no stubble.

4. Summary and Conclusions

Standing cereal stubble changed the in-crop microclimate near the soil surface by reducing soil temperatures, solar radiation, wind speed, and potential evapotranspiration. These effects were more much pronounced for tall versus short stubble. Crop water use efficiency generally increased as the stubble height increased. The exception was chickpea for which the shading effects on the relatively small chickpea canopy that did not rise above the tall stubble may have limited grain yield and water use efficiency compared with short stubble. Compared with cultivated stubble, the average yield advantages for seeding into short stubble was 6%, 10%, 9%, and 12% for spring wheat, chickpea, field pea, and lentil, respectively, while the corresponding yield advantages for seeding into tall stubble were 12%, 5%, 9%, and 21%.

Figure 1. Yield benefit of seeding into tall relative to short cereal stubble versus crop harvest index.