Agronomy Journal 92:808-814 (2000)
© 2000 American Society of Agronomy
SPARSE CANOPY SYMPOSIUM INTRODUCTION
Optimizing Crop Water Use in Sparse Stands of Pearl Millet
William A. Payne
Texas Agricultural Experiment Station, 6500 Amarillo Blvd. West, Amarillo, TX 79106 USA
w-payne{at}tamu.edu
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ABSTRACT
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Most of current theory on crop water use–yield relations has been developed for intensively managed crops with well-developed canopies. In most agricultural systems of semiarid West Africa, low input levels and endemic environmental stress predominate. In farmers' fields, leaf area index (LAI) of the staple grain, pearl millet [Pennisetum glaucum (L.) R. Br.], may never reach 1. In contrast to dense canopied crops, pearl millet yield is little correlated with ET . Reduced LAI decreases ET efficiency (kg dry matter mm-1 evaporation from crop and soil surfaces) because evaporation (E) from the soil surface constitutes a large portion of ET. Additionally, atmospheric water vapor pressure deficit (e*-e) increases within sparse canopies due to sensible heat transfer from the soil surface, and small and irregular roughness length of the canopy. Greater (e*-e) further decreases crop T efficiency (kg dry matter mm-1 transpiration) and therefore ET efficiency. Under low-input conditions, pearl millet ET efficiencies are roughly one-third of those obtained under intensive management, suggesting that T efficiency is also reduced by environmental stress, especially soil nutrient deficiency. Environmental stresses also cause poor root development, which results in reduced crop water supply, and increased resistance to water uptake. Optimizing crop water use of sparse pearl millet stands will require some form of nutrient input. Other appropriate technologies include certain forms of intercropping and agroforestry that have been traditionally practiced in parts of West Africa. These can improve soil nutrient availability, increase effective crop cover, and reduce canopy (e*-e).
Abbreviations: LAI, leaf area index • E, evaporation from the soil surface • ET, crop transpiration + E • e*-e, atmospheric water vapor pressure deficit • ET efficiency, kg dry matter/ET • T efficiency, kg dry matter/T • z0, crop roughness length • Y, crop yield • m, crop water use coefficient
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INTRODUCTION
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Pearl millet is a staple cereal in the semiarid West African tropics, where such environmental stresses as drought, high temperature, and poor soil nutrient availability are endemic. Subsistence farmers typically plant pearl millet at very wide spacings as a perceived measure to reduce risk of crop failure. As a result of these factors, maximum LAI of pearl millet in most farmers' fields may never reach 1 (e.g., Rockström, 1997). Even in more intensively managed fields, LAI seldom exceeds 2, and the period during which LAI exceeds this value constitutes only a small portion of the entire growth period (Fig. 1)
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Fig. 1 Leaf area indices (LAI) of pearl millet grown with nominal fertilizer inputs at a population of 30000 plants ha-1. The experiment was located within a windbreak shelter of Neem (Azadirachta indica A. Juss) trees. Even with these added inputs, LAI never reached 2. Redrawn from Wallace et al. (1993)
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Yield-et relations of pearl millet
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One striking feature of low LAI-pearl millet in semiarid West Africa is that grain yield is largely independent of ET (Fig. 2) . Within any given year, although yield responds strongly to a wide range of management options, ET is relatively conservative. Not surprisingly, therefore, it has been difficult to predict pearl millet yield from ET in West Africa, e.g., in Senegal (Cissé and Vachaud, 1988) and Niger (Klaij and Vachaud, 1992). The lack of a linear relation contrasts yield/ET data of many dense canopied crops, for example, those of grain sorghum [Sorghum bicolor (L.) Moench] (Stewart, 1989) and maize (Zea mays L.) (Hanks, 1983).
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Fig. 2 Relation between pearl millet grain yield and total (soil + crop) water evaporation (ET) in 1983, 1984, 1985, and 1990 at Sadoré, Niger. Total ET was very conservative within any given year, whereas yield varied according to level of management and amount of rainfall. From Payne (1997)
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The poor correlation between yield and ET is largely due to the relative insensitivity of ET to management for crops with leaf area indices <2 (Ritchie, 1983). For predictive purposes, Forest et al. (1990) and Payne (1997) used different empirical linear equations to relate pearl millet yield to ET as management intensity changed. Forest et al. (1990), for example, estimated pearl millet yields in Senegal using values of 3 kg grain ha-1 mm-1 ET for the best of farmers' conditions, and 10 kg ha-1 mm-1 for intensive, researcher-managed conditions. Such empirical equations, developed using qualitative categorization of management intensity, may have practical applications, such as forecasting regional food production. However, they are of limited scientific value.
When LAI is small, a large fraction of solar radiation is intercepted by the soil surface rather than by the plant canopy. Therefore E typically constitutes a much larger fraction of ET than would normally be observed in dense canopy crops, especially when the soil surface is wet (Ritchie, 1972). For this reason, in sparse stands of pearl millet in semiarid West Africa, transpiration constitutes a relatively small fraction of ET (Wallace et al., 1993; Daamen, 1997).
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Yield-t relations of pearl millet
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Diffusivity profiles of (e*-e), much like those of wind speed and temperature, are determined by turbulence transfer due to frictional retardation of the wind as it passes over the soil or canopy. Diffusivity profiles are generally calculated from empirical equations that are usually developed for ideal surfaces, rather than from a consideration of canopy architecture (Saugier, 1977). An example for windspeed is
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where 
is windspeed at the top of the canopy, u* is the `eddy' or `friction velocity,' z is the height of measurement, d is the zero plane displacement, k is von Karman's constant, and z0 is the "roughness length" of the crop (Monteith, 1973; De Wit, 1978). The effectiveness of turbulent transfer over a particular surface is related directly to z0 (Thom, 1975).
Although diffusivity profiles cannot easily be calculated for sparse vegetation with irregular z0 (Campbell, 1977), intuitively one would expect z0 to be greater and less variable in dense canopies, where only small eddies can develop and less mixing occurs, compared to z0 in sparse canopies (Saugier, 1977). The profile of (e*-e) within a sparse canopy can therefore be expected to be more heavily affected by soil, wind, and other conditions. For example, in the presence of a dry soil surface, greater momentum flux density results in greater sensible heat dissipation from the soil, and therefore greater within-canopy (e*-e) (Ritchie, 1972; Hanks et al., 1971). The topic of mass and energy exchange in sparse or heterogeneous canopies has been treated in much more detail by Wallace and Verhoef (1999), Kustas and Norman (2000)(this issue), and Daamen and McNaughton (2000)(this issue).
It has long been recognized that increased atmospheric evaporative demand reduces T efficiency, (e.g., Kiesselbach, 1910; Briggs and Shantz, 1913). De Wit (1958) used mean daily free water evaporation to relate crop growth to transpiration for high radiation environments with the equation
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where Y is dry matter production, T is transpiration, m is a crop coefficient, and E0 is mean daily evaporation. Drawing upon transport theory for leaf CO2 and H20 exchange, Bierhuizen and Slatyer (1965) modified Eq. [2] for field crops by substituting (e*-e) at canopy height for E0:
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Thus, as (e*-e) increases in sparse canopies relative to that of dense canopies, T efficiency, or Y/T, is linearly reduced. This added "clothesline" affect (Ritchie, 1983, citing Tanner, 1957) is manifested by data of Azam-Ali et al. (1984) for pearl millet (Fig. 3)
. As plant spacings increased from narrow to wide spacings, the slope Y/T decreased. Data of Payne (1997) showed similar trends.
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The coefficient m
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The coefficient m in Eq. [3] is equivalent to the experimental approximation of "field k" that Tanner and Sinclair (1983) used to test a more sophisticated transpiration model for a dense
crop canopy with sunlit and shaded leaves. De Wit (1958), Tanner and Sinclair (1983), and others have concluded that m is largely dependent upon the crop species' photosynthetic pathway. However, as discussed below, there is an increasingly large body of evidence to suggest that m is also affected by environmental factors, including those which can be influenced by management. This is especially of interest for pearl millet in the Sahel, because many of the stresses that cause poor canopy development may also affect m.
One environmental stress is drought. De Wit (1958) recognized that m of drought-stressed plants was likely to change when much of the dry matter was produced during periods of partial stomatal closure. Such a scenario would likely occur in drought-prone areas of West Africa, such as the northern latitudes of the Sahel region (Do et al., 1996; Winkel et al., 2000). In both large (Payne et al., 1992) and small (Boukar et al., 1996; Brück et al., 2000) container experiments, as well as in a field experiment (Payne et al, 1995), drought increased pearl millet m. Some of the physiological responses that might increase m during water stress include increased conversion efficiency of photosynthates to biomass because of greater starch production (McCree et al., 1990) and the proportionately greater effect of partial stomatal closure on reducing H20 flux than on reducing CO2 flux (Nobel, 1983).
Many studies in West Africa have concluded that low soil fertility is more of a production constraint than low water supply ( Cissé and Vachaud, 1988; Payne et al., 1990a; Klaij and Vachaud, 1992; van Duivenbooden and Cissé, 1993). Therefore, the effect of plant nutrition on m is of particular interest. It was recognized long ago that m decreases when nutrient deficiency is severe (Briggs and Shantz, 1913; De Wit, 1958). But when is it severe enough to reduce m?
A somewhat arbitrary assumption, based on pot studies, has been that m is unaffected until nutrient deficiency has reduced dry matter yield to about half that obtained in "well-fertilized soil" (De Wit, 1958; Tanner and Sinclair, 1983). Since both Payne (1997) and Forest et al. (1990) estimated that pearl millet ET efficiencies in farmers' fields were about one-third those reported for intensive management conditions, this would suggest that m is reduced under farmers' conditions in West Africa.
Of the plant essential minerals, P appears to be the most critically lacking in West African soils (e.g., Hafner et al., 1993). The reduction of m due to phosphate deficiency was recognized in early studies by Briggs and Shantz (1913). For pearl millet, a reduction in m due to phosphate deprivation was well correlated with the ratio of photosynthesis to transpiration (Payne et al., 1992). In that study, an increase in internal CO2 concentration was observed as photosynthesis rates decreased, suggesting metabolic rather than stomatal limitation (Payne et al., 1996a). Using C isotope discrimination, Brück et al. (2000) identified CO2 leakage from the bundle sheath chloroplasts as a probable contributing factor to decreased m in pearl millet plants subjected to phosphate deficiency.
The availability of other plant-essential nutrients also appears to affect pearl millet m. Boukar et al. (1996) reported an increase in pearl millet m due to increased soil N supply. This is consistent with results for sorghum (Onken and Wendt, 1989) and wheat (Triticum aestivum L.) (Parameswaran et al., 1981). In a field study, Payne et al. (1995) found that the addition of nutrients in the form of manure also increased m.
It seems plausible that other environmental stresses can also affect pearl millet m under semiarid West African growing conditions. Boukar et al. (1996) found that infestation by the angiosperm root parasite Striga hermonthica reduced pearl millet m. The ability of plants to resist Striga infestation and maintain greater m values was dependant upon N availability. The results of Boukar et al. (1996) were consistent with reports by Press et al. (1987), who found proportionally greater reduction of photosynthesis than stomatal conductance due to S. hermonthica infestation.
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Rooting systems of sparse canopied crops
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Because of the close linkage between shoot growth and root growth, environmental stresses that lead to poor canopy development can also lead to poor root development. This affects water relations because plants have a smaller soil volume from which to extract water (Cissé and Vachaud 1988; Payne et al. 1990a), and increased resistance to water uptake (Campbell, 1977). Payne et al. (1996b) observed a pronounced increase in root length density of pearl millet in most soil depth layers in response to P application. Relative increases in root length were greatest for the deeper soil layers, from which water would be needed in the event of drought. Increased rooting depth and root length density have been associated with reduced percolation of soil water beyond the root zone (Payne et al., 1990a; Payne and Brück, 1996) and greater soil water extraction from upper (Payne et al., 1996b) and lower (Payne and Brück, 1996) soil layers during dry spells.
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Suggestions for improved management of sparse stands of pearl millet
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There are several ways by which water use can be optimized in sparse stands of pearl millet. They vary according to economic and social practicality, and to their level of management intensity.
Improve Nutrient Supply
From a purely technical point of view, the easiest way to improve crop water relations of sparse canopied pearl millet would be to simply make the canopies denser. Since nutrient deficiency is seen as a key factor to the efficient use of water for pearl millet production, any practice which increases nutrient availability—even marginally—is likely to increase canopy density and soil cover, and possibly m. Small increases in P availability have had dramatic effects upon pearl millet production (Bationo et al., 1989). The easiest way to increase soil nutrient availability would be to add mineral fertilizer. But even when it is available, mineral fertilizer application has not always been found to be economically feasible for relatively low-value crops such as pearl millet.
Traditionally, soil nutrient supply in semiarid West Africa has been managed by addition of manure, or by leaving lands fallow for several years to rebuild soil fertility. With today's population pressures, however, fallow is becoming less and less practiced. Obviously manure and other sources of organic matter should be used to the extent possible, but in semiarid Niger, 5 Mg ha-1 of cow manure were required to obtain pearl millet yields equal to those obtained with a mere 20 kg ha-1 of single superphosphate (Bationo et al., 1989). For most small landholders in West Africa, this is an unrealistically large amount of manure. Despite a need that has grown increasingly obvious and urgent over recent decades to increase the amount of mineral fertilizer used in low-input cropping systems of semiarid West Africa, there has been very little progress towards this end.
Reduce Evaporation from Soil
Under traditional management conditions, about 35% of the season's rainfall is lost from sparse millet stands as E (Wallace et al., 1993). Probably more than half of rainfall is lost as E during seedling establishment and early growth phases in June or July (Charoy, 1974; Payne et al., 1990a), when atmospheric demand is very high and LAI very small. Any reduction of E during this and subsequent periods would not only increase water supply for crop growth, but reduce the risk of resowing due to crop failure.
Unfortunately, practical methods of reducing E to conserve water are lacking in West Africa (Gregory et al., 2000, this issue). The use of crop residues as a mulch during the growing season is desirable, but not practicable because livestock eat most residue during the long dry season, and residues are removed from fields for other purposes, such as home construction. The use of plastic mulch has been shown to be technologically feasible (Zaongo et al., 1994), but is unlikely to be adopted since such materials are too expensive and generally unavailable. Mulching at the end of the growing season to reduce evaporative loss of any postharvest residual soil moisture has proven ineffectual, because water rapidly drains in these coarse soils during the dry season (Payne et al., 1990b).
Theoretically, E can be controlled by modifying soil albedo to warm the surface and to set up a downward-acting thermal gradient, or by decreasing the conductivity of the soil surface zone through tillage (Hillel, 1982). Payne (1999) found that pulverizing the soil surface after rainfall events with a traditional, shallow-cultivating hoe reduced soil surface reflectance and increased soil temperature at 0.05 m by as much as 12°C. This increased soil water storage in the root zone, reduced seasonal ET by 45 mm, and nearly doubled ET efficiency. Practical exploitation of this effect with current technology is doubtful, however, because of labor constraints. Nonetheless, the study demonstrated principles upon which an animal-drawn implement might be designed.
Increase Plant Density
Canopy cover can be increased by increasing plant density. Despite popular perceptions that wide plant spacings decrease the likelihood of crop failure during drought years, Payne (1997) found that increasing density from 5000 to 20000 "hills" ha-1 increased yield and ET efficiency significantly even under low fertility conditions. Under these experimental conditions, there was no evidence of increased risk of crop failure even during 1984, the driest year on record. There appears to be no justification, at least in terms of crop water use, to the use of wide spacings.
Canopy cover can also be increased by the introduction of an intercrop. In semiarid West Africa, pearl millet is most often intercropped with cowpea [Vigna unguiculata (L). Walp.] Intercropping with cowpea has been reported to increase pearl millet grain yield by 15 to 103% in Mali (Hulet and Gosseye, 1986). Increased production has been associated with greater ET efficiency (Nouri and Reddy, 1990; Grema and Hess, 1994), probably due to greater effective LAI achieved through complimentary phenologies (Fig. 4) and canopy architecture.
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Fig. 4 Leaf area indices (LAIs) for pearl millet, cowpea, and intercropped pearl millet/cowpea. Data illustrate how cowpea increased effective LAI compared to the pearl millet monocrop. Mean ET was slightly greater for the pearl millet/cowpea intercrop compared to monocropped millet, but differences in ET were not significant. Redrawn from Nouri and Reddy (1990)
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Rooting systems of pearl millet and cowpea can also be complimentary. In some (Nouri and Reddy, 1990) but not all (Grema and Hess, 1994) water use studies, soil water extraction was greater for pearl millet/cowpea intercrop than for monocropped pearl millet. This may be related to complimentary root distributions and morphologies (Gregory and Reddy, 1982). In pot studies, Petrie and Hall (1992) found that pearl millet roots were clumped locally along the root axis and globally within the profile, with the highest root length densities at the surface. Cowpea root lengths were only half as dense as millet along the root axis, but distributed fairly uniformly throughout the pot. Pearl millet also developed substantially lower predawn leaf water potentials and overall greater root length densities than cowpea. The ability of legumes to contribute biologically fixed N and to use sparingly soluble sources of phosphate are another advantage of this cropping system (Johansen et al., 1995).
There are other species that can be intercropped with pearl millet. Garba and Renard (1991) observed greatest ET efficiencies (in terms of total biomass) when intercropping pearl millet with such alternative legumes as Sesbania pachycarpa D.C. or Stylosanthes hamata (L.) Taub.
Agroforestry
Another management practice that can increase effective canopy cover in sparse canopies of pearl millet is agroforestry. Much of the recent agroforestry research on pearl millet has centered on windbreaks or hedgerows, using such tree species as Azadirachta indica, Acacia nilotica, and Acacia holosericea (Long, 1989; Smith et al., 1998). Windbreaks reduce wind speed near the soil surface on the leeward side, and thereby reduce wind erosion and improve the microclimate for crop growth (Brenner et al., 1995). However, intensive management of the tree foliage (Smith et al., 1998) or even roots may be required to avoid competition for soil resources. The extent and severity of competition depends on local conditions such as rainfall, depth of water table, and soil fertility. Some tree species will not tolerate intensive pruning.
Parkland systems offer an alternative form of agroforestry to windbreak systems in semiarid West Africa, although they too may require intensive management. Faidherbia albida (Del.) A. Chev (syn. Acacia albida) is an ideal parkland tree species because it avoids competition with crops for water and nutrients by shedding its leaves during the cropping season, and regrowing them during the long dry season. The yield of pearl millet and other crops increases near Faidherbia albida trees. This effect has been attributed to improved soil nutrient availability (Charreau and Vidal, 1965; Depommier et al., 1992; Payne et al., 1998), reduced solar irradiance, and lower soil and air temperatures (Dancette and Poulain, 1968; Payne et al., 1998). Reduced air temperature implies reduced (e*-e), and therefore increased T efficiency. The improved soil and microclimate conditions under Faidherbia albida have been associated with greater leaf area, yield, and soil water extraction for pearl millet (Payne et al., 1998).
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Summary
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For most pearl millet farmers in semiarid West Africa, inputs required to obtain full canopies are lacking. As a result, pearl millet water use/yield relations are unlike those of more intensively managed crops. Because pearl millet LAI is almost always <2, yield and ET tend to be uncorrelated, and E constitutes a major portion of ET. Low canopy roughness length and sensible heat dissipation from the soil surface increase (e*-e), thereby decreasing T and ET efficiency. Endemic environmental stresses, and particularly soil nutrient deficiency, probably reduce the value of m so as to further decrease T and ET efficiency. ET efficiency can be improved under such conditions if soil amendments, such as manure or mineral fertilizer, can be applied in sufficient amounts to at least maintain m at its normal physiological value. Beyond this, measures should be taken to increase effective canopy cover to reduce interception of radiation by the soil surface and (e*-e). These include narrower plant spacings, introduction of an intercrop, and the use of a suitable parkland agroforestry species such as Faidherbia albida. The control of E is theoretically possible, and would further increase ET efficiency. However, lack of crop residue availability and appropriate technologies would seem to render this option unlikely at present.Deuson Sanders 1988; Hervouët 1992; Payne Lascano Hossner Wendt Onken 1991; Rees 1986; Saugier 1976; Sowers Issoufou 1992; Thom 1971
Received for publication September 15, 1999.
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ABSTRACT
INTRODUCTION
