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Agronomic options for improving rainfall-use efficiency of
crops in dryland farming systems
Neil C. Turner*
CSIRO Plant Industry, Private Bag No. 5, Wembley, WA 6913 and Centre for Legumes in Mediterranean Agriculture,
University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Received 18 December 2003; Accepted 27 February 2004
Abstract
Yields of dryland (rainfed) wheat in Australia have
increased steadily over the past century despite rainfall
being unchanged, indicating that the rainfall-use effi-
ciency has increased. Analyses suggest that at least
half of the increase in rainfall-use efficiency can be
attributed to improved agronomic management. Vari-
ous methods of analysing the factors influencing dry-
land yields and rainfall-use efficiency, such as simple
rules and more complex models, are presented and the
agronomic factors influencing water use, water-use
efficiency, and harvest index of crops are discussed.
The adoption of agronomic procedures such as mini-
mum tillage, appropriate fertilizer use, improved weed/
disease/insect control, timely planting, and a range of
rotation options, in conjunction with new cultivars,
has the potential to increase the yields and rainfall-
use efficiency of dryland crops. It is concluded that
most of the agronomic options for improving rainfall-
use efficiency in rainfed agricultural systems decrease
water losses by soil evaporation, runoff, throughflow,
deep drainage, and competing weeds, thereby making
more water available for increased water use by the
crop.
Key words: Crop management, fertilizer use, harvest index,
modelling, rotations, tillage, transpiration efficiency, water use,
water-use efficiency.
Introduction
While the Green Revolution resulted in the development of
new cultivars of wheat and rice suited to high inputs of
fertilizer and water, many regions of the world still rely on
dryland (rainfed) farming for food production. The advent of
increasing water scarcity in this century (Seckler et al.,1999;
Turner, 2001), particularly for agriculture, and the already
scarce availability of new land for agriculture will see less
irrigated land available for crop production than in the past.
While supplemental irrigation can benefit yields and water-
use efficiency in water-limited environments (Oweis et al.,
2000; Turner, 2004), the potential for even limited supple-
mental irrigation is decreasing, with competition for water for
urban and industrial uses and in order to maintain environ-
mental flows. Thus, agriculture will become increasingly
dependent on rainfall as its sole source of water, and
maximizing the efficiency of its use to produce a crop will
be paramount. What, then, are the possibilities of increasing
crop production in dryland farming systems without further
inputs of water; that is, what are the possibilities of increasing
the rainfall-use efficiency of dryland crops?
An analysis of the yield trends of wheat production in
Australia showed that yields have increased by an average
of 12–13 kg ha
1
year
1
over the past six decades (Turner,
2001), despite rainfall not changing and irrigated wheat
contributing only a very small proportion to total pro-
duction. A more recent analysis of wheat-yield trends in
Australia and the various states of Australia has shown
(Fig. 1) that since the early 1980s there has been a more
rapid increase in yield of over 30 kg ha
1
year
1
(Stephens,
2002). In Western Australia, where wheat is not irrigated
and rainfall has probably declined over the last 25 years
(Indian Ocean Climate Initiative, 2002), the increases
shown in Fig. 1 arise solely from increases in rainfall-use
efficiency. In Syria the increases since the early 1980s of 60
kg ha
1
year
1
have been even more dramatic (Turner,
2004), but such increases are not inevitable as increases in
wheat yields in Morocco over the same period have been
very modest (Turner, 2004).
A comparison of the genetic improvement in yields
arising from the release of new cultivars in Western
* To whom correspondence should be addressed. Fax: +61 8 9387 8991. E-mail: neil.turner@csiro.au
Journal of Experimental Botany, Vol. 55, No. 407, ª Society for Experimental Biology 2004; all rights reserved
Journal of Experimental Botany, Vol. 55, No. 407,
Water-Saving Agriculture Special Issue, pp. 2413–2425, November 2004
doi:10.1093/jxb/erh154 Advance Access publication 10 September, 2004
Australia (Perry and D’Antuono, 1989) and England
(Austin et al., 1980, 1989) suggested that about half of
the increase over the past 12 decades, up to the early 1980s,
was from the introduction of new cultivars and half from
improved management (Turner, 1997). Stephens (2002)
suggested that two-thirds of the rapid increase in wheat
yields in Australia since the early 1980s has been due to
improvements in management and one-third to improved
genotypes. Indeed, Turner (2004) has suggested that, as in
the case of the Green Revolution, it has been the combi-
nation of improved agronomy coupled with suitable geno-
types that has led to the increased yield trend and
increased rainfall-use efficiency in Australia’s wheat pro-
duction since the early 1980s. While Miflin (2000) and
Araus et al. (2003) argue that genetic improvements are
likely to bring the greatest increases in yield, and hence
rainfall-use efficiency, in water-limited environments in the
twenty-first century, the role of management in increasing
yield in the past and in the future should not be overlooked.
Thus, this review will focus on the agronomic factors that
have the potential to increase yields and rainfall-use
efficiency in dryland farming systems that rely entirely on
rainfall as their source of water. The role of genotypic
improvements in yield can be found elsewhere (Turner,
1997, 2003; Richards et al., 2002; Araus et al., 2003).
Dryland farming environments
Before considering agronomic options for the improvement
of yield and rainfall-use efficiency in dryland farming
systems, it is necessary to know the environmental con-
ditions under which the dryland crops are grown and the
likely incidence(s) of water shortage. In Mediterranean
dryland farming systems, annual crops are generally sown
in the autumn when rainfall commences, grow during the
cool wet winters, and set seed in spring and early summer as
temperatures and vapour-pressure deficits rise and rainfall
decreases (Fig. 2). High temperatures and lack of rainfall
preclude any significant summer cropping without irriga-
tion in Mediterranean-type climates. Although the winters
are wet and rainfall usually exceeds evaporation (Fig. 2),
cool temperatures and low incoming radiation because of
cloud cover often limit growth in these months. In more
continental, Mediterranean-type environments, frost is also
common. One of the features of Mediterranean-type cli-
mates is that rainfall is more reliable than in other semi-arid
environments (Turner, 2004). For example, the standard
deviation for annual rainfall in the Mediterranean-climatic
region of south-western Australia is 23–25% compared
with standard deviations of 30–33% in the area of northern
New South Wales that has a similar annual rainfall,
but predominantly summer rainfall (Asseng et al., 2003).
Nevertheless, even in Mediterranean cropping regions the
growing-season rainfall can vary markedly from year to
year. For example, at a site with an average annual rainfall
of 460 mm, year-to-year rainfall varied from 200 to 800
mm (Asseng et al., 2001a). This leads to the large variation
from year to year in the wheat yields observed in Fig. 1.
In subtropical environments, dryland crops can be grown
in the warm summer (rainy) season, and also in the cooler
dry (post-rainy) season if the water-holding capacity of the
soil is sufficient to enable the crop to mature. The high
temperatures in the rainy season ensure rapid crop de-
velopment, but erratic rainfall can lead to water shortage,
particularly on shallow or coarse-textured soils. These
periods of water shortage can occur at any time during
Fig. 1. Changes with time in the yield of wheat over the past seven decades in Australia and Western Australia (adapted from Stephens, 2002, with
permission from the Department of Agriculture of Western Australia).
2414 Turner
crop growth. Using long-term weather data (temperature
and rainfall), soil water-holding characteristics, and a crop-
water stress index (or relative transpiration) it is possible to
estimate crop-water use and by cluster analysis to classify
similar types of water-deficit scenarios that are likely to
occur at a particular location. For example, Wright (1997)
did this for one location in Queensland, Australia, and from
85 years of weather data concluded that five different water-
shortage scenarios were possible for peanut (groundnut)
production in this environment: two with terminal water
shortage and three with water shortages at different times
during crop growth (Fig. 3).
In temperate regions, dryland farming is less likely to be
constrained by water shortage than by other factors such as
low radiation, cold temperatures, or frost. In parts of North
America, Eastern Europe, and northern Asia, crop pro-
duction is restricted to the warmer summer months and the
season is constrained by cold soil temperatures in spring
and frost in autumn. Where the winters are less severe,
crops can be sown during the autumn and are well estab-
lished when the soil and air temperatures rise in spring,
ensuring rapid and earlier growth in the spring compared
to a spring-sown crop.
A framework for yield improvement in
water-limited environments
Passioura (1977) suggested a framework for the consideration
of factors affecting yield in water-limited environments:
Yield = Water use3Water-use efficiency3Harvest index
where water-use efficiency is the biomass produced per unit
evapotranspiration (transpiration plus soil evaporation) and
harvest index is the ratio of harvested yield to total above-
ground biomass. This relationship applies to both agro-
nomic and genetic factors affecting yield. As rainfall falling
at a particular site can be transpired by the crop, transpired
by weeds, lost by soil evaporation, deep drainage, runoff,
or throughflow (subsurface flow), or stored in the soil
for subsequent use by a crop, the yield and rainfall-use
efficiency in dryland cropping systems can be improved by
decreasing losses of water from the soil and weeds, and
maximizing the water use (transpiration) by the crop itself.
Taking into account the water losses by the system other
than crop transpiration, the above equation then becomes:
Yield = ðRainfall Losses from soil and non-crop speciesÞ
3Transpiration efficiency3Harvest index
Fig. 3. Changes in calculated relative crop transpiration with growing degree days based on long-term (85 years) climatic data from Kingaroy, Australia,
for (a) two groups of years showing terminal drought, and (b) three groups of years showing intermittent drought. The percentages in parentheses are the
proportion of years in the particular group (from Wright, 1997, with permission from DPI & F Publications).
Fig. 2. The mean daily precipitation (P), pan evaporation (E), and annual-
plant transpiration (T) in a Mediterranean-type climate (adapted from
Fischer and Turner, 1978, with permission).
Agronomic options for improving rainfall-use efficiency of crops 2415
where transpiration efficiency is the biomass produced per
unit of water transpired. Many of the agronomic options for
improving the rainfall-use efficiency and yields in dryland
cropping systems involve minimizing losses from the soil
and weeds and maximizing the proportion of rainfall that is
transpired by the crop. Nevertheless, agronomic options for
improving the transpiration efficiency and proportion of the
crop that is harvested exist and will be discussed briefly.
An alternative framework that has been widely adopted
by advisers and producers in southern Australia is that
proposed by French and Schultz (1984a, b). From a series
of yield and water-use measurements made at a total of 61
sites over a period of 11 years, French and Schultz (1984a)
suggested that, in the Mediterranean-type environment of
South Australia, the potential grain yield of wheat increased
by 20 kg ha
1
mm
1
of water use (transpiration) above
a minimum value of 110 mm, which was assumed to be the
amount of water lost by soil evaporation (Fig. 4a). A
potential transpiration efficiency of 20 kg ha
1
mm
1
has
been observed to apply in a number of field and glasshouse
studies in Australia (Passioura, 1976; Gregory et al., 1992;
Zhang et al., 2004). Since water use is strongly correl-
ated with growing-season (April–October in the southern
hemisphere) rainfall in this water-limited, winter-
rainfall environment, French and Schultz (1984b) used
growing-season rainfall to compare the performance of
wheat crops in farmers’ fields to the potential yield set by
rainfall and showed that rarely did actual yields reach
potential yields (Fig. 4b). The yield potential of 20 kg ha
1
mm
1
of growing-season rainfall (i.e. the rainfall-use
efficiency) has provided a useful yardstick for farmers to
compare the on-farm performance of their wheat crops.
Similar potential-yield yardsticks have been developed
for annual pastures (Bolger and Turner, 1999), canola
(Hocking et al., 1997), and four cool-season grain legumes
(Siddique et al., 2001).
However, the methodology of French and Schultz
(1984b) should be used with caution as it assumes that all
the growing-season rainfall, except for losses by soil
evaporation, which vary with soil type (French and Schultz,
1984a), is used by the crop. This is not always the case as
losses by deep drainage, runoff, and throughflow can occur
at wetter locations and in wetter years (Bolger and Turner,
1999; Eastham and Gregory, 2000), particularly in coarse-
textured soils (Asseng et al., 1998a). Moreover, the
methodology assumes that pre-sowing rainfall, that is,
rainfall before April in the southern hemisphere, does not
contribute to yield. Thus the yardstick provided by French
and Schultz (1984b) is primarily for environments where
the crops rely on current rainfall and where the growing-
season rainfall is less than 500 mm.
An alternative methodology for estimating potential
yields and rainfall-use efficiency in water-limited environ-
ments is simulation modelling. Asseng et al. (1998b) have
developed a simulation model, APSIM-wheat, which has
been widely validated (Asseng et al., 1998b, 2001b), and
predicts potential yields and water use for wheat in a range
of environments and soil types, taking into account the
weather (rainfall, radiation, and temperature), water and
nitrogen movements in the soil, and restrictions arising
Fig. 4. The relationship between the yield of wheat and (a) water use, and (b) growing-season (April–October in the southern hemisphere) rainfall for
experimental sites and farmers’ fields in South Australia. The sloping line gives the potential yield from water transpired, after allowing for a loss of 110
mm for soil evaporation, and the vertical lines give the responses to nitrogen (solid squares), phosphorus (inverted solid triangles), control of root
nematodes (closed triangles), multiple factors (open diamonds), time of sowing (open triangles), weeds (open squares), and waterlogging (open circles)
(adapted from French and Schultz, 1984a, b, with permission from CSIRO Publishing).
2416 Turner
from waterlogging in the rooting zone. A comparison of the
yields predicted by the APSIM-wheat model and by French
and Schultz (1984b) showed that the latter’s yield potential
was useful for the environment in which it was developed,
but that factors such as soil type and rainfall distribution
during the growing season play major roles in determining
the yield potential and rainfall-use efficiency of wheat in
any one year (Fig. 5). In particular, deep drainage and soil
evaporation varied markedly depending on rainfall distri-
bution and soil type (Asseng et al., 2001b). For example,
using 80 years of weather data, Asseng et al. (2001b)
showed that water in the soil at sowing and rainfall
distribution through the growing season had major influ-
ences on predicted potential yield and rainfall-use effi-
ciency in semi-arid Mediterranean-type environments.
French and Schultz (1984a) showed that water use
before anthesis determined the number of grains set in
wheat and hence had a major influence on final grain yield
Fig. 5. Relationship between simulated wheat yields and growing-season (April to October in the southern hemisphere) rainfall for a sandy (open circles,
closed circles) and a clay (open triangles, closed triangles) soil in the (a) high (mean annual rainfall = 460 mm), (b) medium (390 mm), and (c) low
(310 mm) rainfall zones with zero (open circles, open triangles) and 150 (closed circles, closed triangles) kg N ha
1
. The sloping line gives the potential
yield line from Fig. 4 (from Asseng et al., 2001b, with kind permission of Springer Science and Business Media).
Agronomic options for improving rainfall-use efficiency of crops 2417
in Mediterranean-type environments. This contrasts with
studies where high water use prior to anthesis in wheat
resulted in ‘haying off’ of spikelets and poor yields (van
Herwaarden et al., 1998) and conclusions that water use
after anthesis was an important determinant of yield. Turner
(1997), using data for barley from Syria (Shepherd et al.,
1987), showed that, while there was an increase in yield at
sites and in seasons with greater water availability after
anthesis, factors that affected early growth and water use
before anthesis could vary yields at maturity by more than
2-fold (Fig. 6). The analysis by Asseng et al. (2001b)
showed a similar general increase in potential yield with
water use after anthesis, but the scatter was very large when
predicted yields were simulated over 80 years of weather
data (Asseng and Turner, 2003). Simulation modelling can
be a powerful tool for predicting potential yields for a range
of environments and soil types, and for analysing historical
weather data to determine the risks associated with any one
management option or combination of options (Asseng
et al., 2001b).
Agronomic options for improving water use by
the crop
One of the major ways to increase the water use of the crop
itself is by increasing the depth of rooting. In many dryland
environments, crops do not use all the water available in the
soil profile because of restrictions to root growth. These
restrictions may be physical, chemical, or biological.
Agronomic practices that reduce the physical impedance
to root growth can benefit yields of dryland crops in water-
limited environments. Deep ripping to about 30 cm has
been shown to increase yields and hence rainfall-use
efficiency on deep sandy soils (Jarvis, 1982; Delroy and
Bowden, 1986; Asseng et al., 2002; Asseng and Turner,
2003). Other physical soil constraints such as compacted
subsoils can be alleviated by the application of gypsum to
flocculate the soil particles, and to increase water penetra-
tion and root growth (Hamza and Anderson, 2002, 2003).
Chemical constraints are not as easily remedied, but soil
acidity at depth that constrains root growth can be amelio-
rated by liming, particularly with deep placement of lime.
However, soil alkalinity that restricts the growth of lupin
roots (Atwell, 1991; Tang et al., 1992), soil sodicity, and
boron toxicity are more difficult to ameliorate agrono-
mically and may need to rely on the use of different species
or tolerant genotypes (Tang et al., 1993). Finally, root
diseases and nematodes can constrain root growth and are
most easily controlled by rotations to reduce the disease-
and nematode-incidence and by cultivation techniques that
minimize fungal activity (Roget et al., 1996).
It should be noted that deeper roots are not always
beneficial. In environments in which the seasonal rainfall
and soil characteristics are such that the depth of soil
wetting is restricted, deeper rooting will be of no benefit. A
simulation analysis by Asseng et al. (2002) showed that
deeper roots gave the greatest benefit on sandy soils,
particularly in the high-rainfall zones where nitrogen can
leach below the root zone, and had smaller or even negative
effects on yields for wheat growing on clay soils with
limited wetting to depth (Smith and Harris, 1981). The
analysis also demonstrated the role of nitrogen application
in overcoming restrictions to rooting depth, particularly in
sandy soils (Asseng et al., 2001b).
Rotations also provide an opportunity to increase water
use by a crop. Roots of some species have the potential to
penetrate deeper into the soil than others (Hamblin and
Hamblin, 1985), and this may provide ‘biopores’ for
a subsequent crop. It has been suggested that both narrow-
leafed lupin ( Lupinus angustifolius) and canola/oilseed
rape (Brassica napus) develop ‘biopores’ in the soil that
allow easier root penetration by the water and roots of
a subsequent crop (Angus et al., 1991; Cresswell and
Kirkegaard, 1995). However, results have been equivocal.
Nevertheless, there is considerable evidence that lucerne
(Medicago sativa) has roots that penetrate deep into the soil
over 2–3 years and allow deeper water penetration and
deeper root penetration by a subsequent crop (Ward et al.,
2002).
However, the major impact of agronomic management
on rainfall-use efficiency has not arisen from increasing
total water use by the crop in evapotranspiration, but from
increasing water use by the crop itself in transpiration at the
expense of water loss by weeds or from the soil by soil
evaporation, deep drainage, surface runoff, or lateral
throughflow. This increase in water use by the crop at the
expense of other losses generally results in significantly
increased yields, with only a 5–10% increase in total
evapotranspiration (Asseng et al., 2001c).
Fig. 6. The relationship between grain yield and water use between
anthesis and maturity for unfertilized (open circles) and fertilized (closed
triangles) barley. The line gives the mean regression (from Turner, 1997,
reproduced with permission from Elsevier).
2418 Turner
Agronomic options for decreasing losses from
the soil and weeds
Figure 2 shows that transpiration (T) by annual crops in
Mediterranean-type climates is offset or delayed in relation
to incoming rainfall. Earlier planting to more closely match
incoming rainfall and reduce soil evaporation will increase
yield and rainfall-use efficiency (French and Schultz,
1984a; Anderson et al., 1995; Siddique et al., 1998;
Asseng et al., 2001c; Riffkin et al., 2003). Eastham et al.
(1999) and Eastham and Gregory (2000) showed that
earlier planting of wheat and lupin crops in a Mediterra-
nean-type environment did not affect the total evapotrans-
piration, but reduced soil evaporation, particularly early in
the season before the leaf area of the later-sown crop
reached full ground cover. In some cases, this resulted in
higher yields and water-use efficiency (and rainfall-use
efficiency) of the early-sown crops (Gregory and Eastham,
1996). With the use of herbicides to control weeds, farmers
in some parts of southern Australia are sowing into dry soil
(dry seeding) so that the seeds emerge on the opening rains
of the season and thereby gain several days’ more growth
than would be the case if they waited to sow until after the
rain. However, early planting is not always an advantage
(Eastham and Gregory, 2000). If appropriate cultivars are
not available, early planting increases the risk of damage by
frost during flowering and there is a greater vulnerability to
terminal drought due to increased biomass and water use
by anthesis, thereby reducing yields (Anderson et al., 1995,
1996; Riffkin et al., 2003). Indeed, Gregory and Eastham
(1996) found that early planting of wheat only gave yield
benefits in 1 out of 3 years because of increased disease
incidence and earlier water deficits in the early-planted,
high-biomass wheat in the other 2 years. Likewise, early
planting of field peas is not recommended in southern
Australia as this raises the risk of increased disease
incidence and lower yields (Bretag et al., 1995) from black
spot (Mycosphaeralla pinodes and Ascochyta pisi), a dis-
ease for which resistance is not currently available in field
peas. Indeed, in west Asia chickpea crops are generally not
sown in autumn, but in late winter or early spring in order to
avoid the endemic Ascochyta blight for which there is
currently little disease resistance (Abbo et al., 2003), but
this is at the cost of yield and rainfall-use efficiency
(Keatinge and Cooper, 1983; Singh et al., 1997).
Fertilizer use can also have a very marked effect on crop
yield and rainfall-use efficiency. Nitrogen nutrition and
phosphorus nutrition have both been shown to increase the
early growth of cereals in water-limited Mediterranean
environments (French and Schultz, 1984b; Shepherd et al.,
1987; Asseng et al., 2001b). Asseng et al. (2001b) showed
that nitrogen fertilizer input increased the water use by the
crops and reduced soil evaporation so that total evapotrans-
piration was little changed, thereby increasing yields and
rainfall-use efficiency (Table 1). Similar effects on the
balance of crop transpiration and soil evaporation were
observed by Gregory et al. (1984) and Shepherd et al. (1987)
with fertilizer use on barley in Syria. While fertilizer
increases biomass and water use prior to anthesis, the
additional ears produced by the increased fertilizer result
in greater sinks for assimilates and higher yields even with
lower amounts of water available in the post-anthesis period.
As mentioned previously, Turner (1997) showed that while
yields increased with increases in water available after
anthesis, there was at least a 2-fold increase in yield at high
fertilizer rates at any one level of water use, and that the
increased yield occurred with little or no increase in water
use; that is, the fertilizer increased rainfall-use efficiency
(Fig. 6). Rotations are also important means of increasing
fertility. Use of legume-rich pastures or grain legume crops
provides nitrogen to a subsequent cereal or oilseed crop
(Rowland et al., 1988, 1994; Fillery, 2001; Angus et al.,
2001). The quantity of nitrogen supplied depends both on the
proportion of legume in the pasture (Peoples and Baldock,
2001) and the amount of nitrogen removed in the seed of the
legume crop (Evans et al., 2001). However, high nitrogen
levels can reduce yields through ‘haying off’ due to excess
water use in the pre-anthesis period, leaving insufficient
water for post-anthesis grain filling (van Herwaarden et al.,
1998). Fischer (1981) suggests that in dryland environments
there is an optimum biomass at anthesis, depending on
available water, to maximize grain yield. While this appears
to be true for heavy-textured soils, on sandy soils high
nitrogen levels do not induce lower yields (Halse et al.,
1969; Turner, 1987; Asseng et al., 2001b).
High plant density increases crop-water use and reduces
soil evaporation in Mediterranean-type environments, but
Table 1. Simulated yield, evapotranspiration, soil evaporation, crop transpiration, and transpiration efficiency for a wheat crop in
Western Australia
The crop was growing on two soil types and given two levels of nitrogen fertilizer at a medium-rainfall (390 mm annual rainfall, 322 mm growing-season
rainfall) site (adapted from Asseng et al., 2001b, with kind permission of Springer Science and Business Media).
Soil
type
Nitrogen treatment
(kg N ha
1
)
Grain yield
(kg ha
1
)
Evapotranspiration
(mm)
Soil evaporation
(mm)
Crop transpiration
(mm)
Transpiration efficiency
(kg ha
1
mm
1
)
Sand 0 1170 214 168 46 25
150 2090 229 138 90 23
Clay 0 1630 269 188 81 20
150 1820 286 138 148 12
Agronomic options for improving rainfall-use efficiency of crops 2419
the compensation provided by growth of tillers in cereals and
branching in pulses results in a broad range of planting
densities producing similar yields (Anderson and Sawkins,
1997; Johnston et al., 2002; Seymour et al., 2002; Regan
et al., 2003) and hence similar rainfall-use efficiencies. Low
planting density and uneven planting can result in low yields
and a greater proportion of small seeds (pinched grain) that
are discarded at harvest (Turner et al., 1994), resulting
in poorer rainfall-use efficiency. However, where crops are
grown on stored soil moisture, low planting densities are
frequently used to provide a greater source of water per plant
and hence increased yields per plant and per hectare.
Gregory (1989) reported studies with pearl millet in Niamey,
Niger, grown at three row widths but with the same within-
row density. The studies showed that at the low planting
densities (wider row spacing) the crop continued to extract
water longer into the dry period and had greater dry matter
accumulation and root growth than the plants at the high
planting density. Moreover, there was evidence that at low
planting densities the year-to-year variation in yield was less
where rainfall was erratic (Gregory, 1989). This contrasts
with Mediterranean-type environments in which high seed-
ing rates have been shown to incur no greater risk than
low seeding rates (O’Connell et al., 2003). Thus, planting
density depends on rainfall distribution, with low densities
being detrimental in Mediterranean-type environments, but
low densities being preferred where the crop relies on stored
soil moisture, particularly in areas with little or no rainfall
during the growing season and little soil evaporation from
the dry soil surface. It also depends on the degree of risk that
a farmer is prepared to take to gain advantage of the above-
average years.
Competition for water by weeds and the impact of weed
growth on yields is well recognized (French and Schultz,
1984b). Likewise root diseases, insect damage, and root
nematodes all reduce yields and rainfall-use efficiency
(French and Schultz, 1984b). To reduce the influence of
these factors, herbicides, fungicides, insecticides, and
nematocides can be used. However, in low-yield, water-
limited environments, rotations and agronomic manage-
ment practices in the previous crop are often utilized. For
example, ‘take-all’ (Gaeumannomyces graminis) can be
carried over in the residues of the previous crop, but also by
grass weeds in the previous crop. Removal of these weeds
in the previous crop or pasture will reduce the incidence
of the disease in the cereal crop. Likewise, broad-leaved
weeds can be removed in a previous cereal crop more
easily than with selective herbicides in a pulse crop.
Brassica crops such as canola and Indian mustard have
been shown to produce isothiocyanates and other break-
down products of glucosinolates from their residues,
leading to biofumigation of the soil that reduces the
incidence of ‘take-all’ and other soil-borne pathogens,
weeds, insects, and nematodes in the subsequent crop
(Kirkegaard and Sarwar, 1999; Angus et al., 2001). Thus
the better use of rotations in providing nitrogen (Rowland
et al., 1988, 1994; Fillery, 2001; Angus et al., 2001) and
a disease/weed break for the subsequent crop is an
important agronomic management tool for influencing
dryland crop yields and rainfall-use efficiency.
The use of minimum tillage or conservation tillage,
whereby residues from the previous crop are left on the
surface, weeds are controlled by herbicides rather than
tillage, and the seed is sown with minimum disturbance of
the soil surface by the use of narrow tines, has led to
reduced losses of water by soil evaporation and increased
yields (Unger, 1978; Stewart and Robinson, 1997; Cornish
and Pratley, 1991). Further, minimum tillage systems allow
earlier planting as delays resulting from using tillage to
remove weeds are reduced. However, recent studies suggest
that the greater retention of incoming rainfall through
minimum tillage may increase water losses through deep
drainage that are detrimental in a landscape in which
secondary salinity can develop (Sadler and Turner, 1994),
and reduce rainfall-use efficiency.
Finally, fallowing land to conserve moisture has been
widely practised as a means of improving yields in water-
limited environments (Stewart and Robinson, 1997) and
was given credit by Donald (1965) for the increase in wheat
yields in Australia in the first half of the last century.
However, Stewart and Robinson (1997) have pointed out
that only 12–20% of the precipitation in the fallow period is
retained in the soil at seeding. O’Leary and Connor (1997a)
showed that the amount of water stored in the soil and
available to a subsequent crop varied with season, soil type,
and management of the fallow land. At sites with about
250 mm of annual rainfall, the amount of water available at
the time of sowing the subsequent crop varied from 100 to
+100 mm over 4 years, with greater soil water available in
the heavier clay soil, when stubble from the previous crop
was retained, and when the soil was not tilled. On the clay
soil, the greater the soil water in the profile at seeding the
greater the water use and the higher the yield (O’Leary and
Connor, 1997b). However, benefits from fallowing land
were minimal on the sandy soil, whether or not the stubble
was retained or the soil tilled (O’Leary and Connor, 1997a,
b). Moreover, tillage during the fallow period can reduce
the soil organic matter, leading to a decline in soil structure
(Stewart and Robinson, 1997). Indeed crop intensification,
by growing a crop instead of fallowing land, while reducing
yields per crop can improve overall crop yields and
markedly increase rainfall-use efficiency (Jones and
Popham, 1997; Farahani et al., 1998a, b).
Agronomic options for improving transpiration
efficiency
Until the 1980s it was considered that there was no genetic
variation within a species for differences in transpiration
efficiency (Tanner and Sinclair, 1983; Fischer, 1981), a view
2420 Turner
that was dispelled by the development of the isotopic-carbon
discrimination technique to measure transpiration efficiency
(Hall et al., 1994). However, it has long been recognized that
species that were subsequently shown to have the C
4
pathway of photosynthesis had higher transpiration efficien-
cies than those with the C
3
pathway of photosynthesis
(Briggs and Shantz, 1912; Fischer and Turner, 1978). While
C
4
species tend to have a higher temperature optimum and
grow in the warmer periods of the year with high vapour-
pressure deficits, the selection of genotypes with the ability
to grow in cooler temperatures has allowed them to be grown
in temperate regions, where their higher transpiration effi-
ciency can result in higher yields than C
3
species on the same
amount of rainfall, Thus, choice of species can be used to
improve yields with similar water use, that is, to increase the
rainfall-use efficiency. For example, Jones and Popham
(1997) showed that growing sorghum rather than wheat more
than doubled the grain yield and increased precipitation-
(snowfall as well as rainfall) use efficiency in the western
plains of the United States of America.
While low levels of nitrogen in the leaf reduce photo-
synthesis more than transpiration, resulting in low transpi-
ration efficiency, the major agronomic way of increasing
transpiration efficiency is to maximize the growth of crops
during periods of low vapour-pressure deficits (Fig. 7).
Thus in Mediterranean-type climates autumn sowing rather
than spring sowing has a major influence on transpiration
efficiency as a greater proportion of the autumn-sown
crop’s life occurs during the period of low vapour-pressure
deficits in winter (Fischer, 1981; Singh et al., 1997;
Richards et al., 2002).
Agronomic options for improving the
harvest index
Grain yield as a proportion of the total biomass yield, that
is, the harvest index, varies with water use both before and
after the establishment of the floral and seed structures
(Fischer, 1981) and thus can be influenced by management
decisions taken throughout the life cycle of the crop. In
subtropical semi-arid environments where crops are grown
on stored soil moisture, agronomic treatments such as
increased fertilizer use and deep ripping that increase
biomass production and water use prior to flowering can
reduce the harvest index, as insufficient water is available
after anthesis and the number of pods, spikelets, and seeds
is reduced either through low numbers produced or pod
or seed abortion. In Mediterranean-type semi-arid environ-
ments, treatments that increase early growth increase the
harvest index as the head weight at anthesis is often
strongly correlated with the final grain yield (Turner and
Nicolas, 1998). This is particularly true on deep sandy soils,
but too-vigorous early growth of crops growing on heavy
clay soils can lead to a lower harvest index as the water
available after anthesis is too little to fill the grains (Fischer,
1981; van Herwaarden et al., 1998; Asseng et al., 2001b,
2003). In some Mediterranean-type environments in south-
ern Australia excess water in winter can lead to water-
logging and a low harvest index (Gregory et al., 1992;
Gregory, 1998), and management options to alleviate
waterlogging, such as drainage or early planting, can
increase the yield and harvest index of crops (Zhang
et al., 2004). By contrast, water shortage during vegetative
growth has been shown to stimulate reproductive develop-
ment in indeterminate crops such as lupin and cotton, and to
increase the harvest index of the crop (French and Turner,
1991). Indeed, intermittent shortage of water on deep,
sandy, coarse-textured soils may account for the success of
lupin production in Western Australia (French and Turner,
1991) and the large variation in the harvest index from
season to season.
Conclusions
Donald (1965) reviewed decadal wheat yields in Australia
from 1860 to 1960 and showed that until the turn of the
twentieth century yields decreased as nutrients were ex-
hausted. The introduction of the practice of leaving land
fallow to conserve soil moisture, the use of shorter-season,
better-adapted cultivars, and the use of superphosphate
fertilizer and rotations, including legumes, for the supply
of nitrogen produced a steady increase in wheat yields
between 1900 and 1950. Angus (2001) and Angus et al.
(2001) have extended Donald’s (1965) findings to the year
2000 and suggest that from 1950 to 1980 wheat yields
increased as a result of better rotations and more timely
sowing because of mechanization, and increased again in
the 1980s and 1990s as a result of the introduction of
herbicides and break crops such as lupins, other pulses, and
canola into the rotation (Fig. 8). The introduction of
Fig. 7. The relationship between transpiration efficiency of wheat and
pan evaporation for various months in the southern hemisphere (left of the
line) and the northern hemisphere (right of the line) (adapted from
Fischer, 1981, and Richards et al., 2002, with permission).
Agronomic options for improving rainfall-use efficiency of crops 2421
semi-dwarf varieties in the 1970s also had a role in
increasing yields and enabled agronomic management,
such as increased fertilizer use, to benefit yields too.
Similar conclusions on the role of agronomic changes have
been drawn from studies of the Broadbalk experiment at
the Rothamsted Experiment Station (Rasmussen et al., 1998;
Miflin, 2000). These increases in yield have occurred as
rainfall has remained unchanged, resulting in significant
improvements in rainfall-use efficiency. While the early
analysis suggested that until 1980 the increase in rainfall-
use efficiency could be attributed half to new cultivars and
half to increased agronomic practices (Turner, 1997), the
surge in yields and rainfall-use-efficiency in the past two
decades is considered to be one-third attributable to new
cultivars and two-thirds attributable to agronomic man-
agement (Angus et al., 2001; Stephens, 2002). Indeed, the
combination of agronomists working with breeders to
develop appropriate agronomic packages for new cultivars
and the ability of modern cultivars to respond to increased
agronomic inputs is probably the reason for the recent surge
in rainfall-use efficiency in wheat crops in Australia. It is
clear that it is not just one factor that has led to the higher
rainfall-use efficiency, but rather the combination of ap-
propriate fertilizer use, improved weed/disease/pest control,
timely planting, and the increased adoption of a range of
rotations. This is the basis of the ‘Green Revolution in
Rainfed Environments’.
Acknowledgements
Financial support by CSIRO, the Australian Centre for International
Agricultural Research, the Grains Research and Development
Corporation, AgraCorp Pty Ltd, and the Centre for Legumes in
Mediterranean Agriculture is gratefully acknowledged. Drs Senthold
Asseng and Heping Zhang are thanked for their comments on this
paper.
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