Drought-Related Characteristics of Important Cereal Crops - Monitoring and Predicting Agricultural Drought: A Global Study - page 23

of sorghum is about 0.015 Mg ha -1 mm -1 (Kreig and Loscano, 1990),
which is similar to maize grown in arid or semiarid environments. The
advantage of sorghum and millet lies in their capacity to yield something,
albeit little, under conditions with insufficient moisture for maize to pro-
duce anything.
For sweet sorghum, in which biomass production is more important
than grain production, Mastrorilli et al. (1999) found that the sensitivity
to water deficit was higher during leaf production, which reduced biomass
production by 30% and WUE by 16%. For grain sorghum, Camargo and
Hubbard (1999) measured water use with a neutron probe in the field
to estimate evapotranspiration. They found that the ratios of evapotran-
spiration and transpiration to potential evapotranspiration varied from
0.317 to 0.922 across growth stages and irrigation treatments. Calculating
a drought index from the ratio of actual to potential yield, they found that
sorghum grain yield was about five times as sensitive to drought during
heading and grain-filling phases than it was to drought during the vege-
tative phase. Hybrid sorghum varieties show relatively broad adaptation
to drought-prone environments. For example, Haussmann et al. (2000)
found that hybrid sorghum out-yielded parents by 54% across eight semi-
arid macro-environments of Kenya.
For millet produced in the Sahel, Winkel et al. (1997) found that
drought, when it occurred before flowering or at the beginning of flowering
phases, severely reduced both biomass and grain yields, whereas drought
at the end of flowering phase did not affect either biomass or grain yield.
Clearly, farmers must select appropriate varieties and planting periods
for pearl millet to escape drought where there is a significant probabil-
ity of severe drought. Mechanisms of drought resistance in sorghum and
millet include leaf waxiness to prevent desiccation, plastic developmental
responses to stress, and drought escape. Because they are already highly
drought tolerant, efforts to improve sorghum and millet through breeding
have focused more on resistance to parasites and diseases than on drought
resistance (Krieg and Lascano, 1990).
Using a multiple regression approach, Kumar (1998) analyzed grain
yield of pearl millet in India as a function of planting date and rainfall
distribution. He found that yield was significantly reduced by the delay in
sowing in the arid region of India.

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Ir rigation
If irrigation resources are available, farmers generally prefer to use them for
more profitable crops, so sorghum is mostly irrigated only where it is grown
as a component of a rotation system. Sweeney and Lamm (1993) found that
irrigation increased sorghum grain yield by 1 Mg/ha for a three-year field
experiment with different timings of irrigation. Because early irrigation
increased seeds per head, whereas later irrigation increased weight per seed,
irrigation timing did not affect grain yield.