Agriculture Reference
In-Depth Information
on the portion of the aboveground biomass that is grain. The ratio of the grain weight
to the total aboveground biomass weight is the harvest index. The harvest index is
employed as a criterion for screening cultivars and is a genetic characteristic. Prihar
and Stewart (1990) suggested that the maximum harvest index value for corn was
about 0.60 and about 0.50 for wheat. Evans (1980) stated that the yield increase of
wheat through 1976 came from the increase in harvest index, which rose from 0.43
to 0.50. Pingali and Rajaram (1999) reported that wheat architecture had evolved
over the past 45 years to sustain growth in genetic yield potential. Wheat grain yields
increased by at least 15% in semidwarf wheats compared to yields of tall varieties
largely due to an increase in harvest index.
Prihar and Stewart (1990) reviewed many experiments of wheat, corn, and grain
sorghum and showed that the harvest index of these crops decreased with increasing
stress, particularly water stress. Under severe drought conditions, the harvest index
could approach zero because there would be little or no grain produced. Therefore,
any management practice that reduces the stress of growing grain crops tends to
increase the grain yield. Successful management practices under nonirrigated con-
ditions include those involving plant densities, plant geometry, and row spacing.
Under irrigated conditions, timing and amounts of water additions are critical. All
of these practices will affect the harvest index values and therefore affect water use
ef iciency.
Passioura (1977) found a linear relationship between harvest index and the per-
centage of the seasonal water use after
anthesis
, the period during which a flower
is fully open and functional. Harvest index values ranged from 0.2 when water use
after anthesis was less than 5% of the season total to about 0.5 when water use after
anthesis was 25 to 30% of the total. This shows that grain filling is a critical stage
for producing grain. Bandaru et al. (2006) compared grain sorghum plants growing
in clumps to those equally spaced in rows in a semiarid region under drought condi-
tions and showed the plants in clumps used less water during the vegetative growth
period, leaving more soil water for use during grain filling. Yields were increased by
use of clumps, and harvest index values increased from less than 0.3 to more than
0.4. Although total aboveground biomass values were slightly less for the clumped
plants, grain yields and water use efficiencies were higher because of the higher
harvest index values.
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When water is added to the soil by rainfall or irrigation, there are five things that
can happen to the water. It can evaporate, percolate below the root zone, remain
stored in the soil, be lost as runoff, or be used by transpiration by plants. Estimates
of how precipitation in different climatic zones is distributed to runoff, evaporation,
and evapotranspiration are presented in Figure 19.5. As the climate becomes drier,
a much greater proportion of the total precipitation is lost to evaporation, and this
reduces the water available for evapotranspiration. As discussed, the water evapo-
rated from the soil during the growing season and the water transpired by the grow-
ing crop is evapotranspiration, and the agronomic yield of a crop increases linearly
with increasing amounts of evapotranspiration as illustrated in Figure 19.1. The
