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moisture affects wheat development is believed
to depend upon the stage of growth at the
time it occurs. Reported effects include a reduction
in the number of tillers, the fertility of
the spikes, and the size of the grain.
The effects of flooding on wheat development
have been studied at the Northwest
Agricultural Research Center (CIANO) Mexico,
during the past two seasons. The soils in this
region are heavy clays and farmers have an
ample supply of low cost irrigation water;
these conditions suggested that an excess of
soil moisture after irrigating might be a yieldlimiting
factor.
In 1969-70, wheat production in normally
irrigated plots was compared with that in plots
flooded for 4 or 8 days at different times during
the growing season. The soil was a heavy
clay with a bulk density that varied with depth
from 1.1 in the 0-30 cm horizon, to 1.35 in the
60-90 cm horizon, to 1.20 at the 120 cm depth.
Apparently the more dense horizon below 60 cm
was almost impermeable to water, as little
penetration occurred below that depth, even
in plots flooded for as long as 4 days. Oxygen
diffusion rates were measured as soon as possible
after flooding and the measurements
were continued until the rates were adequate
for normal growth. After a normal irrigation,
the oxygen diffusion rate remained below 0.2
micrograms/cm 2 /min at a soil depth of 15
cm for a period of 9 to 12 days. Following
4 days of flooding, the oxygen diffusion rate
was less than 0.2 micrograms/cm 2 /min at a
15 cm depth for 12 to 15 days.
Flooding was found to affect plant development
in several ways:
Flooding for 4 days at planting time reduced
plant emergence by 17 percent and plant
height at 26 days by 11 percent. The number
of heads .and grain yield decreased very
slightly.
Flooding for 4 days, at 15 and 30 days
after planting, increased plant height and the
number of tillers per plant at 53 days. Straw
yields were increased by about 16 percent.
At 45 days after planting, flooding reduced
plant height, tiller density at 53 days, grain
size, grain yield and straw yield; it did not
change the number of grains per head.
Flooding for 4 days at 60 and 75 days after
planting had no significant effect on wheat
development.
Flooding for 8 days at 65 days after planting
slightly increased both grain size and
yield.
The INIA variety used in this study flowered
at 61 days after planting.
Evidently, as far as grain yield was concerned,
the INIA wheat variety was most sensitive
to a low oxygen diffusion rate at planting
time and about 45 days later. The effect
of flooding at 45 days was similar to that
observed in 1968-69, when flooding at 38 days
reduced both plant height and straw yields.
Surprisingly, flooding for four days at 15, 30,
60 and 75 days after planting, did not reduce
grain yields; while flooding for 8 days, at 65
days after planting, slightly increased yields.
Similar results were obtained in a growth
chamber study at the University of California
at Riverside, which found that maximum production
of wheat grain occurred at atmospheric
oxygen levels about one-fourth normal
level.
Results obtained in studies conducted during
the past two years clearly indicate that
present wheat varieties can tolerate several
days of flooding without important reductions
in yield. Further information is needed on the
quantitative effects of low oxygen diffusion
rates occurring about 45 days after planting.
Timing of the Second Irrigation for Wheat
Field studies conducted in India over the
past 5 years have shown that maximum wheat
yields are obtained when the first irrigation
after planting is made 20 to 25 days following
planting, when crown root development
begins. CIANO experiments in 1969-1970 have
obtained more information on this point. Second
irrigations in both years were applied 15,
22, 29, 36 and 43 days after planting. The varieties
used in the 1969 experiments were Sonora
64 and Tobari, while Sonora 64 and INIA
were used in 1970.
Grain yields of Sonora 64 and Tobari were
5.00 and 4.78 ton/ha, respectively, in 1969.
Timing of the second irrigation did not affect
yields significantly.
The INIA variety produced equal yields of
3.67 ton/ha in 1970 when the second irrigation
was applied 15, 22, 29, and 36 days after
planting. Delaying the second irrigation until
43 days after planting reduced the grain yield
to 3.46 ton/ha.
The Sonora 64 variety produced the highest
yields, 4.30 ton/ha, when the second irrigation
was applied at 22 or 29 days after planting.
Yields were slightly less (4.00 ton/ha) when
the second irrigation was applied at 15 or 36
days after planting.
These results are not conclusive, but, in
the case of Sonora 64, do provide some evidence
that application of the second irrigation
at about 25 days after planting is essential
for maximum grain production.
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