Agronomic Practices to Stretch Limited
Water Supplies
Crop production considerations for producers facing limited water include plant populations, residue management, water timing, and soil fertility.
Plant Populations
Plant
populations for dryland production have traditionally been less than for
irrigated production. Populations are reduced to better match precipitation
and stored soil water to crop ET. However, populations on irrigated corn
must be reduced to less than 18,000 plants/acre to reduce ET significantly.
Lamm and Trooien (2001) found that corn grain yields generally increased
as plant populations increased from 22,000 plants/acre to 34,000 plants/acre
for varying irrigation capacities. Little yield penalty was observed at
higher plant populations compared to lower populations when no irrigation
was applied. Therefore, if corn is grown for irrigated production, even
limited, then producers should stay with their normal populations. If
the intent is to grow dryland corn with no irrigation, then a dryland
population (12,000 to 18,000 plant/acre) is the best option.
Residue Management
The goal when working with limited water is to capture every possible
source of water in the production system. These sources include rainfall,
snowfall and irrigation water. Residue management can have a significant
impact upon increasing the availability of water. Runoff from precipitation
and irrigation is also reduced when surface residue is present. Residue
acts as small dams that slow water movement and allow more time for the
water to infiltrate into the soil. Residue also reduces the impact of
rainfall and irrigation upon surface sealing which increases infiltration
rates. As droplets impact the soil surface, they destroy the surface structure
which will seal the soil surface and reduce infiltration rates. Residue
protects the soil surface from the impact of these droplets. Many benefits
of increased residue on evaporation losses and stored soil moisture are
covered in more detail in articles
by Klocke and Nielsen
in this issue of Agronomy News.
Crop Rotations and Water Timing
Crop rotations that have lower water use crops (see article by Schneekloth)
can reduce irrigation needs. Schneekloth et al. (1991) found that when
limited to 6 inches of irrigation, corn following wheat yielded 13 bu/acre
(8 percent) more than continuous corn. The increased grain yield following
wheat was due to increased stored soil moisture during the non-growing
season that was available for ET during the growing season. Crop rotations
also spread the irrigation season over a greater time period as compared
to a single crop. When planting multiple crops such as corn and winter
wheat, the irrigation season is extended from May to early October as
compared to continuous corn, which is predominantly irrigated from June
to early September.
Some systems can never meet crop ET, even with normal precipitation. O’Brien et al. (2001) found that when irrigation system capacity was increased from 0.1 inches/day to 0.2 inches/day yields increased by 28%. To achieve this change in capacity per irrigated acre, a producer would have to reduce irrigated acres by 50%. Profitability of increasing the irrigation capacity by reducing irrigated acres increased net returns per irrigated acre by nearly 4 times. Though only half of the acres were irrigated, profits were more than twice that of irrigating the entire acreage.
Timing of water is critical to crop response. A great amount of research has been done on this subject in irrigated regions. The general finding is that the greatest response to water is during the reproductive growth stages for most crops. A table of critical growth stages for some Colorado crops is provided in the fact sheet “Crop Water Use and Growth Stages, no. 4.715” available online at http://www.ext.colostate.edu/pubs/crops/04715.html. In most cases, grain crops can incur some stress during the vegetative growth stages without significant yield loss, but will decline rapidly with stress during reproductive growth.
Soil Fertility
Although the focus of this newsletter is on limited water, it is important
to remember that yield potential can be limited by a variety of other
factors as well (insects, disease, heat units, soil fertility, etc.).
During dry years the goal of crop production is to maximize water use
efficiency (WUE) defined as yield divided by water used. Fields that are
deficient in one or more nutrients are less able to tolerate water stress
and will have a lower WUE than fields with sufficient soil fertility.
The key is to match fertility requirements to yield potential determined
by water supply. As in water sufficient years, the most reliable method
to determine soil fertility needs is through soil sampling and analysis.
In-season tests may have the most potential for return on fertilizer dollars
this year because our knowledge of water supply will improve as the season
advances. In-season testing is described in the February-March
2001 issue, Vol. 21 of Agronomy News. Articles in this newsletter
also address coping with high nitrogen fertilizer prices, which is also
becoming an issue for the 2003 growing season.
Balanced soil fertility should also be a consideration during dry years. Research has shown an improvement in WUE when phosphorus (P) fertilizer is applied to deficient soils. Phosphorus may increase WUE for a variety of reasons. One is that P is not mobile in soil and with limited water, an adequate supply within plant roots may explain part of the benefit. Another is the possible root stimulation under P fertilization. Regardless of the reasons, growers should evaluate whether their fertility program has adequate P, K, and other nutrients besides N. A balanced fertilizer package basing N, P, K on soil test results and adjusting N for a potential yield decline under drought conditions will produce the best return under limited water supplies.
