June 4, 2010

More Crop Per Drop.

That was the theme of this year’s UNL International Water for Food Conference and has been a longstanding area of study for researchers at UNL’s West Central and Panhandle research and extension centers.

As part of this, deficit irrigation research is particularly important in western Nebraska, where growers face increased challenges compared to those in eastern Nebraska due to lower rainfall, sandier soils and higher elevation. Average rainfall in the Panhandle is approximately 15 inches a year compared with 30 or more inches in eastern Nebraska. This difference means there is no “one size fits all” irrigation management recommendation. However, UNL research, as outlined below, shows how adopting water conservation measures in western Nebraska (as well as the rest of the state) can reduce the number of irrigations needed, and thus, keep a little more money in your bank account.

The data provide an excellent basis for determining the economic value of irrigation water and show the potential of no-till limited irrigated systems to sustain higher levels of productivity than most producers would deem possible with much less water than they have become accustomed to.

Research Specs

Since 2005 limited irrigation research has been conducted at Scottsbluff, building on earlier research conducted at North Platte. The primary objectives of this study are

1. to determine yields from deficit-irrigated corn, winter wheat, dry beans, and canola grown in a no-till cropping system versus full irrigation
2. to determine the agronomic feasibility and problems encountered in using no-till with crops that have primarily been grown under conventional full tillage

The soil at the Scottsbluff site is a Tripp, very fine, sandy loam with plant available water of 1.5 inches per foot. The 30-year average precipitation at Scottsbluff (elevation 3900 feet) is 15.5 inches.

Each phase of the rotation was present each year under a linear move sprinkler irrigation system and the same irrigation level is repeated over time, so low water levels are low year after year. The irrigation levels for the crops were 4, 8 and 12 inches for dry beans, canola and wheat and 5, 10 or 15 inches for corn. The highest irrigation level was designed to be near the long-term average non-ET limiting irrigation.

If rainfall is sufficient, not all the water is used. This has happened only once in the last six years. A no-till drill was used to plant winter wheat and canola. Dry beans and corn were planted no-till in 30-inch rows. Plant populations were not changed across irrigation levels for dry beans, canola, and winter wheat. Corn plant populations for the low, medium and high irrigation levels were 16,000/ac, 24,000/ac and 32,000/ac.

The lowest level of deficit irrigation for corn was usually not applied until tassel emergence based on earlier research. Irrigations of 1 to 2 inches per week were applied from late vegetative stage until water was used. This approximates farmer practices in the area. For the higher irrigation levels, irrigation was started earlier in the vegetative period. Similar strategies were used for other crops.

Research Results

Table 1 shows corn yields for the five-year study.. These are representative good yields for the Panhandle. (Remember, this is not central Nebraska where there are deep silt loam soils, a two-week longer growing season and 24 inches of annual rainfall.)

During 2007, a late freeze on June 8 caused severe damage, but plants did recover. Maximum corn yield in a given year is used to calculate a relative yield. As an example, in 2008 the high yield of 183 bu/ac would be 100% (183/183). For the 5-inch irrigation, the relative yield would be 115/183 = 63%. Relative yields are plotted versus irrigation applied to create a production function (Figure 3).

Our research confirmed much of the previous research on limited irrigation in higher rainfall regimes. The shape of the irrigation response functions (relative yield versus irrigation) was generally curvilinear, however those from the Scottsbluff research were much steeper than those from the North Platte research.

The shape of these irrigation production functions varies drastically across Nebraska because of differences in soils and rainfall. Figure 4 presents data from North Platte and Scottsbluff for drier years and includes a simulated curve for Central Nebraska.

In three of the five years studied precipitation was only 66% of the 30-year average, creating a severe test and much higher responses between the medium and high irrigation level than in our North Platte research. At the lowest irrigation levels, most crops yields were only 45%-50% of maximum yield.

Applying the Data – By the Numbers

If we are very efficient with irrigation and capture maximum amounts of natural precipitation using good residue management and reduced tillage or no-till, we can expect to get by with 14 to 15 inches of irrigation in Scottsbluff. In wetter years, the production functions show that we can get by with 10 to 11 inches or irrigation.

At North Platte, our research showed that in drier years we needed about 10 inches of irrigation for full yield, but in wetter years 7 to 8 inches was adequate. Similarly, in Central Nebraska where there is more rainfall, if producers follow similar water conserving strategies and practices, they should be able to get by with 7 to 8 inches of irrigation for fully irrigated corn. In wetter years it would be 5 to 6 inches.

This research shows how growers in western Nebraska, as well as others, can get “more crop for drop” – higher productivity and thus a higher economic return for their irrigation investment and use of reduced tillage systems.

Gary Hergert
Extension Nutrient Management and Soil Quality Specialist
Panhandle REC, Scottsbluff