Season-Long Deficit Irrigation of Sugarbeets / Results of Three-Year Nebraska Study / By C. Dean Yonts*

    Within the Western Sugar Cooperative growing region**, surface water supplies for irrigation are limited whenever drought occurs.  As a result, delivery of surface water may (1) be delayed at the beginning of the season, (2) restricted during the season, and/or (3) shut off before the irrigation season is over.  Since adequate water may not be available to meet the crop’s total needs, surface water users thus have a difficult decision to make in selecting which crop(s) to irrigate.

    For those growers who depend on groundwater for irrigation, drought increases the amount of water they need to pump — resulting in increased pumping costs.  In many locales, however, irrigation pumping has resulted in a decline in the groundwater table.  In response, government agencies have imposed pumping restrictions in an attempt to extend the useful life of affected aquifers.
    During the early 2000s, a time when restrictions on groundwater pumping were put in place and there was a drought-induced lack of surface water, many producers felt that sugarbeet’s viability as a crop could be in jeopardy.  Without an abundant supply of water, beet production levels likely could not be sustained, necessitating a switch to other crops.
    But society is demanding that irrigated water for the production of all agricultural crops be delivered both effectively and efficiently throughout the season.  Deficit irrigation strategies are being developed for many crops to deal with water shortages throughout the growing season.  One of them must be sugarbeets so that beet production can be maintained.  That was the motivation for a three-year (2008-10) study at the University of Nebraska’s Panhandle Research and Extension Center at Scottsbluff.

    The Scottsbluff study compared nine different irrigation treatments under a small-plot sprinkler irrigation system.  The treatments — each of which was replicated six times — were as follows:
1.    100% of full irrigation
2.    75% of full irrigation
3.    50% of full irrigation
4.    25% of full irrigation
5.    0% (no irrigation)
6.    100% of full irrigation until August 15; then 50% of full
7.    75% of full irrigation until August 15; then 25% of full
8.    50% of full irrigation until August 15; then 100% of full
9.    25% of full irrigation until August 15; then 75% of full

    Two varieties (Betaseed 66RR70 and Hilleshog/Syngenta 9027RR) were planted, with the sprinkler system installed each spring immediately after planting.  (A severe storm and stand-reducing frost necessitated replanting of the 2010 plots on May 18.)  Overall sprinkler irrigation treatments spanned the periods of June 30-Sept. 29, 2008; July 9-Sept. 25, 2009; and July 2-Sept. 24, 2010.
    Approximately 7.7, 8.5 and 7.4 inches of rainfall were received during the 2008, 2009 and 2010 growing seasons, respectively.  Rainfall events occurred at relatively even intervals throughout the 2008 season.  During 2009 and 2010, though, rains fell primarily during the early part of the season, prior to the initiation of irrigation.  In those two years, only about 1.0 inch of rain fell during the heavy-water-use months of July, August and September.
    The soil type in the Scottsbluff plots is a Tripp very fine sandy loam, with a water-holding capacity of 1.8 inches/foot.  The sugarbeets were planted in 22-inch rows.  All plots were irrigated simultaneously, with the water treatments based on nozzle size.  Plots were irrigated no more than once every three days, with a maximum of 0.8 inch of water being applied during any single application.  The sprinkler system simulated a 125-acre center pivot with a 600-gallon/minute well.
    Leaf area and plant height were measured during each season to determine the influence of deficit irrigation on canopy development and architecture.  Also, a neutron probe was used periodically throughout each growing season to measure volumetric water content at one-foot increments.

    What were the findings of this study? 
    Only minor visual differences in plant canopy mass and water-stressed leaves were observed during the growing season among the nine treatments.  In particular, only the 25% and 0% treatments routinely showed visual signs of water stress. 
    Leaf area index was statistically less (by about 17%) for the 0% (no irrigation) treatment, compared to all other treatments.  Plant height for that 0% treatment was 5 inches less than the 25% water treatment and nearly 9 inches less than the 100% irrigation treatment.
    Tare tended to decrease as the irrigation amount decreased.  That’s likely due to fewer fine root hairs on beets in the drier treatments, thereby reducing the amount of soil that could cling to the roots.  Also, drier soil would tend to separate from the roots easier at harvest than would soils containing greater water content.
    Average sucrose content across the three years ranged from a low of 14.9% for the 0% irrigation treatment up to 16.0% for the 75%-25% treatment.  Sucrose levels were similar across all treatments — with the exception of the 50%-100% and 0% treatments.  Those two were statistically lower than the others, but similar to each other.  The sucrose content results would indicate that as soil water conditions become drier during the growing season, sugar levels likewise are reduced.
    Average sugarbeet root yield decreased from a high of 31.1 tons/acre for the 100% irrigation treatment down to a low of 20.4 tons for the 0% irrigation treatment. 
    Per-acre sugar yield was similar for the 100%, 75% and 50% treatments; but compared to the 100% treatment, sugar per acre declined by 14% and 38%, respectively, for the 25% and 0% irrigation treatments.  The split treatments that applied more water early in the season (100%-50% and 75%-25%) had a statistically higher per-acre sugar yield than the split treatments in which more water was applied later in the season (50-100 and 25-75).
    Sugar loss to molasses averaged 1.3, varying from a low of 1.2 for the 100% irrigation treatment up to 1.7 for the 0% treatment.

What bottom-line conclusions can be drawn from this three-year irrigation study?  There are two:
    First, sugarbeets can perform satisfactorily under deficit irrigation.
    Second, sugarbeets respond better to mid-season irrigation treatments than to late-season irrigation.  (This assumes there is adequate water that limits moisture stress during early season growth when plants are being established and developing a strong root system.)   

* C. Dean Yonts is associate professor of biological systems engineering at the University of Nebraska Panhandle Research and Extension Center, Scottsbluff.

** The Western Sugar Cooperative growing region includes western Nebraska/ northeastern Colorado/southeastern Wyoming and south central Montana/ north central Wyoming.  This study was funded by the Western Sugar-Grower Joint Research Committee.