Limited Irrigation: Its Effect on Sugar Yield

Can sugarbeets in the Central High Plains be grown under a deficit (limited) irrigation system? Of course they can. But should they, if one wants to achieve competitive sugar yield at harvest?

That’s what a three-year study at the University of Nebraska Panhandle Research and Extension Center in Scottsbluff set out to determine. The main question behind the 2007-09 investigation was: What is the impact on sugar yield of water stress during the growing season?

The impetus for this study was the multi-year drought that affected much of the Central Plains region during the first several years of this decade. While the drought has been alleviated by higher rainfall during the past two years, water restrictions are becoming an increasingly common fact of life among crop irrigators. Making the most efficient use of available water has become more important than ever for the region’s agricultural producers.

The 2007-09 sugarbeet water stress study was actually preceded by a somewhat-similar investigation conducted during the 1995-98 growing seasons. That study looked at the effect of limiting irrigation water during the latter part of the growing season (mid-August and on).

When the results were combined across both sprinkler and furrow irrigation plots over four years, they showed only a minor effect (not statistically significant) between full and limited irrigation. The full-irrigation treatments produced an average of 7,880 pounds of sugar per acre, while limited-irrigation treatments averaged 7,770 pounds. (Cutting off all irrigation after mid-August resulted in an average sugar yield of 7,340 pounds per acre.)

The 2007-09 investigation featured several additional dimensions, with the irrigation system operated similarly to a center pivot. The treatments included:

• Full irrigation

• 75% of full irrigation

• 50% of full irrigation

• 25% of full irrigation

• Full irrigation until July 15; then 50% of full irrigation

• 75% of full irrigation until July 15; then 25% of full irrigation

• 50% of full irrigation until July 15; then full irrigation

• 25% of full irrigation until July 15; then 75% of full irrigation

• No irrigation during the season.

All of the plots were irrigated simultaneously, with the various water treatments achieved by using different nozzle sizes. The irrigation water was applied on a three-day cycle, with a maximum application of 0.8 inch (equivalent to a 600-gpm well on a 125-acre field).

What was the bottom line of this investigation? When averaged across the three years, the most productive treatment, not surprisingly, was “full irrigation.” However, some of the other treatments were not statistically different in terms of pounds of sugar produced per acre. (The LSD — Least Significant Difference — is 466 pounds. That means the difference in yield ewtween any two treatments must be greater than 466 pounds in order to say the yields are truly different and not due simply to measurement variation.) Here are the specific numbers, by treatment:

• Full irrigation — 9,610 lbs.

• 75% of full — 9,190 lbs.

• 50% of full — 9,380 lbs.

• 25% of full — 8,350 lbs.

• Full until July 15; then 50% of full — 9,330 lbs.

• 75% of full until July 15; then 25% of full — 9,440 lbs.

• 50% of full until July 15; then full — 8,920 lbs.

• 25% of full until July 15; then 75% of full — 9,170 lbs.

• No irrigation — 6,490 lbs.

The results of this work tell us that sugarbeets can produce good yields even if the water supply is limited.

In 2007, furrow irrigation was used in this study to ensure germination and early growth. As a result, a significant amount of water was applied through June. Minimal rainfall occurred during the remainder of the season, but the “no irrigation” treatment still produced 75% of the “full irrigation” yield.

In 2009, rain was plentiful during early and mid-season growth while August rainfall was limited. The “no irrigation” treatment resulted in 85% of the “full irrigation” yield.

In contrast to those years, in 2008 the “no irrigation” yield was only 45% of “full irrigation.” But there also was much less rainfall that year (under 3.0 inches from April through mid-July, while August and September recorded nearly 5.0 inches of rain).

When growers recently faced drought and pumping restrictions, there were feelings of pessimism for the future of sugarbeet production in the region. But after positive grower experience in water-short situations — and research to support the ability to maintain sugar yield with reduced irrigation input — confidence in the sugarbeet industry should be as bright as with any other crop.

There is one that thing we can’t do: i.e., predict when and how much it is going to rain. On the other hand, we can react to the timing and volume of rainfall. Knowing when and how to irrigate with a limited water supply was the primary goal of this research study — and the answer is to “stress late and irrigate early.”

C. Dean Yonts is agricultural engineer and irrigation specialist with the University of Nebraska, Scottsbluff.