REPLANTING THRESHOLDS FOR SUGAR BEETS OR ASSESSING
Replanting a field of sugar beets always takes added expense, labor, time, and usually a healthy degree of emotional involvement. Assessing the need of whether to replant is the start of this effort. When the crop is almost entirely destroyed by frost or wind, the decision to replant can be relatively easy to make. Usually, however, the decision is much more involved and less conclusive than we would like, taking its toll on nerves, knees, and pocket book after spending hours in the field trying to make the decision.
WHAT WE KNOW
To start the discussion of when to replant, a review of what we know is required to grow a successful crop of sugar beets may be helpful. We know that a when crop is established as early as conditions are favorable for plant growth, there is more time for growth and ultimately for sugar production. Plants that are established as early as conditions allow, start root growth and produce a full canopy earlier. The result is usually higher tonnage and more time to produce and deposit sugar in those tons.
Temperature and Moisture for Germination and Emergence
Using 37° F as a basis, sugar beets require 90 degree-days in order to achieve 50% germination. Therefore, if the daytime highs were 50° F, and the lows around 35° F, youd need about 16-17 days before youd see half of the crop above ground. Even when we plant around the first of April, you dont see many plants emerge until after about the 20th, depending on temperature. The crop is also in a state of uncertainty because of the potential for frost, wind and whatever else the weather may deal us this time of year.
Maximum germination occurs with low moisture stress and warm temperature. By maintaining about 50% available moisture in silt- and clay-loam soils, we can expect an emergence of at least 60%. Irrigation or rainfall tend to cool the soil and slow germination. Soil conditions result in less capillary movement of water to the seed and a slowing of the germination process. Soil temperatures affect the rate of emergence, but not necessarily the final number of plants emerged.
Plant Population for Optimum Production
The basic goal is to achieve maximum light interception by the plant leaf surfaces which will result in maximum growth. Light is the energy that drives the photosynthetic process. Photosynthesis results in the forming of sugar that is the fuel for all plant functions, including growth.
The differences in growth potential resulting from light interception at the growth stage of maximum leaf surface area (lay-by), usually about mid-July to early August, can be compared:
These differences correspond directly to growth because light interception is directly related to growth! The more dense population put on 14 % more root mass per week. Differences in final yields are directly measurable in terms of light radiation interception!
Studies of light interception by sugar beets show that the optimum practical arrangement of plants in the field is a row spacing no greater than 20 inches apart. In-row plant spacings range from 5.9 inches to 15.75 inches, with an average of 10-11 inches, resulting in a total of about 30,000 plants/acre. With our most common row spacing of 22 inches, the average in-row spacing should be 9.5 inches with a range of 5-6 inches to 15-16 inches. This spacing also results in about 30,000 plants/acre, or 126 plants/100 ft of row. Light interception and sugar production can be optimum with both arrangements.
Plant populations of up to 150 plants/100 ft of row (35,000 plants/acre) have been recommended in recent years. This high population can be supported by good quality soil and better utilization of nutrients and water, which results in higher sugar production per acre. Not all soils can efficiently support these high plant populations. Lighter colored, shallower soils usually produce better with stands of about 120-130 plants/100 ft. row. Another observation with 150 plants/100 ft row is that approximately 10-15 % of plants often never reach harvestable size. Removing 15 % from a 35,000 plant/acre population leaves 29,750 plants/acre, or 125 plants/100 ft. of row. Now isnt that interesting!
WHEN TO REPLANT
What minimum percent stand is good enough not to replant? The old "rule of thumb" has been that if there are 70 plants/100 ft of row and they are reasonably spaced, the field should not be replanted. The assumption is that a reasonable yield can be obtained by keeping a field with this beet population instead of replanting.
Several European studies showed that the way gaps between plants combine to provide a growing space for each plant could be predicted from expansion of the polynomial theorem (Scott and Jaggard 1993). This information, combined with knowledge of the relationship between the weight of the individual plant and population density, allows predictions to be made of the effect of seed spacing and plant establishment on sugar yield. At practical seed spacings, maximum yield would only be achieved if more than 70 % of seeds planted produced established plants. When this model was applied in field tests it was accurate. Average sugar yields were always within 3 % of predicted values, and 70% plant establishment repeatedly was shown to be required for maximum sugar yields.
If 30,000 plants per acre is the desired plant population for all growth considerations, 70% plant establishment equals 21,000 plants per acre, or 88 plants/100 ft. of row. Near optimal yield can be achieved with only 70% stand even though 100 % stand (30,000 plants/acre) would be more favorable for yield, quality, and utilization of resources. Sugar beets, as is well known, have great compensating capability because of the profuse canopy, a deep rooting system to forage for nutrients and moisture, and their hardiness once established. Based on this information, a new "rule of thumb" for minimum stand may be not to replant if there are 88 plants/100 ft of row.
The next obvious question which is most often asked about achieving necessary stand population is what seed drop is needed. This is one of the hardest to answer because of the many variables that affect emergence. As discussed above, 88 plants/100 ft of row is minimum and 126 to 150 plants/100 ft of row is optimum, depending on soil fertility, soil type and other cultural factors.
About three years ago I put together a chart that shows plant populations and plants per 100 ft. of row at various seed spacings and percentages of emergence (Fig. 1). The most practical approach to determine the seed spacing required to achieve the desired population is for each grower to review historical stands resulting present seed spacing and adjusting the spacing to achieve an average emergence of 30,000 plants/acre.
Fig. 1. Number of plants/acre and /100 ft of row resulting from various emergence percentages and seed drops based on 22 inch rows (23,760 ft of row/acre).
KLAMATH BASIN STUDY ON SUGARBEET PLANTING DATE
The Klamath Basin is located on the Oregon-California inland border and has approximately 200,000 acres of irrigated agriculture. Sugar beets have been raised there since 1989. Soil types range from sandy loams with 1% organic matter to mucky clay loams with 13% organic matter. Temperatures during the spring emergence period often fluctuate from 5-10 ° F in one area of the basin to another. Planting dates and harvest period are similar to those in Eastern Oregon and the Snake River Plain of Idaho.
A series of field studies were conducted from 1991-1996 by researchers at the University of California Intermountain Research and Extension Center in Tulelake, California, and the Oregon State University Klamath Experiment Station in Klamath Falls, Oregon, evaluating the effects of plant population and planting date on sugar yield. This was a cooperative effort of these two stations that are only about 25 miles apart from each other. Planting dates in the study ranged from April 4 to May 31 and plant populations from 7100 to 37,000 plants per acre. Following are the conclusions of the study.
The following chart was developed from the Klamath Basin study to help determine the approximate value of replanting by comparing the percentage of expected sugar yield per acre (Fig. 2). The percentages in the rows for each are those of yield expected for each plant population compared with the yield expected with 88 to 126 plants/100 ft row. This data from this chart supports the above discussion on minimum and maximum plant populations necessary for optimum production.
Fig. 2. Klamath Basin Sugar Beet Replant Guide. Sugar yields expected with varied plant populations and planting dates in the Klamath Basin. Sugar yields are expressed as percentages of maximum sugar yield per acre (Carlson et. al.)
How to use the replant guide
First take stand counts and note the percent yield in the row of the original planting date and column for the stand count. Compare this yield with the yield in the row for the replant date and column for expected plant population after replanting.
The benefit of replanting must be evaluated in relation to the probability of achieving significantly better stands by doing so. If this cannot be achieved, then a healthy stand that is already established and growing might do as well or better.
The shaded area in the upper right of the chart shows that replanting because of complete loss of the crop within the first 2-3 weeks of original planting date can result in about the same yield if re-establishment and subsequent growth are good. For the following examples, refer to Fig. 2.
This information is not absolute and the uniformity of plant spacing in the row needs to be considered. Gaps should be no greater than 15-16 inches between plants and there should not be many such gaps in 20 feet. A good way to assess the stand count is to simply start somewhere in the field, mark out 5 rows 20 foot in length in a block, and count the beets in them. There should be 25 in each row for a perfect stand of 125-130 plants/100 ft of row, 20 in each for 100, 17-18 in each for "the minimum rule of thumb" of 88, 14 for 70, etc. If there are more than 2 out of the 5 rows with fewer than 10 plants, the count, the length of gaps, and beets directly across in the next rows must be evaluated. In addition, the health of the beets, the possibility of additional emergence, and the reasons for delayed emergence must be assessed. The final stand count may then be an estimate based on all these assessments, and the replant guide be used to make the judgement on whether to replant.
Carlson, H. L., Rykbost, K. A., Kirby, D. W. and Dovel, R. L. Effects of sugarbeet planting date and plant population on sugar yield in the Klamath Basin. University of California Intermountain Research and Extension Center, and Oregon State University Klamath Experiment Station.
Scott, R. K. and Jaggard, K. W. 1993. Crop physiology and agronomy. Pages 179-237 in: The Sugar Beet Crop: Science Into Practice. D. A. Cooke and R. K. Scott, eds. Chapman and Hall, London.
Presented at the Snake River Sugarbeet Conference on January 13-14, 2000.
David Elison, Agronomist, Amalgamated Sugar Company, PO Box 700, Paul, Idaho 83347-0700.