Carlyle D. Holen and Alan G. Dexter
IPM Specialist-Northwest Minnesota, University of Minnesota, Crookston, MN and Extension Sugarbeet Specialist, North Dakota State University and University of Minnesota, Fargo, ND
Temperature is considered the primary factor determining the rate at which plants develop although other factors including daylength, moisture and light may modify the effects of temperature on the plant. Attempts to understand these relationships have spawned an enormous number of models and equations that quantify the amount of heat that is available for crop growth. The simplest of these methods is the growing degree day (GDD) concept which uses the daily high and low temperatures to calculate an index of the useable heat produced each day. These daily values are summed for a cumulative value of heat available for development over a period of time and can be used to predict growth stages for individual fields or for entire regions.
GDD equations have proven to be useful tools for researchers, crop managers and crop producers to predict plant development rate and growth stage. In certain crops this information is used in scheduling harvests and as an aid in planning for crop management decisions such as the timing for irrigation or pesticide applications. These equations are simple, use weather information that is readily available and allow a daily tracking of plant development.
Accurate predictions from GDD equations require 1) the temperatures used in the equation are the same as the temperatures experienced by plants in the field and 2) the plants are correctly staged. Errors in measurement between temperatures at regionally placed sensors and fields can be due to differences in field elevation or relief and level of crop residue on the soil surface.
Staging sugarbeet is not difficult but a uniform system for designating stages has not been proposed in the sugarbeet growing regions of the United States. Sugarbeet is a unique crop as it is a biennial plant grown as an annual for its sucrose reserves. In the first year of growth it remains in a vegetative state and produces an indeterminate number of leaves. Under Belgium conditions (110 day growing season) sugarbeet produced an average of 40 leaves and under full canopy maintained an average of 25 leaves with new leaves replacing ones that died. Fortunately, it isn't necessary or even useful to precisely know the sugarbeet leaf number beyond the eight- to ten-leaf stage when herbicide applications usually end because management decisions will not be improved by knowing crop leaf stage later in the season
The objectives of this research were to determine the GDD requirements of sugarbeet up to the nine-leaf stage and to refine the staging system used for sugarbeet.
Field experiments were conducted at the Northwest Experiment Station, Crookston, MN in 1991, 1992 and 1993. Number of plants measured and planting dates by year are presented as follows:
| Year | Number of plants measured | Planting dates |
| 1991 | 28 | May 17, 20, 22 June 17, 21, 28 July 5, 8, 11 |
| 1992 | 15 | June 1, 11, 22, 26 July 6 |
| 1993 | 18 | May 6, 14, 25 June 4, 8, 14 |
Plants selected from untreated plots from Betanex treatment experiments with multiple planting dates were evaluated two to three times per week using a 6-inch dial caliper. Medium-sized seed, variety Beta 1745, was planted at 3.8 lb /A one-inch deep. Counter insecticide was applied in a band application at 11.9 lb/A.
Leaf number, length and width were recorded for each plant. Leaves were considered mature and were counted when the leaf was completely unrolled and unfolded. The interval between mature leaves was further subdivided with a decimal fraction to reflect the percentage of leaf unrolled of the next emerging leaf. Ambient air temperatures were recorded with a Campbell Scientific CR10 data logger that was within one mile of the experimental site. Daily minimum and maximum temperatures were measured in a 24 hour period from midnight to midnight. Growing degree day values were calculated with the equation:
Daily GDD = (Tmax + Tmin) - Tbase
2
Tmax and Tmin are the daily maximum and minimum temperatures (F) respectively. Tbase is an individual crop or plant specific value that represents the minimum temperature for crop development . Two maximum temperature limits were evaluated. This temperature limit represents the maximum or optimum temperature for crop development. The following base and maximum temperature limits were used to predict leaf appearance rate.
| Base temperature (F) |
Maximum temperature (F) |
| 34 | -- |
| 40 | -- |
| 45 | -- |
| 50 | -- |
| 34 | 86 |
| 40 | 86 |
| 45 | 86 |
| 50 | 86 |
| 34 | 90 |
| 40 | 90 |
| 45 | 90 |
| 50 | 90 |
Sugarbeet Staging
Sugarbeet emerges from the soil with a pair of cotyledonary leaves. These leaves serve as a temporary source of food reserves for the developing seeding. As food reserves are removed, the cotyledonary leaves will yellow and drop from the plant, usually by the fifth-to sixth-leaf stage. The next leaves to emerge from the crown are the first two true leaves. Although these leaves appear as a pair and seem to be oppositely arranged, they are alternate and one of the leaves is developmentally behind the other. Stage separation between the first and second true leaves does not occur. Thereafter, all leaves emerge from the crown in an alternate pattern and are arranged in a 5/13 phyllotaxus. Phyllotaxus is a term used by botanists to describe the pattern of leaf emergence of a plant. A 5/13 arrangement means the leaves emerge from 13 vertical ranks around the crown and there are five turns around the crown as you follow leaf emergence before another leaf emerges in the same vertical rank.
Table 1 is a proposed staging system for sugarbeet up to the nine-leaf stage. This staging method designates the leaf stages as vegetative stages V1.0 through V9.0. Leaves are counted when the leaf blade is fully unrolled. This system also uses decimal fractions of each leaf stage to allow better separation of leaf stages and increased accuracy of GDD predictions. The decimal fractions are used to represent the percentage or amount of the next emerging leaf that has unrolled. For example, if the plant has three fully unrolled leaves and the fourth leaf is approximately 60 percent unrolled the stage is V3.6. At later leaf stages, when several unrolled leaf initials may be present, use the most advanced leaf in the estimate. This staging method has at least one major deficiency. As crop development progresses beyond the V2.0 leaf stage, two or more developing leaf initials always are present. This means true V3.0 to V9.0 stages do not exist. For example, as the fifth-leaf fully unrolls, the sixth leaf initial is present and the earliest V5 stage is V5.1 or V5.2
Choosing the correct base and maximum temperature limits for use in a GDD equation is important in minimizing prediction errors. Identifying the most appropriate temperature limits for plants has been elusive and it is believed this is because temperatures have been selected on the basis of observation rather than on underlying growth or development processes. Table 2 shows the R2 and Mean Square Error (MSE) values for each of the base and maximum temperatures evaluated. Selection of the best temperature limits can be done by looking for high R2 and low MSE values. Overall these data indicate there were only very small differences between the temperature limits evaluated. Each year indicates a different temperature base may be slightly superior. In 1991, the warmest year of the three years in which the experiments were conducted, the base temperature of 34 F without a maximum temperature appears best. This is the only year where the average daily temperature exceeded 90 F (on 11 dates June - August). In 1992 and 1993, a cool and cold year, a base of 50 and 40 F respectively appear best. In neither year did the daily maximum temperature reach or exceed 90 F which explains why the inclusion of a 90 F maximum limit in the equations are equal to the base temperature alone. When all years are combined, the 34 F base temperature without a maximum temperature limit appears superior. Research conducted in England also concluded, based on results of growth chamber studies, that a 34 F base temperature without a maximum temperature limit produced the most appropriate values for a GDD equation.
The response of sugarbeet to temperature was strongly curvilinear with early sugarbeet leaf stages requiring more GDD to advance in stage than later leaf stages. The cumulative GDD's required for sugarbeet up to the nine-leaf stage are shown in Figure 1 and the cumulative number of calendar days up to the nine-leaf stage are shown in Figure 2. The predicted number of GDDs and calendar days for each leaf stage are given in Table 3. The coefficient of determination for calendar days is surprisingly strong (0.952) which raises a question about using calendar days to predict leaf stages e.g. is it just as good as using GDDs? Under somewhat normal weather patterns, calendar days should provide a reasonable estimate of sugarbeet development, but when prevailing temperatures are either above or below normal then calendar days should be used with caution. This is simply because time in itself is not important in plant development but it is the heat available each day that allows plants to advance in stage. In research on crops such as corn, calendar days provide poor prediction of growth stages, but these estimates are made over the entire growing season. Calendar days may be more accurate in predicting sugarbeet leaf stages because the typical interval from V1 to V9 is less than 30 days and the average number of GDD per day do not vary widely. The mean number of calendar days per stage from V4 to V8 is 2.3 days.
GDD should be a more reliable predictor of leaf stages than calendar days and accurately account for development over a wider range of temperatures. The cotyledon and two-leaf stages of growth require nearly 47 percent of the total GDD necessary for the plant to reach the nine-leaf stage. After this establishment phase, new leaves develop rapidly with each new leaf requiring somewhat fewer GDD than the preceding one. The mean number of GDD for each leaf from V4 to V9 is 72.
The division of each leaf stage into decimal fraction allows for more accurate predictions of plant development. For example, if a plant is correctly staged at V2.5 but you ignore the decimal fraction and call it a two-leaf plant or V2.0 there are 70 GDD of development not taken into account (141 GDD from V2.0 to V3.0 vs 70 GDD from V2.5 to V3.0).
1996 Sugarbeet Research and Extension Reports. Volume 27, pages 152-157.
