Planting Date and Sorghum Flowering
A study was conducted at the Maricopa Agricultural Center to determine the influence of planting date on time to flowering of sorghum hybrids. Sorghum was planted on March 19, April 16, May 14, June 18, July 2, July 16, and July 30. A total of 17 sorghum hybrids varying in maturity groups from early to late were planted at each date. The number of days from planting to flowering was greatest at the March 19 planting date and decreased with each planting date thereafter. Growing degree days required to reach flowering likewise decrease as planting was delayed. In order to avoid the heat during pollination in the early part of the summer, early to medium maturity hybrids need to be planted in mid-March at Maricopa. July planting dates resulted in flowering occuring in late August and September.
Sorghum acreage in Arizona has increased in recent years and current information on production practices is needed. The optimum planting date, in particular, has been questioned. The optimum planting date for sorghum grown in Arizona is detailed for various elevations in the state and for various sorghum maturity groups (Dennis, 1981). Sorghum can be grown in two seasons in Arizona: spring-summer or summer-fall. In the spring-summer season sorghum should be planted as soon as possible, usually in March, so that the crop pollinates before the hot, dry weather occurs in June. In the summer-fall season, sorghum is typically planted in July, and pollination occurs at the end of August or in September when the weather has cooled somewhat. The crop matures before cool temperature occurs in November. Since sorghum is often planted after small grains, much of the sorghum in the state is planted in the summer and grown in the summer-fall season. Sorghum is sometimes planted in June which is usually too early and the heads are subject to blasting, or in August which is too late and the crop does not have time to mature.
The original purpose of this study was to provide information on yield and maturity of sorghum as affected by planting date. However, severe bird damage forced us to abandon any hope of obtaining meaningful yield results, and the only reliable data we were able to obtain was date of flowering.
A planting date experiment was conducted at the Maricopa Agricultural Center on a Casa Grande sandy loam soil. The previous crop was barley. The experimental design was a randomized complete block with 17 hybrids and 4 replications combined over 7 planting dates. Seeds were planted with a cone planter into flat dry soil in five rows spaced 24 inches apart in plots 10 ft x 25 ft. Seeds were planted at a rate of 1000 seeds per plot or 174,000 seeds per acre, which, at an establishment rate of 70% would result in a final stand of 122,000 plants per acre. The planting dates were March 19, April 16, May 14, June 18, July 2, July 16, and July 30. Germination irrigation was applied on the day of planting. Ammonim sulfate was applied preplant at a rate of 52 pounds of nitrogen per acre. UN32 was applied in the irrigation water at a rate of 35 pounds of nitrogen per acre for the first three post-emergence irrigations for a total nitrogen rate of 157 pounds of nitrogen per acre. Irrigation amount was about 6 inches per irrigation and the frequency was about every 10 days during the peak of growth. The irrigation schedule for each planting date was as follows: 1) March 19 - 3/19, 3/31, 5/2, 5/14, 5/30, 6/6, 6/17, 6/26, 7/7, and 7/16, 2) April 16 - 4/16, 5/15, 5/30, 6/6, 6/17, 6/26, 7/7, and 7/16, 3) May 14 - 5/14, 5/30, 6/6, 6/17, 6/27, 7/7, 7/16, and 8/5, 4) June 18 - 6/18, 7/9, 7/16, 7/25, 8/5, 8/14, 8/25, and 9/4, 5) July 2 - 7/2, 7/16, 7/25, 8/5, 8/14, 8/25, 9/4, 9/15, and 9/26, 6) July 16 - 7/16, 8/5, 8/14, 8/25, 9/4, 9/15, 9/26, and 10/9, and 7) July 30 - 7/30, 8/14, 8/25, 9/4, 9/15, 9/26, 10/9, and 10/31.
Flowering dates were noted for each plot. The flowering date corresponded to when about half of the heads are blooming or pollinating . Growing degree days were obtained from the Arizona Meteorological Network (AZMET), and were calculated using 50 F as a base temperature and 86 F as a ceiling temperature from a sine curve of maximum and minimum temperature.
The growing season weather conditions are presented in Table 1. From July through October, which may be considered the normal sorghum growing season in Arizona, maximum temperature was below average, minimum temperature was above average, precipitation was below average, and growing degree days (GDD) were close to the average. During the spring-summer season, GDD accumulated faster than nomal and maxiumum temperature in June was below normal.
The influence of planting date on time to flowering averaged over the 17 hybrids is presented in Table 2. The number of days to reach flowering was greatest with spring planting dates and least with summer planting dates. Generally, the longer a crop is in the ground the greater the yield potential is due to the number of days available to capture sunlight and transform this energy into carbohydrates and yields. Of course, this yield potential may not be met if certain conditions are unfavorable, such as cold weather limiting stand establishment or high temperatures limiting pollination.
The number of growing degree days to reach flowering also varies with planting dates. Growing degree day or heat unit models of crop growth are notoriously specific to a certain season of growth or locality. Photoperiod and other factors affect crop development and growing degree days have some limitation in predicting crop development. Less growing degree days are required to reach maturity if planting occurs in the summer compared to the spring.
The hybrids are compared in Table 3 in terms of time to reach flowering averaged over planting dates. The time required for each hybrid to reach flowering at each planting date is presented in Table 4. In order to avoid flowering during the hot, dry weather that may start in mid-June or earlier, early to medium maturity hybrids should be planted in mid-March. If planting is delayed until mid-April, flowering will occur during the latter half of June for all hybrids. Mid-May planting dates resulted in flowering during mid-July and mid-June planting dates resulted in August flowering dates. For flowering to occur in September, planting dates of July are required.
The authors wish to acknowledge the assistance of Dan Gladden and the farm crew at the Maricopa Agricultural Center.
This is a part of publication AZ1059:
"1998 Forage and Grain Agriculture Report," College of Agriculture, The
University of Arizona, Tucson, Arizona, 85721.