An Early Soybean Production System for Drought Avoidance

Late summer drought during seed formation often limits yield of soybean [ Glycine max (L.) Merr.] grown in the southern U.S. A field study was conducted to determine if earlier planting of early‐maturing varieties could avoid periods of drought stress by having their reproductive development coincide with periods of greater available moisture. Varieties in Maturity Groups (MG) III to VII were planted in mid‐April and in mid‐May near Blossom and Hooks, TX, from 1986 to 1988. There were significant year and location effects on yield. Two MG III varieties planted in April 1986 at Blossom yielded eight times more than the MG V to VII varieties. Negative correlations were found between yield and maturity date in 1986 and 1987. Early drought stress in 1988 reduced yields about 30% for the April‐planted MG III and IV varieties, but yield was again negatively correlated with maturity date. Yields at the Hooks site were up to 45.8 bu/acre higher than those at Blossom, but they were also negatively correlated with maturity date in the April 1986 planting. The seasonal rainfall at Hooks in 1987 was less than half that of 1986 resulting in varietal yields from the early planting being reduced from 8.3 up to 47.1 bu/acre. A negative correlation was still found between maturity date and yield. In 1988, however, late‐season rainfall resulted in a positive correlation between yield and maturity for the April planting. Results from this study show that an early soybean production system using adapted MG III and IV varieties can avoid the effects of late summer drought and result in acceptable yields. Research Question Late summer drought stress often limits yield of soybean grown in the southern USA. A field study was conducted to determine if earlier planting of early‐maturing varieties could avoid periods of drought stress by having reproductive development coincide with times of greater moisture availability. Literature Summary In northeast Texas, as in most of the southern USA, the optimum planting time for MG V to VII varieties is considered to be from mid‐May to early‐June. Under such production systems, flowering, pod‐set, and seed filling occur from late July to early September. This period of reproductive development often coincides with prolonged periods of low rainfall and high temperature. It would be advantageous to shift this period of high water requirement to a time of greater moisture availability, as commonly occurs in June and early‐July. Planting MG V to VII varieties earlier (mid‐April) does not achieve this goal because plants still mature within 7 to 10 d of plants seeded at optimum dates. Moreover, the short‐days of April may induce premature flowering that results in reduced plant height and limited yield potential for traditional determinate plant types. While MG V to VII varieties are primarily determinate, MG III and IV varieties are generally indeterminate. Indeterminate types continue to grow vegetatively after flowering begins; thus, plant heights are reduced less than those of determinate types planted in mid‐April. Also, MG III to IV varieties planted in the southern USA mature earlier than MG V to VII varieties. Study Description This research (Table ) was conducted for 3 yr at two nonirrigated locations in northeast Texas. The Blossom site has a clay soil and the Hooks site has a silt loam. Varieties: MG III — Williams 82, Asgrow A3966; MG IV — FFR 441, Crawford, Egyptian, Northrup King RA 451; MG V — Asgrow A5474, Forrest; MG VI — Centennial, Leflore; MG VII — Bragg, Jacob Hartz H7126; Applied Questions Will the planting of early‐maturing (MG III and IV) soybean varieties in mid‐April reduce risks associated with late‐summer drought in the southern USA? The heavy clay soil at the Blossom site is considered low in available moisture and cracks deeply when dry. Furthermore, high temperatures and generally low rainfall in late‐July and August result in drought stress most years. The highest yielding varieties in this test in 1986 and 1987, averaging more than 40 bu/acre, were the MG III varieties Williams 82 and A3966 planted in mid‐April. The average yield of the six April‐planted MG III and IV varieties was 32.3 and 34.2 bu/acre in 1986 and 1987, respectively (Table ). The average yield of the six May‐planted MG V to VII varieties was only 9.4 and 7.6 bu/acre in 1986 and 1987, respectively. Thus, in the first 2 yr, early planting of early‐maturing soybean varieties suxessfully avoided late‐summer drought and resulted in seed yields substantially higher than those obtained with conventional production practices. The Blossom site received much less rainfall in May and June of 1988 than in 1986 and 1987. Therefore, the average yield of the April‐planted MG III and IV varieties was reduced about 5 bu/acre to 28.9 bu/acre and the May‐planted test was lost due to poor plant stands resulting from the dry conditions. The silt loam soil at Hooks holds more available moisture than the clay soil at Blossom. Soybean grown at Hooks showed less drought stress and had higher yields than soybean grown at Blossom. Still, average yield of the six April‐planted MG III and IV varieties was more than the average yield of the six May‐planted MG V to VII varieties (Table ). In 1986, rainfall was plentiful during the growing season. The early‐planted MG III and IV varieties averaged 43.9 bu/acre while the May‐planted MG V to VII varieties averaged 34.7 bu/acre. Half as much rain fell in the 1987 growing season as in the 1986 growing season. The average yield of the six April‐planted MG III and IV varieties was 24.5 bu/acre in 1987, while the May‐planted MG V to VII varieties averaged only 15.3 bu/acre. The first half of the growing season in 1988 was dry at Hooks. The May planting was lost due to the dry conditions, while the average yield of the six April‐planted MG III and IV varieties was 35.4 bu/acre. Even on a silt loam soil, early planting of early‐maturing soybean varieties was superior to the conventional system in all 3 yr of the study. Will yields of MG V to VII varieties be increased by planting in mid‐April rather than mid‐May? Planting the MG V to VII varieties early did not result in higher yields. Average yield from the six April‐planted MG V to VII varieties was similar to average yield from the May‐planting at both Blossom (5.2 vs. 9.4 bu/acre in 1986 and 11.8 vs. 7.6 bu/acre in 1987) and Hooks (40.9 vs. 34.7 bu/acre in 1986 and 11.3 vs. 15.3 bu/acre in 1987). Will yields be unfavorable if planting of the MG III and IV varieties is delayed from mid‐April until mid‐May? At Blossom, the average yield of the six MG III and IV varieties was reduced from 32.3 to 15.7 bu/acre in 1986 and from 34.2 to 15.0 bu/acre in 1987, when planting was delayed from mid‐April to mid‐May (Table ). At Hooks, the average yield of the same varieties was reduced from 43.9 to 34.3 bu/acre in 1986 and from 24.5 to 15.3 bu/acre in 1987 (Table ). However, average yield of the six May‐planted MG III and IV varieties at both locations and in both years was equal to or greater than average yield of the six MG V to VII varieties also planted in May. Agronomic and cultural information for Blossom and Hooks, TX, from 1986 to 1988. Blossom Hooks 1986 1987 1988 1986 1987 1988 Planting dates 16 Apr. 17 Apr. 22 Apr. 17 Apr. 15 Apr. 21 Apr. 15 May 12 May 6 May 14 May 11 May 7 May Row spacing, in. 32 34 34 32 36 36 Fertilizer, N‐P‐K 0‐83‐44 0‐50‐26 0‐50‐26 0‐83‐44 0‐50‐26 0‐50‐26 Rainfall, in. (10 Apr.–30 Sep.) 27.8 21.1 11.8 31.3 14.9 14.4 <jats:table