Soybean Irrigation Serially Timed During Stages R1 to R6. 11. Yield Component Responses

Soybean [Glycine max (L.) Merr.] seed yield components are established and fixed in a temporally ordered sequence during stages R1 to R6. To determine the effect of irrigation timing during R1 to R6 on these traits, we examined the seed yield component responses of 16 soybean cultivars to nine irrigation treatments in a 3-year experiment. Treatments T₁ thru T₇ consisted of only a single irrigation coinciding with the average R stages 1.1, 2.5, 3.7, 4.7, 5.6, 5.9, and 6.4, respectively. ‘Check’ treatments included TO, which was not irrigated, and T₈ which received seven total irrigations coincident with those for T₀ and T₇. Averaged over cultivars and years, the mean number of seeds/plant were: T₀: 87f, T₁: 121a, T₂: lllbc, T₃: 115ab, T₄: 105cd, T₅: 99def, T₆: 102de, T₇: 95ef, and T₈: 122a. T₀ and T₈ thus established the minimum and maximum ‘limits’ in the seeds/plant response, respectively, with declining responses as irrigation was delayed from T₁ to T₇. The 100-seed weight (9) means were: T₀: 19.lb, T₁: 17.8c, T₂: 17.8c, T₃: 18.3c, T₄: 20.3a, T₅: 20.4a, T₆: 19.8ab, T₇: 20.3a, and T₈: 18.3c. Seed size for T₈ was thus smaller than T₀. The T₄ thru T₇ responses were significantly greater than T₀, while the T₁ thru T₃ responses were significantly less than T₀. Average seed yields were greatest for the T₃ and T₄ treatments, although for determinate cultivars, T₈ also resulted in high yields. Only minimal changes in seed number and size were affected by irrigation in the determinate cultivars (in contrast to indeterminates), which may have accounted for the greater yield responsiveness of determinate cultivars to irrigation. Based on both the seed yield and yield component data, it would appear that stage R3.5 to R4.5 represents the reproductive period in soybeans most responsive to irrigation.