Updated November 8, 2019:
2017 (6th year to trial). In 2017, rainfall was below average in June (2.08”), above average in July (6.88”) and August (5.30”), and below average in September (2.25”). Soybeans were initially planted on May 12, however the entire study had to be destroyed and replanted due to significant deer damage. Soybeans were replanted on June 15. The amount of water applied based on the irrigation strategy ranged from 1.5” to 4.0”.

Soybean heights were measured on 7/26, 8/11, and 11/4. Heights ranged from 34.7 in. to 38.7 in. There were no significant differences between treatments on soybean height probably due to the above average rainfall in July and August. Soybeans were harvested on 11/4. The irrigation strategy that produced the highest yield (72.8 bu/A) was the full season irrigation treatment that maintained soil moisture >50%, but was not statistically different from all other irrigation treatment and the no irrigation treatment. It seems the above average rainfall in July and August was adequate to achieve maximum yields, even under no irrigation. It is interesting to note that the below average rainfall in September did not affect yields. Yields ranged from 68.1 to 72.8 bu/A.

The top 3 most profitable treatments based on yield and total irrigation applied were 1) Full season irrigation >50%, 2) no irrigation until R3 then >50% moisture, and 3) full season irrigation >30%.

Individual year results (2012-2017)

2016 (5th year of trial). In 2016, rainfall was below average in June (2.99”), July (3.53”), and August (2.12”). Rainfall total from August 1 to September 15 was 2.98”, which averaged 0.06” per day over that time period. In the full season soybean study, soybeans were planted on June 3. The amount of water applied based on the irrigation strategy ranged from 4.9” to 7.8”. Average yield in plots that received irrigation ranged from 65 to 75 bu/A compared to 55 bu/A in plots that received no irrigation. The irrigation strategy that produced the highest yield (75 bu/A) was the full season irrigation treatment that maintained soil moisture >50%, but was not statistically different from all limited irrigation treatments to later growth stages and no irrigation to R1 and R5. There was a significant difference between the full season >50% irrigation treatment and the no irrigation to R3 and the full season irrigation >30%.

2015 (4th year of trial). In 2015, rainfall was above average in June (6.00”), average in September (4.20”), and below average in July (2.50”) and August (2.25”). Rainfall total from July 1 to September 9 was 4.75”, which averaged 0.07” per day over that time period. Rainfall total from August 12 to September 9 was only 0.21”, which averaged 0.01” per day over that time period. In the full season soybean study, soybeans were planted on May 27. The amount of water applied based on the irrigation strategy ranged from 5.3” to 9.6”. Average yield in plots that received irrigation ranged from 68 to 80 bu/A compared to 25 bu/A in plots that received no irrigation. The irrigation strategy that produced the greatest yield (80 bu/A) was when no irrigation was applied until R3 then applied to maintain available soil moisture >50% until maturity. However, yield attained by this irrigation strategy was only statistically different from the no irrigation treatment (25 bu/A) and strategies that applied no irrigation until R5 then >50% available moisture to maturity (68 bu/A) and limited irrigation to R5 then >50% soil moisture to maturity (72 bu/A).

2014 (3rd year of trial). In 2014, rainfall was above average in July (6.76”) and August (5.76”), but below average in June (2.05”) and September (3.99”). In June, July, and August, rainfall in the last 2 weeks of each month was less than 0.88 in. There was a 4 week period from 8/16 to 9/15, where Harbeson received only 1.56” of rainfall. In the full season soybean study, soybeans were planted on May 20. Average yield in plots that received irrigation ranged from 73 to 77 bu/A compared to 73 bu/A in plots that received no irrigation. There was no significant difference between any irrigation treatments this year. The amount of water applied based on the irrigation strategy ranged from 1.9 in. to 9.0 in. Soybeans planted in 7.5 in., 15 in., and 30 in. row widths yielded similarly to each irrigation strategy, but there was a slight yield difference between row widths. Average soybean yield was 76 bu/A in 7.5 in. rows, 74 bu/A in 15 in. rows, and 74 bu/A in 30 in. rows.

2013 (2nd year of trial). In 2013, rainfall totals in June (10.4”), July (6.9”), August (6.5”) and October (6.1”) were above average, but rainfall was well below average in September (0.7”). In the full season soybean study, soybeans were planted on May 19. Average yield in plots that received irrigation ranged from 65 to 73 bu/A compared to 69 bu/A in plots that received no irrigation. The amount of water applied based on the irrigation strategy ranged from 4.3” to 7.0”. The irrigation strategy that produced the greatest yield (73 bu/A) was when irrigation was applied at a limited amount (>30% available moisture) until the R3 growth stage and then applied to maintain available soil moisture >50% until maturity and was also the irrigation strategy with the least amount of water applied (4.3”). However, yield attained by this irrigation strategy was not statistically different from the no irrigation treatment and strategies that applied no irrigation until R1 and limited irrigation to R1 and R5, where yields ranged from 69 to 71 bu/A. The two irrigation strategies, full season irrigation (65 bu/A) and an ET based program (66 bu/A), that maintained soil moisture >50% all season were the only two strategies that produced less yield than the no irrigation treatment. Soybeans planted in 7.5”, 15”, and 30” row widths yielded similarly to each irrigation strategy, but there was an overall yield difference between row widths. Average soybean yield was 72.8 bu/A in 7.5” rows, 68.6 bu/A in 15” rows, and 66.4 bu/A in 30” rows.

2012 (1st year of trial). In 2012, rainfall totals in May (0.5”), June (2.5”), July (2.5”), and September (2.8”) were below average, but rainfall was well above average in August (10.6”). In the full season study, average yield in plots that received irrigation ranged from 63 to 70 bu/A compared to 54 bu/A in plots that received no irrigation. The amount of water applied based on the irrigation strategy ranged from 5.3” to 9.6”. The irrigation strategy that produced the greatest yield (70 bu/A) was when irrigation was applied at a reduced amount (>30% available soil moisture) until the R5 growth stage and then >50% available soil moisture until maturity. This irrigation strategy also required the least amount of water applied (5.3”). There was no yield advantage in irrigating to maintain >50% available soil moisture until Mid-August this year. Soybeans in all row widths responded similarly to each irrigation strategy, but there was an overall yield difference between row widths. Average soybean yield was 67 bu/A in 15” rows, 64 bu/A in 7” rows, and 61 bu/A in 30” rows.

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Soybean irrigation research was conducted at the University of Delaware Warrington Irrigation Research Farm from 2012 to 2017. Over these years, there were only slight differences in yield between irrigation strategies in full season soybeans. Yield with strategies of limited irrigation (>30% soil moisture) to later growth stages were comparable to strategies that provided full irrigation (>50% soil moisture) all season.

In 2015, 2016, and 2017 studies, new treatments were included to determine if irrigation could be completely delayed until later growth stages (R3 and R5). Averaged over these three years in the full season study, delaying irrigation to R3 provided similar yield to all other irrigated treatments. There was a slight decrease in yield when irrigation was delayed to R5, however yield was significantly higher than the no irrigation treatment