Much of what has been written about when to start irrigating soybeans in relation to reproductive stage stems from early research of scheduling irrigation for May (full-season and not doublecrop) plantings of maturity groups 5 through 7 soybean varieties (conventional soybean production system). That research was conducted during the 1980s throughout the southeastern United States.
The vast majority of results proved that irrigating soybeans before beginning bloom did not increase yields above the level achieved from starting irrigation at beginning bloom.
Soybeans are now planted in March and April, and the varieties are predominantly maturity groups 3 and 4 (early soybean production system). The reproductive period for these plantings begins in early to mid-May rather than early July and later in the conventional soybean production system.
The decision about when to start soybean irrigation, therefore, should be based on the early development in early soybean production system plantings.
In research where tensiometers and weather data were used, irrigation of late March and early April plantings of Maturity Group 3 and Maturity Group 4 varieties rarely began before the beginning of pod development (R3), which occurs after May 20.
Atmospheric water demand was low before that stage, soil water was plentiful, and rainfall was frequent and often replenished the soil profile. There was just no need to irrigate earlier. Yields from plantings grown under that irrigation regime were in the mid-70 to low-80 bushels acre.
There has been conjecture that early-season irrigation of the early plantings will promote more rapid vegetative growth and taller plants, but this is not necessarily so. In fact, soybean growth during the first 30 days of development is relatively slow. Irrigating will not change this.
Soybean plants just need time for nitrogen fixation to reach high levels, and time for enough leaf area to develop to support future rapid growth. Even plantings that will not be irrigated have adequate soil moisture and mild weather that will promote all the growth the plant can achieve during vegetative development in early soybean production system plantings.
In doublecrop plantings that are planted in early June and later, the above discussion does not apply. In fact, irrigation of these plantings may be necessary just to achieve a stand.
Also, irrigation of doublecrop soybeans during vegetative development may promote more rapid canopy development, which becomes increasingly important in a shorter season with hotter days, warmer nights, higher atmospheric water demand, and less time each day for plant growth and development.
Larry G. Heatherly is a retired USDA-ARS Research Agronomist and current crop consultant. e-mail email@example.com