Several major seed companies have recently introduced corn hybrids that specifically target enhanced drought tolerance. Optimum AQUAmax hybrids from DuPont Pioneer and Agrisure Artesian hybrids from Syngenta were commercially available last year. In 2013, Monsanto is conducting a “stewarded” commercial introduction of Genuity DroughtGard hybrids in several Western Corn Belt states.
Most seed companies generally provide drought tolerance ratings for the hybrids they market (e.g. 1= poor; 9= excellent) but the recently introduced AQUAmax, Artesian and DroughtGard hybrids represent an effort to develop hybrids with traits affecting response to water stress. DuPont Pioneer and Syngenta drought tolerant hybrids involve native traits and those from Monsanto involve a transgenic trait. The companies marketing these hybrids emphasize that the enhanced drought tolerance associated with these hybrids is not associated with “yield drag”.
The hybrids are primarily targeted for use in the Western Corn Belt especially in areas where corn growers are looking for ways to reduce irrigation and conserve water supplied by aquifers. Nevertheless AQUAmax and Artesian hybrids adapted to Eastern Corn Belt conditions are available and Ohio growers had an opportunity to assess the performance of these hybrids during the 2013 drought.
Unlike Bt insect or herbicide resistant traits which involve single gene qualitative traits, drought tolerance is more complex and involves many genes. Some of the physiological traits associated with improved drought tolerance include a short anthesis silking interval (ASI, the time between the onset of pollen shed and silk emergence), delayed leaf rolling and senescence (leaf greenness), reduced barren and nubbin ears, better ear fill (greater kernel set and reduced abortion under stress), improved root development and architecture and photosynthetic/transpiration rates under stress.
A corn crop may be subjected to different types of drought stress - droughts may be protracted and season long in duration, or they may be limited to one or more developmental stages ( early to late vegetative growth, early and late grain fill). High temperatures in conjunction with drought during one or more of these stages can magnify the impact of stress. Hybrids may react differently to the varied types of drought stresses. Moreover drought tolerance may be strongly influenced by management practices. Greater corn plant populations promote more stressful conditions, and delayed and late planting dates often result in drier and hotter weather during critical development stages (i.e., flowering, grain fill).
In 2012 we initiated a study to determine if drought tolerant AQUAmax and conventional (non-AQUAmax hybrids) differ in their response to plant population and planting date. Various physiological traits associated with drought tolerance were also evaluated. 2012 was the warmest year on record in Ohio and the summer one of hottest and driest. However the impact of drought and heat varied considerably across the state. These conditions provided us with an opportunity to assess the response of the new drought tolerant hybrids to varying levels of stress.
The study compared four hybrids (two drought susceptible and two drought-tolerant AQUAmax hybrids) at five plant populations (24,000, 30,000, 36,000, 42,000 and 50,000 plants/A) at two planting dates (early May vs. early June). The susceptible and tolerant hybrids were characterized by drought tolerance ratings of 7 and 9, respectively, on a scale of 1-9 where 1 is poor and 9 is excellent. The study was conducted at three OSU-OARDC research stations in southwest, northwest, and northeast Ohio.
Preliminary results indicated that the grain yield of each hybrid had a similar response to plant population at each site but differed due to planting date at each location. At the NW Ohio location for the early planting date (May 3), which was subjected to the greatest drought stress, the drought tolerant hybrids out yielded the drought susceptible hybrids by 8 to 10 bu/A (about 5% averaged across plant populations). At the other locations, where drought stress was limited or absent, the drought tolerant hybrids generally yielded more or about the same as the drought susceptible hybrids.
Drought stress can delay silking and increase ASI. This “asynchronous” flowering can result in poor kernel set and inn extreme situations barren plants. In the 2012 study, measurements of flowering synchrony at the NW Ohio location for the May 3 planting date indicated that the anthesis silking interval was 1 to 3 days greater for the drought susceptible hybrids than the drought tolerant hybrids but no consistent effect of plant population on flowering synchrony was evident. On the later planting date (June 7), no difference in flowering synchrony between hybrids was present but ASI increased as plant population increased.
The preliminary results suggest that under the water stress conditions of the type experienced at the NW Ohio location in 2012, drought tolerant hybrids offered a yield advantage over drought susceptible hybrids (w/ lower drought ratings). Under more favorable conditions, the drought tolerant hybrids showed yields that were similar or slightly higher those of the drought susceptible hybrids.