As we enter the era of herbicide-resistant crops, one of the issues that may come up is whether all the seed in a bag contains the trait that provides resistance. Because of the nature of producing hybrids, if the herbicide resistance is in one parent (heterozygous), there is almost always a small amount of foreign pollen that gets through. In the case of Liberty-Link corn, AgrEvo certifies seed lots as Liberty-Link only if at least 97 percent of the population carries the glufosinate resistance trait. This means that up to 3 percent of the corn crop might not be resistant and, therefore, would be killed by a Liberty application.

Mike White, extension field specialist in southwest Iowa, recently reported a situation where this issue has caused a problem. A farmer planted Liberty-Link seed at approximately 27,000 seeds per acre, and ended up with 20,000 plants emerged. The Liberty treatment killed some corn plants, as it is likely to do, and knocked the population down to a little over 19,000. The grower was unhappy with this stand loss and is questioning whether to use Liberty on the remainder of his fields planted to Liberty-Link hybrids. Would switching herbicides be an appropriate management decision?

ISU research has found there is approximately a 1.2 percent yield loss for every 1,000-plant decrease in population between 22,000 and 18,000 plants per acre. At higher populations, similar stand losses will impact yields less. In the situation described above, the application of Liberty would have killed a maximum of 600 plants (3 percent of 20,000). According to the research data, this stand loss would result in a yield loss of 0.75 percent. Obviously, any stand loss is undesirable, but in most situations a 3 percent loss will not result in a significant reduction in yield potential. Also, it is likely that most seed lots have less than 3 percent non-transformed seed, so this represents a worst-case scenario.

Many growers will not like the concept of their herbicide program causing a stand loss, but in most situations a 3 percent stand loss will not have a major impact on yield potential. When deciding whether to use a herbicide-resistant crop, the effectiveness of the herbicide on the weeds present in the field and the cost of herbicide in comparison to other options should be more important than the presence of non-transformed seed in the bag.