Some farmers perceive that herbicide resistant crops are inherently lower yielding than conventional hybrids due to yield drag or yield lag. The Monsanto Roundup Ready GA21 event was first released to growers on a limited basis in 1997. The event Nk603 was first released to growers in 2000. Both events are approved in all countries except the European Union. Experiments were conducted throughout the Midwest U.S. with the objectives: 1) to determine if Roundup Ready genes suppress yield, and 2) to determine the effect of Roundup Ultra on Roundup Ready hybrids.

University corn agronomists from Wisconsin, Iowa, Nebraska, Missouri, Ohio, North Dakota, Pennsylvania, Minnesota, and Michigan recently tested hybrids containing the Round-up Ready events. In 1999, 10 hybrid pairs, one hybrid with the GA21 Roundup Ready event, the other without (genetic isolines) were compared. In another experiment, seven Roundup Ready hybrids were grown in conventional and Roundup Ultra herbicide systems. In 2001, six hybrid pairs were evaluated.

Averaged across years and locations, no statistical differences were observed between hybrids with the GA21 and Nk603 Roundup resistant events (Figure 1). The Roundup Ready gene or its insertion did not affect yield. Glyphosate (Roundup Ultra) had no effect on Roundup resistant hybrids. Based on these preliminary studies at numerous locations there is no evidence of yield suppression associated with Roundup Ready corn technology.