I received several questions recently on the feed quality of transgenic corn, especially rootworm resistant corn and glyphosate tolerant corn.

Many farms with high producing herds are using these hybrids but it pays to review some of the research that has been done on this topic. A number of feeding trials have been conducted to assess these traits and their effect on forage quality and milk production. In our own studies, we have mostly evaluated the forage quality and have not been able to document any significant differences due to the presence of these transgenic traits.

A number of feeding trials have concluded that the transgenic and conventional versions of Bt, glyphosate tolerant and Bt rootworm resistant corns are nutritionally equivalent to conventional hybrids. A Purdue University study, for example, conducted by Doinkin et al. published in 2003 in the Journal of Dairy Science evaluated milk production, forage quality traits and rumen degradability of Bt and glyphosate tolerant corn and found no effects on milk production when the both the transgenic grain and silage were used in the dairy rations compared to a control using both conventional grain and silage.

Another example is a University of Nebraska study conducted by Grant et al. and published in Dairy Science in 2003 evaluated dry matter intake and milk production due to the presence of one of the Bt rootworm (Mon863) or one of the glyphosate tolerant genes (nk603). They found no effect due to the Bt rootworm trait on milk production or dry matter intake. They did find lower milk yields in this study with the glyphosate tolerant hybrids, but attributed this to the higher dry matter content of the Roundup Ready hybrid at ensiling rather than the gene. Sometimes differences in dry matter content in the field can lead to reduced animal performance and this could be interpreted as an effect from the gene.

The overwhelming majority of studies on transgenic corn have shown nutritional equivalency and no effect of the transgenes on feed quality and animal performance. I suspect most differences observed between conventional and transgenic hybrids are due to differences in management or due to differences in dry matter content as observed in the Grant study cited above.