Jan. 17, 2011

Several farm magazines have reported claims from at least one fertilizer company that current fertilizer use recommendations may not be valid for corn hybrids with Bt rootworm resistance.

Generally, results from the Nebraska Soil Fertility Project support the expectation that because Bt rootworm-resistant hybrids are likely to have healthy root systems, these hybrids are as efficient or more efficient in nutrient recovery than other hybrids.

Bt rootworm resistance was not common during 2002-2004 when these trials were conducted, but the hybrids used in six of the 34 trials had this trait. The research was not designed to compare different types of hybrids. Therefore, in interpreting the following, remember that hybrid effects are affected by field, weather, and management differences.

• Average maximum treatment yield in these trials was 240 bu/ac.
   
• The Bt rootworm hybrids had higher yield with no N applied for trials with corn following corn but not with corn following soybean or dry bean.
   
• The economically optimal N rate (EONR) for the trials with Bt rootworm hybrids was on average 20% less compared to the mean EONR for all trials.
   
• The lower EONR was related to a trend of greater recovery of applied N with Bt rootworm-resistant hybrids (66% compared to 47%; P = 0.19).
   
• Phosphorus uptake was 28% less and sulfur uptake was 8% more with Bt rootworm hybrids than with other hybrids (P < 0.1).
   
• Potassium, calcium, and magnesium uptake were not affected by hybrid group.
   
• Responses to applied P and K were not different for the two groups of hybrids.

These results do not justify a change in fertilizer recommendations in Nebraska for Bt rootworm-resistant varieties of high yield corn. The results, while inconclusive, do indicate greater efficiency of nitrogen uptake, and that the UNL nitrogen recommendations are above the most profitable rate with Bt rootworm resistant varieties. More research is needed to verify these indications.

Generally the UNL results, with the exception of the reduced P uptake, agree with the expectation that because Bt rootworm-resistant hybrids are likely to have healthy root systems, these hybrids are as efficient or more efficient in nutrient recovery than other hybrids.

In summary, the UNL fertilizer use recommendations for high yield corn were well verified by results of the Nebraska Soil Fertility Project and there is no need to apply more fertilizer for Bt rootworm resistant hybrids.

Charles Wortmann, Richard Ferguson, Gary Hergert, Tim Shaver, and Charles Shapiro
UNL Extension Soils Specialists