Abstract
Nitrification inhibitors (NI) can be used with liquid swine manure (LSM) to decrease potential NO3 losses, but knowledge specifying when and where NI can increase corn (Zea mays L.) yields is limited. Eleven on-farm evaluation trials (OET) were conducted in 2009 and 15 in 2010 to identify site-specific factors for using Instinct (an encapsulated form of nitrapyrin) with LSM in Iowa. Farmers injected LSM in the fall in at least three field-long strips with and without NI. Yield responses (YR) to NI were calculated by dividing yield monitor data into 50-m cells within each field. Hierarchical models were used to estimate predictive probabilities of profitable YR for two categories of monthly average rainfall and soil drainage. On average, NI produced no YR in relatively normal 2009 and a 0.15 Mg ha−1 YR in extremely wet 2010. The NI did not change late-season corn N status but half of corn stalk nitrate test (CSNT) samples were N deficient in 2009 and about 65 % in 2010. Fields receiving >90 cm March through August rainfall in 2010 were predicted 65 % more likely to have economic YR (>0.13 Mg ha−1) than fields receiving <90 cm rainfall. Within-field variability in YR was about four times greater than among-field variability, but within field-level factors had no significant effects on YR. The NI effects may not have lasted long enough to increase yields across all OET and predictive probabilities suggest that NI may produce profitable YR only when spring and summer rainfall exceed the long-term averages by more than 40 %.
Highlights
In Iowa, nitrification inhibitors (NI) have been historically used with anhydrous ammonia to retard conversion of NH4 to NO3
A study in Iowa in a relatively wet year showed a twofold decrease in the percentage of nitrification of fallapplied anhydrous ammonia from nitrapyrin when soils had a pH \ 6 than in soils with pH [ 7 and presence of free carbonates (Kyveryga et al 2004)
Observations from on-farm evaluation trials (OET) and analyses using hierarchical modeling showed that the nitrification inhibitor called Instinct had no effect on grain yield or late-season corn N status in 2009, a year with about average rainfall
Summary
In Iowa, nitrification inhibitors (NI) have been historically used with anhydrous ammonia to retard conversion of NH4 to NO3. Numerous experiments were focused on studying the efficacy (often expressed as timing and length of inhibition) of this NI and its effects on corn (Zea mays L.) yield. Economic performance (i.e. potential N fertilizer saving and additional yields) of nitrapyrin in corn production can be extremely variable (Blackmer 1986). A study in Iowa in a relatively wet year showed a twofold decrease in the percentage of nitrification of fallapplied anhydrous ammonia from nitrapyrin when soils had a pH \ 6 than in soils with pH [ 7 and presence of free carbonates (Kyveryga et al 2004). Positive YRs to nitrapyrin were observed mostly in high pH, calcareous soils (Ellsworth 2001), where nitrification rates of anhydrous ammonia were much higher (Kyveryga et al 2004)
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