Following the drought – nitrogen fertilization in 2013

Farm Forum

We hope this year brings more precipitation particularly in those areas of the state that were hit hard last year by drought. Average soil tests from those areas indicate 100 to 125 lbs. of available nitrogen carryover from severe-drought affected corn fields. Should we take advantage of that available N by planting a non legume crop?

For eastern SD, the choices are limited – mainly wheat or corn on corn. Wheat is usually not recommended following corn because of increased incidences of head scab (Fusarium head blight). Many growers will consider corn following corn. However, long term studies indicate at least a 10% decline in corn yields following corn compared to following soybean. On stressful sites/years this decrease can be over 20%. For example, a five year study at Beresford, comparing rotations of corn after soybean, corn after corn (stover retained), and corn after corn (stover removed) indicated a 26 and a 20% yield reduction from the stover retained and stover removed rotations, respectively, compared to the corn after soybean rotation.

Successful growers of corn on corn usually use their most productive fields and studies have indicated lower residue levels usually favor higher yields when attempting corn on corn. If we run the numbers, how much do we save by planting corn on corn vs. soybean after corn with available nitrogen levels of 110 lbs/acre? If planting soybean on this field, we would “waste” about 110 – 40 (typical carryover after soybean) = 70 lbs of soil available N that the soybean would use instead of fixing from the atmosphere. At $0.65 N, this is 70 lb N x $0.65 = $45.50/acre of nitrogen value. If our corn after soybean normally produces 180 bu/a, but our corn after corn produces 162 (10% decline), we are losing 18 bu/a at $6 per bu = a $108 dollar loss per acre. Obviously, to just make up for the carryover nitrogen, this isn’t a profitable situation. However, there are many growers who grow corn after corn and make more profit than corn after soybean even with a yield decline. New growers to corn on corn should choose fields with a history of high yield potential and that have lower residue cover and higher N carryover.

There were numerous reports last year of corn on corn fields yielding much lower (over 30% in some cases) than neighboring corn after soybean fields. Corn water use efficiency is decreased when grown after corn versus soybeans. Additionally, corn has the potential to deplete stored soil moisture to a greater depth than soybeans. As a result, the corn on corn yield penalty can be greater in drier years. Therefore, a prospect of another dry year does not bode well for this rotation. In any case, utilization of available carryover nitrogen – although it looks good on paper – is not as simple as it seems.