Just Like Holding Sand In Your Hand

"I can't believe you two talked for half and hour about nitrogen and the only thing I got from it is that weather determines the right rate." (Yawn)
These were Cathy's words after she sat through a lunch meeting we had with Paul Sullivan last Monday.  Paul is a highly respected, independent crop consultant and 2011 CCA of the Year from eastern Ontario.  I am not sure what Cathy was expecting because when you put two crop advisers at a table on a sunny day in March the conversation is automatically going to migrate to nitrogen use in corn.
Forgetting genetics for the moment, nitrogen usage has been for the most studied and debated topic in the history of corn production.  Yet we still have unanswered questions.

Most corn producers in my experience are happy with a general rate recommendation that gives them a realistic chance to getting through the growing season without shorting the crop.  There is a general consensus that 140 to 180 lbs of N per acre will accomplish this goal for the majority of producers in Ontario.  The consensus falls apart when we start to push the boundaries of N rates, both at the low and high ends of the scale.  When you add application timing, the ability now to apply variable rates and toss in a dash of environmental concern you get to the point Paul and I were at last Monday and the point where Cathy falls asleep.  I assured her that agronomists live for moments like this.

Let's review what we know for sure about nitrogen,
1. The soil's ability to supply nitrogen is extremely variable both within the field in the same year and from one year to the next. Soil health plays a huge roll in how much nitrogen is available during the growing season.  Organic matter breakdown is the main source of soil nitrogen, but OM is not consistent across a field.  Organic matter breakdown is for the most part driven by temperature.  Hot weather speeds up the process, cold weather slows down the process.
2. Nitrogen applications of fertilizer and manure are highly susceptible to losses that occur before and during the period when corn needs the nitrogen to be most available.  Moisture is the main contributor to losses.  Too much rain leads to high losses across all soil types, denitrification being the main N loss mechanism in clay soils and leaching in sand.  Another type of loss, known as volatilization occurs when we leave nitrogen on the soil surface without incorporation.  I have believed for a long time that incorporation or injection is absolutely necessary for maximum nitrogen efficiency.  Research by Dr Craig Drury presented at the CCA Conference in January confirmed this. If side dressed nitrogen fertilizer, both urea and 28% was not injected, there was a lower % of applied nitrogen that ended up in the corn plant.  The use of nitrogen inhibitors helped reduce surface application losses, but still was not as efficient as nitrogen that was injected into the soil    
3. Modern corn hybrids are more efficient at utilizing N.  Depending on which research paper you look at hybrids developed after 1990 will produce 14-28% more corn per unit of N applied than hybrids developed prior to 1990.  Practical experience over 3 decades backs this up.  We tended to apply 130 -150 lbs of N in the 80's because we thought it took one lb of N to produce one bushel of corn. Today that same rate can easily produce 200 bu of corn if weather permits.
4. Modern corn hybrids are also taking up more nitrogen after pollination than older hybrids.  They can tolerate high populations better and stay greener longer.  Approximately 66% of the total nitrogen taken up by a corn plant goes toward grain production. This nitrogen comes from two sources, soil nitrogen uptake and nitrogen from the plant's stalks and leaves.  Older hybrids tend to use nitrogen already taken up by the plant.  Researchers refer to this process as nutrient mobilization, but I think it is easier to think of it as cannibalization whereby the plant consumes itself.  I was taught years ago that part of the maturity process was the fact that the plant was consuming its reserves faster than it could make new reserves.  In modern hybrids a greater percentage of the nitrogen found in the grain comes from uptake of soil supplied nitrogen.  This has the potential to change nitrogen management strategies from both a yield and environmental point of view.

To quickly review, nitrogen availability can be extremely variable thanks mostly to environmental conditions.  This has been a source of frustration my whole career and will continue for the foreseeable future to give corn producers headaches when trying to figure out the best nitrogen rate to apply.  The game has been changed by the fact that modern hybrids help us out by being more efficient, but require more nitrogen to be available late in the growing season.

How do we insure adequate nitrogen availability late in the season?

One way is to start loading up by applying more nitrogen at planting time and crossing our fingers that enough of it will still be around in September for the plant to find.  This has appeal from a simplistic point of view, but is irresponsible from an environmental point of view.  Dr. Sandy Endicott, Pioneer's Agronomy Research Manager reported last week that the city of Des Moines Iowa is taking agriculture to court over nitrate contamination in drinking water.

DesMoines Waterworks Sue Counties Over Nitrate Pollution

Corn producers in Iowa love applying nitrogen in the fall and early spring before planting, which gives them the summer off to pursue other interests.  While it may be a convenient management practice for the farmer, the farmer should not be too surprised when this style of management comes under scrutiny. The farmer's defense has been to say it wasn't me, it was someone else.  Plus he really has shown a preference to go fishing in the summer rather than spend time applying fertilizer.  Try explaining that to a judge.  For this same reason I do not believe applying more nitrogen at planting time is an acceptable method of nitrogen management.

A second approach is to apply a base amount of nitrogen at planting and then supplement it later in the season.  For my whole career there has been the debate about the value of sidedress nitrogen and it was always my opinion that sidedressing nitrogen in June did not deliver much real value.  There was no impact on yield and while you could reduce your total nitrogen rates, the reduction was barely enough to cover the cost of application.  Now we have high clearance sprayers that are set up to apply nitrogen into late July.  Ideally as already discussed this nitrogen should be injected, but it requires a big investment when we are a long way from being convinced that it pays.  The less expensive option is the Y-Drop system that dribbles nitrogen at the base of the plant.  We do have to cross our fingers that moisture will move this surface dribbled nitrogen into the plant.  A dry summer will present a big challenge for this delivery method.  Can this later sidedress timing change the game? We don't know for sure, but it creates an opportunity to either deliver more yield by placing nitrogen closer to an important point of the corn plant's life or deliver a similar yield at a significantly reduced rate of nitrogen.  Either way it is a positive step that if nothing else demonstrates good environmental stewardship.

A third approach that we already have confidence in is the use of cover crops to capture and release nitrogen in a more timely fashion.  Red clover has been demonstrated time and time again to be a perfect match for releasing nitrogen which coincides nicely with a corn plant's needs.  Red clover is out of favour with many because of inconsistent establishment, but it is still the best and least expensive option for the first try at establishing the cover crop.  We get a second chance after the wheat comes off and can fill in with crimson clover and possibly vetch to provide a more uniform cover.  David Start at Woodstock has worked for 20 years to identify the best nitrogen rate in corn following red clover.  With a good stand of red clover his typical nitrogen rate is 30 - 60 lbs per acre. How many of you have grown 190 bus of corn on 30 lbs of applied nitrogen? David has. More than once.

For the last word on nitrogen, I turn back to Dr Craig Drury.  He said trying to hold nitrogen at a uniform level in the soil profile is just like holding sand in your hand.
I wish I had thought of this line when Cathy was telling me how bored she was listening to Paul and I at the lunch table.  Just think dear, romantically walking hand in hand while trying to hold sand in our hands paints a beautiful picture.  Right dear?