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Post-seed nitrogen applications can be successful

At least 50 percent of N should be applied at time of seeding to reduce risk.

April 8, 2008  By Bruce Barker

With high nitrogen (N) prices, farmers have expressed great interest in managing N input costs by trying to match application timing to the time of greatest crop need. Traditionally, that has meant applying N at or just before seeding. The idea behind split N applications is that by delaying some of the N application until after crop emergence, farmers could have a better sense of yield potential and expected growing conditions, and then top up N fertility to match expectations.

In cereals, the time of greatest crop use extends from the start of elongation until heading, with peak uptake during flag leaf extension. In canola, this period is from the start of flowering to the end of pod formation.

Research conducted by Guy Lafond with Agriculture and Agri-Food Canada (AAFC) at Indian Head, Saskatchewan, earlier this decade supported the theory and found that post-emergence N applications using UAN solutions and surface dribble band applications were feasible. However, the practice was not without risk and was shown at that time to be never better than putting all the fertilizer down at time of seeding as is currently done in one pass no-till seeding systems. “The risk of post-emergence  application is that if the weather turns dry, the N will be stranded at the surface and yields will be lower,” explains Lafond. “We saw that in 2003.”

Applying 50 percent of N at seeding helped reduce the risk of split applications. Photo By Bruce Barker.

To strike a balance between post-emergence N application and the risk of dry weather stranding N at the soil surface, some N needs to be applied with the seed – but at what level? For example, if the weather turned dry and the post-emergence N was stranded at the soil surface, would the crop have had the same yield potential with only 33 percent of the N placed at time of seeding, or would a higher level be required at seeding to satisfy the crop’s drought-reduced potential? That is the question that Lafond set out to answer.

Lafond and others conducted field trials in Manitoba and Saskatchewan to determine the proportion of N fertilizer that should be applied at seeding to reduce or even eliminate the risks associated with in-crop applications of N. The trials were conducted on wheat at three locations from 2003 to 2006, and at two locations for canola from 2004 to 2006. The treatments consisted of applying 100 percent, 67 percent, 50 percent, 33 percent or zero percent of the targeted N rate at seeding with the balance in-crop at the 1.5, 3.5 or 5.5 leaf stages in spring wheat, and at the five to six leaf stage, bolting or start of flowering stages in canola.

During the years of the study, a wide range of growing conditions and precipitation were experienced, which provided a good evaluation of the concept of in-crop applications of N.

At least 33 percent of N application required at seeding for wheat
In study number one with spring wheat, no differences in yield were observed when 33 percent of the target N was applied at seeding with the balance in-crop at the 1.5 or 5.5 leaf stages compared to where all N was applied at seeding. There was some yield loss, though, at the 3.5 leaf stage, which Lafond says indicates that a higher proportion, such as 50 percent would be more appropriate to minimize risk. When all N was applied in-crop, yield was always lower than applying all N at seeding, regardless of crop stage.

Similar observations were noted for grain yield in the second wheat study. Lafond says that it is important to note that although yield of spring wheat was similar when at least 33 percent of the target N was applied at seeding with the balance in-crop compared to all N at seeding, grain yields were never higher with split applications in-crop.

Canola requires 50 percent at time of seeding
With canola, applying some N at seeding with the balance in-crop also maintained grain yields relative to all N in-crop. However, a minimum of 50 percent of the target N was required at seeding to maintain equivalent yields to when all N was applied at seeding.
Stage of application was also important for canola. Post-emergence N applied at or before the start to mid stage of bolting was successful in maintaining yields, but applying the N at start of flowering resulted in some yield losses even when 67 percent of the target N was applied at seeding.

Lafond says that adding at least 50 percent of N at seeding for both canola and wheat will greatly minimize the risks associated with in-crop applications of N. “If we were to assume that available water will be at least 75 percent of normal, application of N fertilizer at seeding at 50 percent or more of the target yield potential may be a cost effective management option when combined with in-crop application of N at the appropriate rate and time,” explains Lafond.

The best proportion of starter N application may also vary by growing zone. In wetter areas, less N could be applied, while in drier zones, more N might be required. The starter proportion would also vary by each individual’s risk tolerance. For lower risk tolerance, a higher proportion of starter N, such as 66 percent, might be considered to help ensure that the crop’s full yield potential is achieved in a drier year when post-emergence N is less successful.

Still, since applying the entire N at seeding was never worse than a split application, the question begs to be answered: “Why take the chance and split apply?” The answer likely is
found at the farmgate, where each farmer has a different take on seeding capabilities. For example, does he have the ability to apply all N at seeding with either a pre-seed banding operation or a side-band/mid-row operation? Additionally, risk management comes into play and split applications might afford the opportunity to apply 50 percent of the fertilizer at seeding, and then assess the crop’s potential and the need for subsequent fertilizer.

What Lafond’s research also supports is that if a farmer chooses to split apply N, the risk can be reduced and the practice can be successful if at least 50 percent of the target N is applied at seeding. -end-

The Bottom Line
Split application of nitrogen fertilizer would certainly provide another management tool for
producers to reduce risk. One must also take into account nutrient and application availability in season as well as the cost of the product and the cost of the second application. Perhaps with the new polymer coating technology just recently introduced, new recommendations will need to be researched. Warren Kaeding, Churchbridge, Saskatchewan.


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