Winter wheat growers who are hoping to maximize yields should keep one thing in mind when considering spring nitrogen applications: Timing is everything.
According to Ken Gross, winter wheat agronomist with Ducks Unlimited, applying spring nitrogen early, right at green up, ensures the greatest benefit in terms of yield and financial returns.
“This is what everyone should focus on, is getting their nitrogen on early, right at green up, or right around when the crop is starting to green up,” said Gross.
“Why? It’s clearly because (early application) makes a huge difference yield wise.”
During a recent webinar on winter wheat fertility, Gross said winter wheat yields are determined early in the spring.
“The seed head is produced very early in the spring … and it sets the number of spikes and spikelets and the size of the head very early,” he said.
“So if you fertilize early, you’ll have a healthier plant and you’re going to grow a (crop with) healthier, larger seed heads and bigger yield potential.”
“If you wait until later, you’re starving that crop and you’re going to end up with a smaller seed head.”
Gross cited University of Saskatchewan research that suggested a three-week delay in applying spring nitrogen can produce crop yields that are 30 percent smaller compared to fields that receive nitrogen at optimal timing.
Similar research conducted in Ontario suggested that a four-week delay reduces grain yields by as much as 40 percent.
Nitrogen is by far the most important nutrient for winter wheat yields.
“There are different formulas for figuring out how much N you should put on,” Gross said.
“South Dakota States University says 2.5 pounds (of nitrogen) per bushel (of yield, others) … say 2.25….
“I like to use as a guideline 2 1/4 lb. of N per bu., so if you’re looking at 100 bu. as a yield goal, you’d need 225 lb. of N, minus what ever is in your soil (as residual nitrogen).”
In most cases, residual nitrogen levels are relatively low in areas where winter wheat is commonly grown. Actual levels should be tested to ensure proper application rates.
Gross cautioned that much of the research that examines the link between yield response in winter wheat to nitrogen rates was conducted using older winter wheat varieties that have been around for several years.
Many newer winter wheat varieties that are available today have greater yield potential and therefore may also have greater nitrogen requirements.
Another mistake that some growers make is assuming that spring wheat and winter wheat have similar nitrogen requirements.
“A lot of producers still use the same rate of N (on their winter wheat) as they do on their spring wheat,” said Gross.
“But winter wheat tends to have a … 10 to 40 percent higher yield potential than spring wheat and that’s mostly because (winter wheat) makes very good use of spring moisture.”
That said, the spread between potential spring wheat yields and potential winter wheat yields is likely to be reduced in years when spring moisture is plentiful.
Historically, research that looks at the relationship between winter wheat yields and nitrogen application timing has shown that spring applied nitrogen generally produces higher yields, although sometimes not significantly higher.
More recently however, research suggests that split applications — some in the fall and some in the spring — may provide the greatest yield boost.
Gross cited research conducted by winter wheat breeder and researcher Brian Beres that suggests split applications rarely result in a yield penalty but usually produce yields that are as good as or better than fall applications or spring applications alone.
“If you look at every single form (of nitrogen that can be applied), there’s really not much of a penalty, if anything, from putting a split app down. And sometime there’s a benefit,” Gross said.
“With a split app, you’re not going to pay a penalty on yield.”
Split applications can also reduce risks associated with weather conditions that can delay spring applications.
For example, growers who prefer spring-only applications may be hindered by wet field conditions, resulting in delayed applications and lower grain yields.
Similarly, a split application can provide growers with the flexibility to adjust spring rates in cases where spring conditions are dry or where the crop did not come through the winter in optimal condition.
“It gives you some flexibility in dry years,” said Gross.
“If you can put something on in the fall and you have a dry spring, then … maybe you’re not going to shoot for a 90 bu. per acre (target yield) on that crop. Maybe (because of dry spring conditions) it’s only got a … 60 to 70 bu. yield potential. Now, you can dial down your second application, put a bit less on and save yourself a little bit of money.”