Reducing nitrogen (N) supply to hasten maturity in crops is sometimes called “premature death”. It might more correctly be termed “limiting growth” — the aim being to reduce vegetative growth so the crop can get into its productive stage earlier. “This year we had a wet and cool spring so a lot of producers were seeding very late,” says Patrick Mooleki, nutrient management specialist with Saskatchewan Ministry of Agriculture. “In late seeding situations, excessive N could extend the vegetative and reproductive growth periods and thus leave too little time for seed growth and ripening before fall frosts end the growing season. Crops normally will look at the resources they have and the time remaining for them to grow. Somehow in their DNA there is a time clock.”
If a late seeded crop is given the same amount of nutrients that would have been given if it had been seeded earlier, it will continue to grow for too long into the season. “If you have too much fertilizer and an advanced season when temperatures are much hotter earlier in the plant’s growth cycle, it will continue to grow vegetatively such that by the time it is flowering it may already be late July or early August, and the grain is not going to mature on time.”
Under the above circumstances the recommendation is often to reduce N.
“We have studies in Manitoba on corn and dry beans showing that proper N fertilization has great effect on increasing yield and little impact on delaying maturity,” says provincial soil fertility specialist, John Heard. “In fact our corn studies indicate that it is under-fertilized corn that matures slower and yields less.”
Reducing N isn’t something he sees farmers doing very often, says Murray Hartman, an oilseed specialist with Alberta Agriculture & Rural Development. “Late seeding isn’t normal, and fertilizer N may already be banded the previous fall or even in the spring before very wet conditions arise,” he says. “In situations where seeding gets delayed, and fertilizer must be applied during seeding, then some cutback in N rates may happen. But the cutback is mainly due to reduced yield potential of late seeded crops, and secondarily for reducing maturity.”
Canola and other crops can often suffer a yield loss of around 20 per cent when seeded in late May versus early May in the main canola growing areas. Late seeded crops are also at higher risk of grade loss due to fall frosts.
In areas that normally experience dry weather, late seeding may not induce a cut in N since the producer may feel higher spring moisture than normal creates above average yield potential, says Hartman. Areas with the longest growing seasons do not have as much pressure to cut N rates when seeding is delayed.
The question really comes down to the crop variety and its days to maturity, says Mooleki.
Doing the math
“If you’re seeding a late maturing canola crop which matures in 105 days and you’re in an area where frost is expected to come between August 24 and September 1, and it’s already May 31, you only have on average 84 to 92 days remaining before it is normally expected to frost,” says Mooleki. “If you need 105 days to maturity and you only have an 85 day window there is a very high risk for you to plant that crop. So you might look at changing to a variety that matures in around 80 days.”
The downside, adds Mooleki, is that earlier maturing varieties may not produce enough biomass to give as high a yield, If farmers want to risk seeding a later maturing variety, reducing N might help, but it’s still a big gamble.
And that gamble may only buy a few days, says Hartman. “The cut in days to maturity [in canola] is rather modest, generally one or two days. In general this strategy best fits delayed springs in short growing season areas, especially when canola prices are low to moderate and fertilizer prices are moderate to high.”
Modest reductions in N fertilizer rates (for example 100 pounds N/acre cut to 75 lbs.) have been shown to save one or two days maturity in canola, while larger cutbacks (for example 120 lbs./ac. to 60 lbs.) may save three to five days.
There are other things that farmers can try to help push maturity, says Hartman. “The small effect of N fertilizer reductions on saving maturity is similar to that from increasing phosphorous fertilizer by 15 to 25 pounds, or increasing stand density from three or four plants per foot to around 10 plants per foot,” he says “In cases where seed placed N fertilizer has caused thin stands, then maturity may be delayed by a week or so — and then the effect can be moderated by decreasing the amount of seed placed N or using special forms that enhance seed safety.”
Producers should be cautious before they cut N rates under most conditions. “It’s challenging to come up with a definite recommendation since some of these late-seeded fields may be low in nutrients such as nitrogen to begin with, due to leaching or denitrification losses,” says Mooleki. “However an abundance of nitrogen can delay crop maturity.” †