Farm and Field

Timing the application of fall nitrogen

By Herald-Whig
Posted: Nov. 19, 2017 12:01 am

An agronomist at the University of Illinois urges farmers to wait to apply fall ammonia not only until soil temperatures are 50 degrees or lower -- but are likely to stay cool.

"Almost everyone is on board with waiting until soil temperatures are at or below 50 degrees before applying ammonia. Cool soil, along with use of a nitrification inhibitor, lowers the rate of nitrification and helps preserve nitrogen in the ammonium form. Nitrogen present in the soil as ammonium is safe from loss," Emerson Nafziger said.

Some producers would like to start before soil temperatures reach 50, assuming they will go down as air temperatures start to decline.

"But if we apply when soil is at 60 degrees and soil temperatures don't drop quickly, or if they rise again after application, nitrification will continue and will persist as long as soils stay warmer," Nafziger said. "In fact, nitrification does not stop dead at 50 degrees; as a biological process, its rate drops off as the temperature falls, but temperatures need to be near freezing for nitrification to stop completely."

In Illinois, it's typically been safe to apply ammonia on or after Nov. 1, but that's not a sure thing. In the past two years, soil temperatures have risen above 50 degrees at least once between November and February.

The problem with using only the minimum soil temperature is that it doesn't represent the soil temperature in the ammonia application zone. Minimum soil temperatures on clear days are typically five degrees or so less than the average soil temperatures for the day.

Average fall temperatures in Illinois have been trending slowly upward for some time, so with warm weather in late October, it might make sense to wait a little longer.

"Otherwise, patience in waiting another 10 days will likely be rewarded, even if -- as is often the case when doing the right thing -- the reward isn't very visible," Nafziger said.

Phytase response

Canola meal, which is included in diets fed to pigs as a protein source, also is relatively high in phosphorus. But most of the phosphorus in canola meal is bound to phytic acid, and microbial phytase is often added to diets to help make more phosphorus available to pigs.

New research from the University of Illinois shows that not all kinds of canola meal respond equally to the addition of phytase.

A research team fed growing barrows diets containing conventional canola meal (CM-CV), canola meal processed at a high temperature (CM-HT), canola meal processed at a low temperature (CM-LT), high-protein canola meal (CM-HP) or soybean meal as the sole source of amino acids and phosphorus. For each test ingredient, diets were formulated that contained either zero, 500, 1,500 or 2,500 phytase units (FTU) of microbial phytase for a total of 20 diets.

Results indicated that if zero, 500 or 1,500 FTU of phytase was added, there was no differences in phosphorus digestibility among the different canola meals. However, phosphorus digestibility was less in CM-LT than in the other canola meals when 2,500 FTU was added.

Phosphorus digestibility was greater in soybean meal than in any of the canola meals when zero, 500 or 1,500 FTU was added. "However if more than 2,000 FTU of phytase was added to the diets, the digestibility of phosphorus in all canola meal sources except CM-LT was equal to that in soybean meal," research project leader Hans Stein said. "This indicates that with sufficient phytase in the diets, the phosphorus in canola meal can be made available to pigs."

Stein's team developed regression equations from the results, which he says will make it possible to predict how much phosphorus will be released from canola meal or soybean meal for a given amount of phytase.

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