Wednesday, June 15, 2016

Fast Forward on Frost Recovery

Abnormally cold temperatures and frost on 5/14/16 and 5/15/16 caused significant injury to many corn fields in Southeastern Minnesota.  Corn injury was influenced by planting date, with most severe injury on planting dates around April 19th. 

Corn planted around April 19th, sustained serious damage and has had the most difficult recovery.  My infield observations on May 23rd were that stands had not been reduced by much, if at all, by the frost. That said, the crop came back unevenly and upwards of 20,000 of plants per acre were buggy-whipped or tied up as of Monday May 23rd (stand counts remain at 34,000 to 35,000).

Fast Forward to June 8th, stands remain strong and plants have grown through the buggy whipping but growth remains uneven across the areas most severely affected by frost:

We will see if things even out over the next few weeks.
 Ryan Miller

Monday, June 6, 2016

In-season nitrogen fertilizer recommendations

As the corn starts growing, we’re approaching the time where we need to think about whether supplemental N will be necessary.  While anyone who applied N in the fall probably lost a lot from the warm, wet weather during late fall and early spring, spring applications should be pretty well-protected due to lower than normal rainfall so far.  Weather data from Houston, Fillmore, and Mower counties (found here, courtesy of the Root River Field-to-Stream Partnership) shows that while growing degree days so far this season are right around average, most of southeastern Minnesota is several inches below average on rainfall.  Most rain events so far have been less than half an inch in most locations, making it unlikely that much leaching has occurred.  

As of now, it looks like supplemental N should not be necessary for those who applied their nitrogen in the spring.  I’ll keep my fingers crossed that it stays that way.  Nonetheless, since they are for research purposes, we will still be taking presidedress nitrate tests at our U of M nitrogen rate response trials here in southeastern Minnesota.  This test isn’t perfect under Minnesota conditions, but we do know that when nitrate levels are above 20 ppm, you won't get a yield response to added N.  That’s the main value of the test.  Below 20 ppm is still hard to interpret- hopefully having more data on presidedress nitrate tests from rate response trials will help us refine this tool.

A more valuable tool at this point in time is the Supplemental Nitrogen Worksheet, found here or as an online calculator here.  It’s good at helping you think through whether you had the right weather conditions to cause large nitrogen losses, and whether your corn looks nitrogen deficient.  If necessary, the best time to sidedress is right around V6.

Iowa’s Raccoon River: nitrate trends and implications for Minnesota farmers

A research paper about nitrate trends in the Raccoon River in west-central Iowa has been getting a lot of attention recently, as the Raccoon River is the subject of the Des Moines Water Works lawsuit that every Midwestern farmer knows about.  A copy of the article was forwarded to me, and I think there is some interesting information on nitrogen management that came out of it.

The researchers looked at nitrate concentrations flowing through the Raccoon River Watershed to Des Moines from 1999 to 2014.  Here’s some of the key information from their study: 

- 75% of the sites they sampled within the watershed had average nitrate concentrations above 10 mg/L (the drinking water standard) during the months of April to July (the months where leaching and erosion are typically highest from farm fields).  However, it doesn’t look like nitrate levels in the river have been increasing at all from 1999 to 2014.

-The biggest factor that affected nitrate concentrations and total amounts of nitrate in the water from year to year was rainfall.  Because of large changes in precipitation from year to year (ranging from 22 to 46 inches), there was a lot of variability in nitrate losses.  On average, less than one pound of nitrogen was lost to the river per acre of farmland in the best year (2000, a dry year), but an average of 28 lbs of nitrogen was lost per acre in the very wet year of 2013.

- Nitrate concentrations were very high in years following major droughts (including after the 2012 drought).  In the case of 2013, there was a very wet spring with a lot of losses, and there had already been a lot of residual nitrogen in the soil since the corn didn’t take much up in 2012 and it didn’t leach through the soil.  Sampling for residual nitrates after a drought year is really important to help prevent over-applying fertilizer.

-In the Raccoon River watershed, average nitrogen rates on corn after soybean were 159 lbs, or 195 lbs if it came from manure; average nitrogen rates on corn following corn were 189 lbs, or 238 lbs from manure.  This average was above the Iowa State-recommended range for corn following soybean, and within the recommended range for corn following corn.

-Average leaching losses of nitrogen were around 10% of the fertilizer nitrogen applied plus nitrogen fixed by soybeans.  This is pretty typical for the Upper Midwest; in Minnesota, leaching losses typically range from 0-20% of applied fertilizer nitrogen, depending on soil type, rate, and weather.

The researchers tried to understand why nitrate wasn’t increasing in the Raccoon River.  Corn is often implicated as being the biggest source of nitrates from farm fields, but corn acres increased 19% over the 15 years they studied, while nitrate levels did not increase at all.  There were two other notable trends that may be important- total cultivated acres decreased by 2.5%, and soybean acres decreased by 24%.  They thought the decline in soybean acreage was preventing any increase in nitrate leaching from increased corn acres.  Here’s their reasoning:

-Carbon-to-nitrogen ratios are much lower for soybean residue than for corn residue.  As a result, corn residue ties up quite a bit of nitrogen as it breaks down, while soybean residue does not (this is why we can grow corn after soybeans on typically 40 lbs less nitrogen than corn after corn without affecting yield potential).  

-They’ve seen less denitrification occurring in or after soybean years than in continuous corn, making leaching a larger potential loss in corn-soybean rotations.

-There is a lot more tile flow under soybeans that under corn, because corn transpires a lot more water than soybeans do.  We’ve seen this happen consistently in Minnesota as well (you can see some results in this publication), although in Minnesota, nitrogen losses from corn-soybean systems were still lower than continuous corn.

Another possibility is that, on average, nitrogen is being applied in excess to corn after soybean.  If nitrogen efficiency under current practices is higher in continuous corn production, the increase in continuous corn at the expense of corn-soybean rotations probably wouldn’t increase Raccoon River nitrates.  Average rates on corn after soybean were quite a bit higher than recommended, while the average rate on continuous corn was well within the recommended rates, which supports this interpretation. Regardless, it’s nice to see some hard data on the state of water in the Raccoon River.