This year I've been getting quite a few questions on winter manure application, some from farmers, some from the general public, and some from people who saw some manure being applied as they drove by on the road who wondered what my thoughts were on the topic or what the science says about these practices.
So let's start with the law. Many of you have probably heard that Iowa does have a snow covered and frozen ground rule, but are you aware of what it says and who it applies to? Iowa code placed a restriction on application of liquid and slurry manure's from confinement animal feeding operations (basically means animals raised under a roof) with more than 500 animal units. These rules also have effective date ranges, for example it says if the ground is snow-covered between Dec. 21 and April 1 then liquid manure from these operations can't be applied (unless we could inject it or perform an incorporation on the day of application or they had a manure emergency). For completeness the dates on frozen ground portion of the rule run from Feb. 1 to April 1.
So say its January 3rd and there is no snow on the ground could these operations legally apply manure? Yes. However, if its February 3rd, there is now snow on the ground but the ground is frozen can these operations legally apply manure? No. Some of you may be thinking, why those particular dates - why not just just say snow covered fields? Well, a few reasons actually - some science based and some practically based. First the science reasons. What the research has shown is that the amount of nutrient we lose after a manure application depends on lots of factors, but some of the most important are the size of the runoff event and the amount of time after manure application before the runoff event occurs. This means that I worry more about nutrient loss from manure that gets applied closer to spring melt than manures that get surface applied in the fall that get some reaction time with the soil. The other thing to remember is these dates weren't just arbitrarily selected, they were researched in relation to typical soil temperatures, snowfall chances, and runoff risk.
Along with that there are some practical reasons. Giving dates provides some certainty to the rule and lets us plan accordingly. One of the things you might have seen me talk about before (or write about on here) is try to wait until soil temperatures are 50-degrees and cooling before you apply manure (especially ammonium rich manures, like most swine manures) in the fall. This recommendation is based on trying to keep nitrogen in the ammonium form rather than getting it converted to nitrate in the soil - things that matter in this process are time and temperature and the 50-degree recommendation seems pretty appropriate for most of Iowa. However, following this recommendation might mean waiting until mid to late November. What if we get that early snow storm or a soil freeze quickly after that and didn't get our manure application completed? Well, that would certainly put us in a bind and realizing that risk existed we might be more anxious about getting my manure applied in the fall, and as a result choose to apply it earlier to mitigate the risk of getting stuck with a full storage. So essentially what we are really trying to do is balance the risk of two competing recommendations, that is delaying manure application until soils cool but not putting it on frozen ground. Effective dates provide at least a little certainty in this equation - hence I'm a big fan of dates as I feel it puts us in a better position to make the most appropriate decisions.
There are two other things I want you to note about the rule; there is a farm size threshold (500 animal units) before it applies to you. Why is this in there? Again I'd suggest its about risk. In this case the number of acres that would be impacted and the resulting effect that could have on water quality. More manure might mean more impact, since more acres are impacted. Does this mean if you are below this threshold go right ahead and apply manure all winter long? No, we certainly want them to minimize winter application as well especially during riskier times such as during snow melt, but again manure decisions at these farms are a balancing of competing factors - how much it costs to build a larger storage, how long they will continue to farm, and the the economics of animal production.
The other thing to note is that the rule was written to apply to liquid and slurry manure, it does not include solid manure. In some ways this probably makes some sense - if we poor a glass of water on frozen or snow covered soil does it soak in and is it held by the soil? It probably depends to at least some level how wet the soil was when it froze, but we can probably see the risk of it flowing away. With solid manure there isn't this direct runoff concern at the time of application; instead we worry about liquid (rainfall or snow melt that will pick up and move those nutrients). We'd certainly recognize there are a least some differences in risk between solid and liquid manures. Again, that doesn't mean we should be out there applying all winter long - but there are some logistic constraints (only so many days int he fall especially with a late harvest) on manure application that come into play. However, if we are out there in the winter we need to focus on doing what we can to protect water - that means picking the right fields, the right weather conditions, and making sure our equipment and decisions are specific to that environment.
So if the snow covered and winter manure application rule don't apply to your farm are there any regulations that do apply? Yes! There is a Iowa DNR fact sheet that lists "Separation distance for land application of manures from open feedlots and confinement feeding operations of all sizes." (If you need a copy let me know and I'll get you one). This fact sheet says a setback - no application area - of at least 200 feet is required from a sinkhole, abandoned well, drinking water well, designated wetland, or water source (any sort of stream) if manure is surface applied and not incorporated (that applies to both solid and liquid manures) [800 feet if the water is listed as a high quality water resource]. Alternatively, you can maintain a permanent grass buffer between the manure application area (at least 50 feet wide) and those designated area to meet this requirement. No manure in the buffer!
Alright, so we've covered the rules and you still want to apply in the winter what's next. Stop and think about it for one more minute? Okay, perhaps we find ourselves in a situation where we have to apply (maybe its logistic, maybe its a storage emergency, maybe its another reason I missed), but we need to ask is this the best decision we can make based on all the other factors impacting our manure management decision. If yes, its time to really think about water quality and what we are going to do to limit the risk of any winter manure application. It needs to start with identifying the best fields for this choice - look for level ground and places where you have soil erosion controlled. Beyond that its timing and weather - these two factors tend to be the largest drivers of nutrient loss. This means watching the weather forecast and avoiding application before anticipated rainfall and snowmelts.
Additional recommendations include incorporating the manure when you can, avoiding areas of concentrated flow such as waterways, ditches, or similar areas, using setbacks from sensitive areas like stream banks, sinkholes, and similar, and if possible avoiding application near areas that drain to surface tile inlets. If these areas can’t be avoided add protection around drainage tile intakes to prevent entry by manure or runoff water.
If winter manure applications are not avoidable,
- Take into account soil and weather conditions
- Avoid applying before a snowmelt or rainfall event
- Apply to areas of level ground and where soil erosion is controlled
- Apply to areas with less snow cover
- Follow appropriate setback distances
- Update your Manure Management plan to reflect surface application rates and if subject to Master Matrix requirements for injection or incorporation get approval before surface applying