Cover Crops and Manure Management: Do They Change My Application Rates?

 Cover crops are popping up more these days, and for good reason. They’re great at grabbing leftover nitrogen and keeping it from wandering off, especially if you had to put manure on earlier than you’d like. But in some operations, we might start thinking of them as more than just a cover crop; they can also be chopped for silage to feed ruminants and become a production crop.

So, here’s the question I want to dig into today: Does using a cover crop, or growing one for forage, change my allowable manure application rates in my manure management plan?

The short answer is sometimes. The long answer is it depends:

·         Is your manure plan limited by phosphorus or nitrogen?

·         Are you harvesting that cover crop, or just using it as a cover and terminating it?

If You’re Phosphorus Limited

Here’s where cover crops can really shine. Because they hold the soil in place, they lower erosion estimates in RUSLE2. That can improve your Phosphorus Index score. Sometimes, it may bump you from “P-limited” to “N-limited,” which opens the door for higher manure rates.

And if you’re harvesting that cover crop, say as rye silage, you’re pulling more phosphorus out of the field. More removal = more room in the plan for phosphorus application.

If You’re Nitrogen Limited

If you’re N-limited, the story is a little different.

·         No harvest? Nothing’s leaving the field, so your N removal hasn’t changed. That means your maximum manure N rate stays the same.

·         Harvest the cover crop? Now it counts as a crop, and you can add its removal to your N budget.

What Does Rye Silage Remove?

Cereal rye uptake depends on growth stage at harvest, but a good ballpark for removal is:

·         40-60 lb N/ton dry matter depending on harvest stage

·         15 lb P₂O₅/ton dry matter

·         75 lb K₂O/ton matter

So even a couple tons (2-3 tons/dry matter) of rye silage can swing the numbers in your nutrient plan.

An Example in Action

To put some numbers on this, I ran a simple comparison using RUSLE2, the phosphorus index, and N and P-limited application rates. I assumed a liquid dairy manure testing 25 lb N/1,000 gallons (70% available) and 10 lb P₂O₅/1,000 gallons. I kept corn silage yield at 30 tons/acre (65% moisture) and assumed 2 tons/acre dry matter rye silage when harvested.

I compared three rotations:

·         Corn silage only

·         Corn silage with cereal rye as a cover crop

·         Corn silage with cereal rye harvested as silage

Quick note: these numbers are for illustration only. The specifics at your farm could change the outcomes.

Table 1. Effect of rotation on erosion, Phosphorus Index, and allowable manure application rates.

Rotation	Erosion (tons/acre)	P-Index	Allowable Application Rate (gallon/acre)
Corn Silage	2.7	2.26	12,857
Corn Silage - Rye Cover Crop	2.2	1.97	12,857
Corn Silage - Rye Silage	5.1	3.65	19,714

 

What’s the Take-Home?

Adding rye as a cover crop reduced erosion and dropped the P-Index below 2.0, but it didn’t change how much manure I was allowed to apply in this case. Harvesting rye for silage, though, added nutrient removal to the system. That extra removal let me use more manure, even though erosion ticked from extra field activities.

So, the bottom line is that cover crops helped in phosphorus-limited systems, and if you harvest them, they also change the nitrogen and phosphorus removal math. That’s when your allowable rates in the plan start to shift.