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.
