Manure Application Injectors - What do we need?

When it comes to selecting the right manure injection tool for the job there are many variables; the application rate, the amount of power to pull it, the soil type and conditions, the desired amount of residue cover left, or even the speed we can pull it through the field. All these constraints are important to consider, but the one we are going to discuss today is how much space do we need to create in the soil to have room to get the manure in.

It is intuitive that injection tools that create a larger cavity below the ground for the manure are capable of achieving good injection at higher application rates if the soil conditions are right, but they also require more power to pull, so trade-offs are required. One newer example injector is shown below. This one uses a fluted coulter to open an injection cavity.

Figure 1. A fluted disc manure injector followed by two concave discs. The fluted coulter opens the injection trench while the discs close and cover the injection furrow.

When it comes to injection, we want no overflow of manure out of the injection cavity. Two things are to achieve this, the first is that we must not have overflow manure. Overflow manure is when our injection furrow isn’t big enough to hold all the manure and as a result, it bubbles back to the surface. To avoid this the tool capacity has to be greater than the application rate (we’ll discuss in more detail below). The second thing we have to avoid is in-furrow manure; this manure stays in the injection furrow like we want, but we fail to cover up the furrow after putting the manure in it. Avoiding these two conditions limits the manure from air exposure, keeping odor and ammonia volatilization low. The example injector shown in figure 1 demonstrates both of these operations. In this case, the fluted coulter cuts the injection cavity. To be successful this cavity must be big enough to hold the manure we are putting down. The two concave trailing discs then cover the applied manure so we can’t see the furrow. To be successful both parts must be set correctly for the soil conditions and manure application rate we are trying to achieve. Below (in figure 2) you can see two examples of manure injection, the one on the left where the manure is covered, and the one on the right where we coverage of the injection furrow wasn’t achieved.

Figure 2. Good injections as compared to in-furrow manure injection.

So how can we determine how much injection capacity is needed for our manure? Well, it’s based primarily on two factors, the application rate you are trying to achieve and your tool spacing. Higher application rates require more capacity, while narrower spacing reduces required capacity (because each knife has to put down less manure per acre). Next, we will take a look at the requirement for two reasonable manure application rates, a swine finishing manure applied at 3,000 gallons an acre and a dairy manure applied at 12,000 gallons an acre. In both cases, we will assume the manure injector are on 30-inch centers.

The first thing we need to do is calculate the amount of manure each injector will receive. In the 3,000 gallon per acre case this is calculated by multiplying the application rate (3000 gallons per acre) by the tool spacing (2.5 feet), dividing by 43,560 to convert from acres to feet, multiplying by 0.134 to convert from gallons to cubic feet, and then multiplying by 144 to get the injection cavity cross-sectional area in feet. For the swine manure, we need about 3.3 square inches, as the dairy manure application rate was 4 times as much, four times this much area, almost 13.25 square inches, is needed.

What does this mean in practice? Let’s assume we are using the fluted disc (or similar to that shown in figure 1). Based on our soil conditions (current soil moisture, soil structure, residue cover, and the down pressure on our toolbar) it is cutting a cavity 4 inches deep by 2.5 inches wide, is our tool capacity sufficient for these application rates? The tool capacity is equal to the cross sectional area we are cutting so in this case it would be 4 inches times 2.5 inches, or approximately 10 square inches, which is enough for the deep pit swine manure example (3.3 square inches required), but not enough for the dairy manure example (13.25 square inches required).

So what options are there to increase capacity? A few things could be done: (1) We could reduce the manure application rate to be in line with what the equipment can handle (to achieve the desired application rate we would need to apply twice), (2) we could reduce the tool spacing as this reduces the amount of manure each tool needs to inject, (3) we could try running the tool deeper to get a larger cavity, or (4) we could use a tool with a larger area, potentially a knife or sweep.