January 9, 2014 | Pat Beaujot

Comparing a Disc Opener to a Knife Opener in No-Till

When we started working on the Seed Hawk opener in the early nineties, we had already seen the results of disc openers. In those early days, brave farmers were bringing up disc machines from Washington and other Southern states and they were having trouble getting quick, uniform emergence in our cold, wet soils. As a result early zero-till wasn’t very successful. Early on, the ZERO was the focus, and the attitude was to have no disturbance at all, but there was very little success with that in prairie conditions. Soon people realized that some type of knife opener might warm up the soil a little bit, and the research that followed certainly supported that.

I want to go through what I think are the advantages and disadvantages of knife openers versus disc openers. In a no-till field in the spring, temperatures are different all over the field, depending on the direction that the sun hits it (the slope) or how much straw cover there is. If you don’t disturb that field at all, and just use a narrow disc opener all over the field, the seeds will germinate at different speeds and grow at different speeds. The result is non-uniform emergence caused by varying soil temperature.

What has caused 90% of farmers to use knife openers on the Canadian Prairies is that when we disturb that strip of soil where the seed is placed, we turn the soil a bit, the sun warms that blackened that strip. It causes a bit of mineralization of that nice high organic surface soil, which will release some nutrients, and all of that provides a better seed bed. It is still important to place the seed on undisturbed soil, but if the soil is disturbed around and above that seed, then it results in warmer soil, and quicker emergence. Faster soil warming has been well documented.

A study by David Rourke in Minto, MB in 1995 compared a knife opener in no-till conditions to a disc opener in no-till conditions. The results showed that there was a significant difference in the soil temperature at the seed location with a knife opener- the furrow temperature was almost ten degrees warmer after seeding. This brought the soil temperature right up to the temperature of a conventionally tilled field. That really explained how a no-till farmer could plant a crop and get it up and in the bin in the ninety days that we have to work with in Canada. Timing is so critical.

Quick emergence is important anywhere in the world because when you plant your crop you want to get it out of the ground quick so it gets ahead of the weeds. No matter what kind of soil you have, quick, uniform emergence is always the goal.

There are many other things to consider in addition to soil temperature. Knife openers tend to penetrate easier. You can build a lighter frame machine with a knife opener because they don’t take a lot of weight to penetrate. This can be a significant advantage during a wet spring, as you can travel across the ground easier, without causing ruts or getting stuck. A knife opener provides more consistent soil-to-seed contact in all conditions. If the soil is soft and wet, a disc will push the straw into the furrow instead of cutting the straw. As a result you will get soil, seed and straw mixed into the furrow, rather than a nice clean soil around the seed, and that is important for proper germination and disease control.

In terms of maintenance, a knife opener is my choice for several reasons. A knife opener will work properly even if it is partially worn out. A disc opener can lose its cutting ability if it is worn. The cost to replace knifes is relatively inexpensive and not terribly time-consuming. Most knife systems today have a knock-on tip or one quick bolt to replace them. With a disc you are taking off several bolts to remove the disc. The bearing may need to be replaced as well, depending on its quality. There is a fair bit more time and expense involved in maintaining a disc drill than a knife drill.

Another advantage is that a knife opener causes a small furrow in the ground and there are some advantages to that furrow. In really dry conditions, the moisture might be an inch and a half below the soil surface when we go to seed. If we create a furrow that is an inch deep, then our plant, if it is placed inside the wet soil at the bottom of the furrow, will benefit from both that the good moisture, and from shallower placement of the seed (only a half inch rather than an inch and a half without the furrow).

For a crop like canola, if you can plant it at a half or three-quarters of an inch deep in the moisture and it only has to reach that far to get to the sunlight, it is going to survive quite nicely. If you have to plant it at an inch and a half or more with a disc opener, which doesn’t leave a furrow, then suddenly that plant has to have enough energy to push its way to the surface, and sometimes that won’t work. The other advantage of a furrow is that it will collect water from a light rain, whereas on flat soil, that moisture might evaporate away. Every extra bit of moisture will help.

The machine purchase price is going to be higher on a disc machine. There are more bearings, more parts on it, and more weight to it. There is certainly a lot more steel on the frame of a disc machine. The disadvantages of a knife opener is that the soil is disturbed a bit. If the goal was not to disturb the soil at all, then a knife opener won’t provide that. Also the field finish will be rougher. If you drive across a field that uses a knife opener, it will be a bumpier ride than with a disc machine. The seeding speed with a knife opener is limited, because a knife opener throws soil as it goes through the ground. If you are driving too fast, the back knives might throw soil over to where one of front knives had already passed and then you are burying the seed that was placed earlier. It is important to maintain the proper speed so that you ensure proper placement and that can be a limiting factor. A disc opener doesn’t throw soil, so if it can penetrate at higher speeds then you will be able to operate at a quicker pace.

The biggest challenge with a knife opener is going through straw. If straw conditions are challenging, for instance if they are wet and heavy or not chopped and spread well, then the knife opener will drag the straw as well and not do as neat a job leaving bunches. A disc opener will roll right through the straw and hopefully cut through it. The fix for this is to address your straw management by cutting shorter, chopping and spreading your straw well. This process is critical to no-till with a knife opener, and with a disc opener as well. Really good management starts with the combine, so that you can provide an optimum seed bed the following spring.

The research chart I mentioned is as follows: