Keep an Eye on Cutting Height to Maintain Forage Stands

– Dwane Miller, Penn State Extension Educator

Now that the sun is beginning to shine (at least on a more regular basis), forage growers have begun to take first cutting hay across Pennsylvania. One of the questions we get when trying to maximize both forage yield and longevity of the stand is “How low can you mow?”

This question came to light with the popularity of the disc mower. One of the features of the disc mower allows the operator to lower the cutting height closer to the soil surface. Because of this, we can expect an increase in the tonnage we harvest. However, there are some negatives associated with cutting closer to the soil surface.

First, we can expect an increase in the ash content of the forages. Forages naturally contain some ash; around 6%. However, as we clip closer to the soil surface, the ash content can increase significantly. Results from the University of Wisconsin forage testing lab have seen samples with 15-18% ash… that’s a lot of extra dirt!

Second, if we cut closer to the surface, we can expect a reduced stand life, especially with a cool season grass forages. This is because in orchardgrass, the energy is stored in the stem base of the plant. Timothy’s energy reserves are stored in the corm, a bulb-like structure just below the soil surface. Repeated mowing of grasses at a low height will significantly lower the life of the stand. Research conducted at the Miner Institute in New York showed that orchardgrass cut at a height of 2” regrew at a much slower rate than a cutting height of 4”. Legumes like alfalfa can tolerate a shorter cutting height, because their energy storage is contained in the taproot. However, quality can be compromised with a shorter cut. The lower part of the plant often contains more stems than leaves, and you run the risk of getting more ash in your forage.

Here are some considerations for cutting height:

  • Alfalfa – can tolerate a lower cutting than grasses. Recent recommendations have been in the 2.5-3” range, to minimize ash content and maximize quality. Keep in mind that frequent cutting at early maturity will deplete carbohydrate reserves and weaken the plant.
  • Grasses– Keep the cutter bar higher to maintain the stand. Minimum of 4” cutting height during the establishment year, and 3” during production years.
  • Mixed stands – In this case, you need to manage for the predominant species. If you consider it primarily an alfalfa stand, mow at a minimum height of 2.5”. For a heavy grass mix, raise the cutter bar up to at least 3” if you want to keep the grass in the mix.