Low Cutting Height Can Result In Loss of Orchardgrass Stands

– Les Vough, University of Maryland Forage Specialist Emeritus

Cutting height may be the cause of loss of orchardgrass stands as much as insect and disease problems. Insects such as grubs and billbugs and leaf diseases are certainly contributors to loss of stands but the advent and popularity of disc mower-conditioners somewhat coincides with the shorter stand life and slower recovery of orchardgrass fields after cutting. There is a tendency to set disc mower/conditioners to cut closer to the ground than with sickle bar mower/conditioners. I see much more scalping of the ground where disc mower-conditioners are used than where sickle bar are used. With disc mower-conditioners, some farmers are cutting as close as an inch or inch-and-a-half, sometimes even lower. This is analogous to overgrazing of pastures and if done repeatedly the outcome can be the same – loss of stands.

In contrast to alfalfa which has large taproots and stores its reserves primarily in the roots, orchardgrass stores its reserves principally in the lower 3 to 4 inches of the stem bases or tillers. When growth begins in late winter or after cutting or grazing, leaf area for photosynthesis is low and stored carbohydrates are used to support new growth. In the case of alfalfa, depletion of carbohydrates in the roots continues until the topgrowth is about 6 to 8 inches tall, at which point there is usually enough leaf area to meet growth and respiration needs. If orchardgrass is cut low and most of the leaves removed, depletion of carbohydrates in the stem bases will continue until there is enough leaf area once again to meet growth and respiration needs.

Grasses store relatively little carbohydrates in the roots. The highest concentration of carbohydrates in grasses is in the leaf sheaths and stem bases in the vegetative stages of growth and in the lower stem in the reproductive stages of growth. Therefore it is critical in cutting or grazing management to leave enough stubble to retain sufficient stored carbohydrate and basal leaf area to support regrowth. Production of the first 1 to 3 leaves requires a substantial amount of stored energy (carbohydrates). If mowing or grazing removes too much of the lower stems and sheath area of tillers, too much of the stored carbohydrates can be removed, not leaving enough to support root maintenance and shoot regrowth. In addition, with summer regrowth cuttings, higher temperatures usually increase respiration rates, so less storage often occurs in midsummer.

If mowing or grazing removes too much leaf area and too much of the stem bases, growth rate is slowed substantially. Because there is not much leaf area for photosynthesis and manufacture of carbohydrates, additional reserves may be required for regrowth until there is sufficient leaf area and photosynthesis to meet growth needs. If much of those reserves have been removed by cutting too low, ultimately the point may be reached where reserves are totally depleted and the plant dies.

Root growth is also affected by heavy defoliation (either mowing or grazing), which makes the plant less competitive and more vulnerable to heat, drought, insect and disease stresses. Orchardgrass plants that are repeatedly cut at 1 to 1.5 inches will have shallower and less extensive root systems, thus less able to obtain moisture from the soil than plants cut at 3 to 4 inches.

Grasses can withstand greater defoliation during early and rapid growth stages in the spring and early summer than they can later in the growing season when less opportunity for growth exists. Forage researchers and farmers in Missouri have observed less damage to plants from summer heat and drought when the first harvest is made early and plants have time to regrow before the stress occurs. Some farmers in southern Missouri have reported almost 100 percent loss of stands when harvest was delayed to the late bloom stage. When little opportunity for regrowth exists during midsummer or drought, sufficient leaf material should be left after cutting or grazing to maintain carbohydrate levels within the plant.

A number of farmers have commented to me over the last several years that their orchardgrass not only doesn’t last as long as it used to but it also doesn’t come back as fast as it used to. They wonder what has happened to orchardgrass varieties today. Well, we are still using some of the same varieties that we used 20 or more years ago and today’s newer varieties are better than they have ever been, so these comments are a signal that something else, like the management practices, has changed, not the varieties. One of the most likely reasons that the orchardgrass is not coming back as quickly is that the cutting height is too low.

If you are presently having problems with slow regrowth and loss of stands and want to see if increasing the cutting height will improve regrowth and stand longevity, try several different cutting heights in the same field next year. Do it within the same field, not different fields due to differences between fields and stand conditions. Use a relatively new stand, preferably one only a year or so old. Make several rounds around the field cutting at your usual height. If your usual cutting height is 1.5 inches, after several rounds readjust the cutting height to 2.5 or 3 inches. After several more rounds readjust the cutting height again to 3.5 or 4 inches. Record the number of rounds made at each cutting height so that the same heights can be used in the same areas of the field for subsequent cuttings the rest of this year and next year. This will show you what impact cutting height has on performance and persistence.

I think you will find that cutting at a height of 3 to 4 inches will help maintain strong root reserves, leading to faster recovery of regrowth and better stand persistence. While we will still face problems from insects and diseases, more healthy and vigorous plants that are not stressed from low cutting heights will be better able to withstand the impacts of the insects and diseases and should help improve yield and stand persistence.