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Cutting forages: How low should you go?

Stephen Foster Published on 14 August 2013
Green Chop

I often receive calls from local producers shortly after the morning coffee-shop crew adjourns for the day. Recently, a caller asked: “What is the best height to mow forages for hay?” I gave him my usual extension educator reply: “That depends.”

When cutting forages for hay, there are a few things we need to clarify:

• Are you harvesting grass hay or alfalfa hay?

• What are your management goals for hay quality and yield?

• What is the condition of your forages? (Are they stressed from soil conditions, lack of water or other factors that affect their growth and ability to recover rapidly from harvest?)

• How level are your fields?

Searching the Internet for research on cutting height of forages, I found numerous studies and articles that included recommendations for both alfalfa and grass hay. However, most of the studies found were conducted before the year 2000.

An article written in 2007 by Daniel Wiersma, Mike Bertam, Ron Wiederholt and Nick Schneider, “The Long and Short of Alfalfa Cutting Height,” documented the following studies:

• Early Wisconsin studies on alfalfa harvested three or four times per season yielded more tons of forage when cut at a 1-inch height versus cutting at 3 inches or taller.

• Research conducted in North Dakota since the mid-1960s demonstrated similar results where shorter cutting heights resulted in higher yields. In a recent North Dakota two-year study using three-cut or four-cut systems, annual alfalfa yields were reported to be 1.6 tons per acre higher when cut at 1-inch compared to 5-inch heights.

• Research at Wisconsin’s Marshfield Agricultural Research Station (MARS) showed a reduction of half a ton of dry matter per inch as cutting height increased from 2 inches to 6 inches.

These studies indicate that forage yield definitely increases when the crop is harvested at shorter heights. The gain in yield isn’t simply from harvesting more of the plants at each cutting but also from changes in how the plants regrow.

Shorter cutting height results in increased growth from stems that originate from buds on the root crown rather than from axillary buds on the lower portions of the stems.

So if increased yield is a benefit to reducing cutting height, are there corresponding disadvantages? The answer is a definitive yes. First, shorter cutting heights increase the risk of scalping an uneven field or one with excessive gopher activity.

Cutting forages at lower heights can damage equipment and increase the ash content (amount of dirt and debris) in the forage. Other important management and economic issues include effects on hay quality and stand longevity.

So the next question is: “How does lowering the cutting height affect hay quality?” A research study conducted in Nebraska evaluated the top half of a full-bloom alfalfa canopy with the bottom half.

Results indicated that the top half of the canopy had a higher percentage of digestible nutrients and less fiber than the bottom half of the plant. Also, the lower two-thirds of the stem decreased in quality at a faster pace than did the upper portion of the stem.

This occurs because the lower-stem sections are more mature and tend to be more fibrous and woody compared to the less mature, upper-stem sections.

A Wisconsin study confirmed hay quality diminishes modestly as alfalfa cutting height becomes closer to the soil surface. Reducing cutting height from 6 inches to 2 inches decreased crude protein by less than 1 percent.

However, this study found that pounds of milk produced per acre increased at shorter cutting heights. The quality improvement obtained by increasing the cutting height does not offset the yield reduction.

The amount of quality gain must be weighed against the yield loss due to leaving taller stubble. If growers are harvesting in a timely manner, there should be no reason to forgo yield for a slight quality increase due to leaving taller stubble.

When changing forage management practices, growers want to be certain stand longevity is not jeopardized. Alfalfa has a deep taproot below the root crown and can store large energy reserves in buds on the root crown for rapid regrowth following a cutting.

Most grasses, however, lack a deep root system and have smaller energy reserves than alfalfa plants. Rapid regrowth of grasses occurs from growing points located at the base of leaf blades, not from buds on the root crown or axillary buds on the lower stem.

Therefore, if you cut grasses too short (below the base of the leaf blade), you are removing the plant’s ability to regrow rapidly and forcing it to use energy stored in basal buds that also is needed to initiate regrowth after the next dormant period.

Cutting alfalfa shorter can reduce stand life if other stress factors exist. Frequent cutting of alfalfa at very early maturity stages often depletes root carbohydrate reserves. When carbohydrate reserves are low, energy for stem regrowth is low and results in poor plant recovery after harvest.

In spite of this, cutting height research shows that early plant decline or death does not occur for alfalfa when proper crop management practices are used.

When plants are healthy and not under stress (properly irrigated, with adequate nutrients and disease-free), it appears that short cutting height will not reduce stand longevity. One exception to lower cutting height for alfalfa is the fall harvest.

For the fall alfalfa harvest, taller cutting heights may be warranted for two purposes. First, more leaf area will result in a quicker replacement of energy reserves and bud formation for growth initiation the following spring.

Second, taller and abundant stubble catches more snow and may help keep the snow cover for a longer period of time. Adequate snow cover can help protect alfalfa plants from cold temperature extremes or fluctuating late-winter temperatures.

Higher incidence of root heaving was documented for 1-inch or 2-inch cutting heights compared to a 4-inch cutting height. From this study, it appears that leaving a 4-inch-or- greater stubble in the late fall may be helpful to prevent root heaving in the spring.

As for grass forages, research conducted at Miner Institute, Chazy, New York, a greenhouse experiment studied the effect of cutting height on orchardgrass and reeds canarygrass.

This work showed that first-year reeds canarygrass was completely killed at a 2-inch cutting height, while the orchardgrass did regrow but at a much slower rate. In contrast, at the 4-inch cutting height both grasses performed fine with the reeds canarygrass measuring 16 inches of regrowth in 21 days.

More prevalent Western grasses, such as tall wheatgrass and basin wild rye, require an 8-inch height to thrive. Intermediate wheatgrass should be cut at 6 inches to maintain stand longevity. In a mixed forage pasture or field, the recommended cutting height should be based on dominant grass in the field.

Here are the take-home points to remember:

• Cut healthy, non-stressed alfalfa fields at 1-inch heights to achieve maximum dry matter and nutrient yields.

• In fields where plants are experiencing moisture or flooding stress, or where the crop has been cut early and frequently, root carbohydrate reserves may be low, and cutting height should be adjusted upward to avoid additional stress or plant death.

• Adjust cutting height to avoid injuring plants, contaminating the forage with soil or picking up rocks.

• When harvesting forages in the fall, growers should leave a 4-inch- or-greater stubble to help capture snow, prevent heaving and provide for quicker storage of energy reserves and bud formation.

• Grasses – A minimum of 3 to 4 inches of stubble should be left to maintain stand longevity.

• Stands could be even more sensitive to cutting height reduction in the seeding year.

• The loss in grass stand productivity from cutting too short far outweighs any yield boost you might get from harvesting a few extra inches in that one cutting.

• In mixed stands, cutting height could actually be used as a management tool for stand composition by choosing a cutting height that either favors grass or alfalfa.  FG

References omitted due to space but are available upon request. Click here to email an editor.

Photo
Hay chopping at Hernkes Farm, Cannon Falls, Minnesota. Photo by Lynn Olsen.

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Stephen Foster
University of Nevada Cooperative Extension

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