The introduction of disc mowers has allowed forages to be cut close to the ground — much closer than sickle bar mowers. Some farmers cut as close to the ground as possible, leaving nothing in the field, in an attempt to maximize yield; however, this approach can have severe consequences to forage stand life and forage quality.
With a higher cutting height, more low-quality stem is left in the field, regrowth is hastened, stand health and long-term productivity are preserved, and the risk for forage soil contamination is reduced for many forage species.
When determining an optimum cutting height, consider how the plant stores energy for regrowth and also when and where regrowth shoots come from.
A plant must store an energy reserve (nonstructural carbohydrates) to produce the regrowth after it has been defoliated (either cut or grazed). Some plants store energy underground, such as in the roots (for example, alfalfa and clovers) or in rhizomes (for example, bermudagrass and some clovers). These plants can be defoliated to near ground level with little effect on regrowth. That’s because new buds will form, and energy is available for the shoots to regrow until they are big enough to photosynthesize their own energy.
There is, however, one caution. If the alfalfa harvest is delayed until flowering, such that new shoots have begun to grow, then forage must be cut above the height of the new shoots. The growing point is in the very top of the new shoots; cutting them off will mean that the plant must start over and produce new shoots for the next harvest. This will delay regrowth and reduce total-season yield.
Cut these high
Some species, including timothy, tall fescue, meadow fescue, smooth bromegrass, and birdsfoot trefoil, store energy in the stem bases. These plants must be harvested above the height of the carbohydrate storage area or growth will be reduced. The recommended cutting height for these and other cool-season and range-grass species is 4 inches. Frequent mowing or grazing at a low height will deplete their energy reserves, resulting in shorter stand longevity.
Cool-season grasses such as ryegrass and Kentucky bluegrass can tolerate lower defoliation. This is why these grasses tend to invade close-clipped or overgrazed areas. Harvesting alfalfa
-grass mixtures at 2 inches or less cutting height will cause the grass to die out in a couple years because it cannot compete with the alfalfa when the grass stem regrowth reserves have been removed.
Stolons are another energy storage structure of some species. Stolons are above ground horizontal stems that are capable of rooting and sending up new shoots at nodes. Species falling into this category include white clover (both common and ladino), kura clover, birdsfoot trefoil, and bermudagrass. If present, mower cutting height should be above the stolons to leave as much energy as possible for regrowth and spreading to fill in thin spots.
There’s more to consider
A second factor to keep in mind is when and where regrowth shoots are formed on the plants. During the period of internode elongation, cool-season grasses have low carbohydrate reserves and no basal axillary tillers present for regrowth. Basal tillers are not formed until early flowering. Timothy and smooth bromegrass often fail to persist in alfalfa when the spring crop is harvested at or prior to the early flower stage of alfalfa. This occurs because these grasses do not form tillers until flowering and are slow to recover after mowing or grazing. Tall and meadow fescues are not as severely affected, so these are better companion grasses to mix with alfalfa.
The following current recommendations regarding cutting height are designed to maximize yield while maintaining high-quality forage, good winter survival, and stand longevity:
Alfalfa or clover: The recommended minimum cutting height is 3 inches. Some literature shows a cutting height of 1 inch will not reduce stand longevity, but a potential higher ash content from lower cutting and the lower quality of the stem base must also be considered.
Frequent cutting at early to mid-bud will continue to deplete carbohydrate reserves. Therefore, allow one cutting of alfalfa to reach the early bloom stage each year. If cutting below 3 inches with a disc mower, use horizontal rather than angled knives. The latter will cut lodged forage better, but they also pick up more soil when the surface is dry.
Birdsfoot trefoil: The recommended cutting height is 3 inches. Research has shown that cutting frequently at a 1-inch height rapidly reduced the stand compared with a 3-inch height during two cutting years. Birdsfoot trefoil must retain some stem and leaves for regrowth since carbohydrates are at a low level during most of the growing season. This is probably because carbohydrates are being used in the continuous production of new top growth.
Cool-season grasses: The recommended cutting height for cool-season grasses is 4 inches. Many cool-season grasses store carbohydrates in the lower 2 to 3 inches of the stem. In addition to removing carbohydrate reserves, a low-cutting height also removes more photosynthetic leaf area.
If cut below 3 to 4 inches, especially on a consistent basis, regrowth is impaired. Frequent cutting of cool-season grasses at a low height depletes energy reserves, shortens stand life, and favors weed encroachment.
Range grasses, sorghum, and sudangrass: The recommended cutting height of bluestem or switchgrass is 6 inches. These grasses also store energy in stem bases. Cutting closer will not give much more hay value because few leaves remain on the stubble. Leaving nodes on the stubble stems will provide sites for axillary tiller formation. These tillers contribute leaf area and energy for fast regrowth.
Mixed alfalfa (or clover) and grass stands: Manage mixed stands for the predominant species. Determine if you have a grass stand with some alfalfa (or clover) or an alfalfa stand with some grass. Cut at 2 to 3 inches if more alfalfa is desired and at 4 inches to keep more grass in a mixed stand.
Bermudagrass: This warm-season grass can be cut as low as the mower and terrain will allow; close clipping will not harm stands. The key to harvesting bermudagrass is to harvest at four- to five-week intervals, which represents the best compromise between forage yield and quality. This harvesting interval produces hay that has a high proportion of leaves to stems and is easy to cure.
Bottom line: Cutting forage stands lower may give more dry matter yield on an individual cutting, but it will result in less yield for the season or life of the stand.
This article appeared in the January 2020 issue of Hay & Forage Grower on pages 26 and 27.
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