Prepared by Marvin H. Hall, professor of forage management, and Jerry Jung, adjunct professor of agronomy. Penn State College of Agricultural Sciences research and extension
Use of Brassica Crops to Extend the Grazing Season
Cool-season perennial grass and grass-legume pastures typically become less productive as the grazing season advances from June to November. Forage brassica crops such as turnip, swede, rape, and kale can be spring-seeded to supplement the perennial cool-season pastures in August and September or summer-seeded to extend the grazing season in November and December. Brassicas are annual crops that are highly productive and digestible and can be grazed 80 to 150 days after seeding, depending on the species (see table on back page). In addition, crude protein levels are high, varying from 15 to 25 percent in the herbage and 8 to 15 percent in the roots, depending on the level of nitrogen fertilization and weather conditions.
Species and varieties
Kale (Brassica oleracea L.)
Varieties of kale differ markedly in winter hardiness, rate of establishment, stem development, and time required to reach maturity. The stemless type of kale (e.g. ‘Premier’) has a faster rate of establishment than varieties that produce stems. Crop height of the stemless type is approximately 25 inches, whereas that of narrow stem kale is 60 inches with primary stems often 2 inches in diameter. Stemless kale attains maturity in approximately 90 days, allowing two crops per year, whereas varieties that develop stems require 150 to 180 days to attain maximum production. ‘Premier’ has consistently survived winters in central Pennsylvania, whereas other varieties of kale usually are winter-killed in December.
Rape (Brassica napus L.)
Mature forage rape is one of the best crops available for fattening lambs and flushing ewes. Rape is a multi-stemmed crop with fibrous roots. The stems vary in length, diameter, and in palatability to livestock. Forage yields of spring- planted rape increase until plants become physiologically mature. Growth slows or ceases at maturity and yields plateau until leaves senesce and die. Varieties differ in when this occurs; however, ‘Rangi’ rape retains its leaves longer than most varieties. Generally, yields of rape varieties in Pennsylvania are maximized with two 90-day growth periods. However, performance of ‘Emerald’ and ‘Winfred’ rape varieties is best with one 180-day growth period, and yields of rape hybrids are greatest with 60 days of growth before the first harvest and a 30-day growth period before the second harvest.
Swede (Brassica napus L.) Like turnip, swede produces a large edible root. Swede yields are higher than those of turnip, although growth is slower and requires 150 to 180 days to reach maximum production. Swede usually produces a short stem (neck), but can have stems 2 1⁄2 feet long when grown with tall crops that shade the swede. Unfortunately, stem elongation is at the expense of root development. The variety ‘Calder’ is cold hardy in central Pennsylvania and thus ideal for stockpiling and for late fall or early winter grazing. In general, all swede varieties are recommended for late fall grazing.
turnip (Brassica rapa L.) or turnip Hybrids
These crops grow very fast, reaching near maximum production levels in 80 to 90 days. Studies in southwestern Pennsylvania showed that turnip can accumulate dry matter in October as fast as field corn does in August. Growing “out of season” (October/November) makes turnip a valuable crop for late fall grazing.
The proportion of tops and roots varies markedly depending on variety, crop age, and planting date. Research by the USDA Pasture Laboratory showed that turnip crops can vary from 90 percent tops/10 percent roots to 15 per- cent tops/85 percent roots. Some hybrids have fibrous roots that will not be readily grazed by livestock. All varieties produce primarily tops during the first 45 days of growth. Sixty to 90 days after seeding, turnip varieties such as ‘Savannah’ and ‘All Top’ continue to produce a high pro- portion of tops. During the same period, other turnip varieties have nearly equal top and root production, except ‘Purple Top’ has a greater root than top production. The significance in the proportion of tops and roots is that the crude protein concentration (8 to 10 percent) of roots is approximately one-half of that in turnip tops. Therefore, greater root production tends to reduce the crude protein yield of the total crop. On the other hand, stockpiled tops appear to be more vulnerable to weather and pest damage than roots. Varieties differ in their resistance to diseases, but this often is not evident until the crop is more than 80 days of age and the plants are reaching full production.
Other Forage Brassicas
Several hybrids of brassica species are also used as forage crops; however, there is limited research information on the production and management of these hybrids. The more common hybrids include a cross between Chinese cabbage (Brassica campesteris sensulato L.) and rape (‘Perko’), tur- nip (‘Tyfon’ and ‘Buko’), and swede (‘Wairangi’).
All brassica crops require good soil drainage and a soil pH between 5.3 and 6.8 for optimum production. Good stands can be established by planting 3.5 to 4 pounds per acre of kale or rape, or 1.5 to 2 pounds per acre of swede or turnip. The higher seeding rates are recommended for spring plantings. The seeds should be planted in rows 6 to 8 inches apart and not more than one-half inch deep. However, brassica seed can also be broadcast and incorporated into tilled seedbeds by cultipacking. When preparing a tilled seedbed for brassica planting, plow the ground several weeks before planting to allow weed seeds to germinate before secondary tillage is completed to form a firm and fine seedbed that is free of weeds. In addition, the preplant incorporated herbicide Treflan (trifluralin) is labeled at 0.5 to 1.0 pint active ingredient per acre for control of annual grass and small- seeded broadleaf weeds in brassicas.
Brassica stands can also be established by no-till planting in grass sod that is suppressed with paraquat or glyphosate herbicides. Read pesticide labels and precautions before using either of these herbicides. Ideally, the grass sod should be grazed through June with the grazing prior to brassica seeding being very close. Approximately two weeks before planting the herbicide should be applied to the grass sod. Another option for no-till establishment would be to apply a manure slurry to the sod, burn the sod back, and then no- till plant the brassica seeds through the slurry. In addition to reduced erosion concerns with no-till planting, there are generally fewer insect problems than with conventionally seeded brassicas. The following recommendations will improve the chances of successful brassica establishment.
1. Attempt establishment only on well drained soils.
2. Do not seed deeper than one-half inch.
3. When seeding into a sod, suppress the sod long enough to allow the brassicas to establish (two to three weeks).
4. Apply 75 pounds of nitrogen at seeding to stimulate establishment and growth.
As previously mentioned, forage brassicas can be grown to supplement perennial cool-season pastures in August and September or to extend the grazing season in November and December. In the first instance, brassicas would be planted in May or early June because spring rains will help ensure production for August and September grazing (Figure 1). Turnip, rape, or stemless kale could be used for this purpose. In the second instance, swede or kale would be planted in spring, or rape, turnip, and turnip hybrids would be planted in late July or early August, and growth allowed to accumulate until November or December.