Forage brassicas are fast-growing annual crops that are highly productive and digestible. Crude protein levels range from 15% to 25% in the herbage and 8% to 15% in the roots, depending on nitrogen fertilization rate and weather conditions. The most commonly used forage brassica crops are rape, turnip, kale and swede. They can be grazed from 80 to 150 days after seeding depending on species (Table 7.12). These crops offer great potential and flexibility for improving livestock carrying capacity from August through December. Spring-seeded brassicas boost forage supply in late summer. Summer-seeded brassicas extend the grazing season in late fall and early winter.
RAPE (Brassica napus L.) is a short-season, leafy crop whose stems and leaves are eaten by the grazing animal; rape can also be greenchopped. It has fibrous roots, and each plant produces many stems. Rape regrows after harvest and is the easiest brassica species to manage for multiple grazings. Mature rape is excellent for fattening lambs and flushing ewes. Yields are generally maximized with two 90-day growth periods, but some varieties yield better with one 180-day growth period while rape hybrids yield best with 60 days of growth for the first harvest, followed by 30 days for the second harvest.
TURNIP (Brassica rapa L.) is a fast-growing crop that reaches near maximum production 80 to 90 days after seeding. Roots, stems, and leaves are grazed. The relative proportion of tops and roots varies markedly with variety, crop age, and planting date. The crude protein concentration of roots (8%–10% CP) is approximately half of that in turnip top growth; however, stockpiled tops are more vulnerable to weather and pest damage than roots.
KALE (Brassica oleracea L.) is a long-season, leafy brassica that produces some of the highest yields of the brassica family when it is spring-seeded. Some varieties are very cold tolerant, which allows grazing of leaves and stems into December and January most years. Stemless varieties reach about 25 inches in height, whereas narrow stem kale grows to 5 feet, with primary stems 2 inches in diameter. Stemless kale (e.g., Premier) establishes quickly and reaches maturity in about 90 days. Narrow stem kale is slower to establish and requires 150 to 180 days to reach maximum production.
SWEDE (Brassica napus L.) is a long-season brassica that produces a large edible root-like turnip. Swede produces higher yields than turnip, but it grows more slowly and requires 150 to 180 days to reach maximum production. Swede produces a short stem when not shaded. If plants are shaded, it produces stems 30 inches tall. Swede does not regrow after harvest.
HYBRIDS OF CHINESE CABBAGE with rape, turnip, or swede can also be used for forage. Research information on the production and management of these hybrids is limited.
Establishment
Brassica crops germinate quickly, and can be planted to provide either summer or late fall/winter grazing:
• Plant rape, turnip, and stemless kale in the spring (midApril through May) to provide pasture in August and September.
• Plant rape and turnips in July and August to provide grazing in November and December.
• Plant swede and kale in the spring for grazing in November and December.
Brassica crops require well-drained soils with a pH between 5.3 and 6.8 for good production. Seeding rates for rape and kale are 3.5 to 4.0 pounds per acre, and turnip and swede are 1.5 to 2.0 pounds per acre. In the spring, use the higher side of the suggested seeding rates. Plant seeds in 6- to 8-inch row spacings at 1/4 to 1/2 inch deep in a firm seedbed. Apply 50 to 75 pounds of nitrogen per acre at seeding to stimulate establishment and seedling growth. Weed competition should be controlled during brassica establishment, or stand establishment failures are very likely.
On conventionally prepared seedbeds, brassica seed can be broadcast and incorporated with cultipacking. No-till seeding into grain stubble or grass sod is recommended, but weeds and sod must be suppressed for two to three weeks to allow the brassicas to establish. Apply either paraquat or glyphosate for sod suppression. Another alternative is to apply a manure slurry or liquid nitrogen solution to burn the sod back and then no-till plant the brassica seeds. Brassicas can also be seeded with rye to provide forage growth and protect the soil after brassicas are consumed.
Fertilization
Determine lime and fertilizer needs by a soil test. Adequate phosphorus and potassium are important for optimum growth. In addition to the nitrogen applied at planting (50 to 75 pounds per acre), another 70 pounds per acre should be applied when multiple grazings are planned with rape and turnips. This second application should be made from 60 to 80 days after seeding. Nitrogen application in a chemically-suppressed grass sward tends to increase the efficacy of the suppressing herbicide. This reduces the proportion of grass in the brassica-grass sward, which is not always advantageous. Avoid excessive nitrogen and potassium fertilization to prevent animal health problems (see Animal Health Concerns with Brassicas).
Harvesting
Although brassicas can be harvested for greenchop, they are most often grazed. Rotational grazing or strip grazing helps reduce trampling and waste by livestock. Graze small areas of brassicas at a time to obtain efficient utilization. Rape is most easily managed for multiple grazings. Leave 6 to 10 inches of stubble to promote rapid regrowth of rape. When turnips are to be grazed twice, allow only the tops to be grazed during the first grazing. Turnip regrowth is initiated at the top of the root. Both rape and turnips should have sufficient regrowth for grazing within four weeks of the first grazing.
Stockpiling these crops for grazing after maturity should only be attempted when plants are healthy and free of foliar diseases. Some varieties are more suited for stockpiling because they possess better disease resistance. Do not grow brassica crops on the same site for more than two consecutive years to prevent the buildup of pathogens that limit stand productivity. Insect problems are not a consistent problem in Ohio.
Animal Health Concerns with Brassicas
Brassica crops are high in crude protein and energy, but low in fiber. The low fiber content results in rumen action similar to when concentrates are fed. Sufficient roughage must be supplemented when feeding brassicas to livestock. If grazing animals are not managed properly, such health disorders―such as bloat, atypical pneumonia, nitrate poisoning, hemolytic anemia (mainly with kale), hypothyroidism, and polioencephalomalacia may occur.
These disorders can be avoided by following two guidelines:
• Introduce animals to brassica pastures slowly, and avoid abrupt changes from dry summer pasture to lush brassica pasture. Do not turn hungry animals into brassica pasture, especially if they are not adapted to brassicas.
• Only two-thirds of the animal’s diet should comprise brassica forage. Supplement with dry hay or allow grazing animals access to grass pastures while grazing brassicas. No-tilling brassicas into existing grass pastures helps reduce the risk of these disorders, if sufficient grass growth is available for grazing.