Brassicas offer high energy and high digestibility for both summer and late fall grazing

by Allan Nation

KANSAS CITY, Kansas: Nebraska grazier, Terry Gompert, said that ruminant animal performance is a function of two factors. Forage quality and quantity. Both of these factors are negatively affected by hot weather.

As temperatures rise, the lignin content of perennial grasses and most legumes increases and digestibility falls. Lignification is why spring cut alfalfa hay brings such a premium price and is also why animal performance on summer grazed alfalfa is low in hot weather.

At 86 F (30 C) cool-season grass growth virtually ceases and severe forage quantity shortfalls result. It is this hot weather near-dormancy that produces the big dip in production between spring and fall peaks. A great deal of the United States is subject to extended periods of temperatures in excess of 86 F. Night-time temperatures in excess of 50 F, allow the grass to grow at night and consume sugars built during the previous day. This results in a low energy grass for the main morning graze. (This is why grass in warm weather should not be cut for silage until after noon.) This is also why animal performance tends to be better in arid climates where cool nights are common. The combination of all three of these factors result in what we call the "summer slump" in ruminant animal production. This is a major problem for dairy graziers and for beef and lamb graziers trying to "finish" animals in August and September.

While warm-season perennial grasses grow well in hot weather and suffer no shortfall in quantity, they too suffer from lignification, and animal performance drops like a rock in hot weather. Warm-season annuals such as crabgrass are better but still not equal to cool-season perennials. However, there are forages that maintain their digestibility through the summer heat and can help overcome this drop in production. One is white clover.

Unlike red clover and alfalfa, white clover does not lignify and stays highly digestible right through the heat of summer. While the benefits of white clover are normally discussed in the context of its high nitrogen-fixing, I believe it is far more valuable for its ability to maintain pasture quality and energy during the hot summer months.

While white clover alone is probably adequate in areas of the country with cool summers, it is unlikely to be adequate in regions with extended periods of hot weather. Highly digestible cool-season grasses such as perennial ryegrass and endophyte-free fescue can easily be killed with continuous grazing when daytime temperatures are above 86F and soil moisture is low.

Maintaining stands of these two forages necessitates long rotations (35 - 45 days) during wet periods to complete removal during extended periods of hot, dry weather. Therefore, highly digestible supplemental forage crops can be very valuable in not only keeping animal production levels high but in maintaining preferred sward stands.

Three forages that fill the bill for both of these needs are rape, turnips and narrow stem kale. These brassicas can grow a lot of grazable tonnage in 80 to 90 days and so are suited for late summer grazing. A May or early June planting assures a grazable stand by August. Only one variety of kale (Premier) will also fit this time window. All three of these have the potential to regrow for later fall season use.

Swedes produce a large edible root like a turnip, but require 150 to 180 days to grow. Consequently, they are better used for fall and early winter grazing. Swedes have a higher yield than turnips and some varieties such as "Calder" are extremely cold hardy.

Of course, yield is dependent upon weather conditions and the amount of nitrogen used, but a well grown acre of these plants should support 160 cows for a day or 1550 ewes. With adequate fertility, brassicas can produce amounts of digestible energy equal to a corn yield of 115 bushels per acre.

Dry matter digestibility generally exceeds 90 percent for all plant parts except kale stems at maturity. This compares with 70 percent digestibility for dairy quality alfalfa hay. Crude protein can range from 30 percent in rape leaves to 8 to 10 percent in turnip roots.

Since turnip tops are twice as high in crude protein compared to roots, a high top/root ratio is valuable for high performance classes of animals such as dairy cows. Research at the USDA Pasture Laboratory at Penn State showed that turnip crops can vary from 90 percent tops/ten percent roots to 15 percent tops/85 percent roots depending upon turnip variety. "Savannah" and "All Top" were far superior to "Purple Top" in their proportion of tops to roots.

Unlike perennial forage crops, the dry matter digestibility of brassicas does not decrease dramatically with increasing plant maturity. This makes them extremely valuable as a stockpiled summer forage for drought insurance.

However because the fiber content of brassica crops are extremely low, they should not make up over 75 percent of the animal's diet. They are truly "supplemental" crops and should be thought of, and used, similarly to a concentrate.

Terry Gompert, a long-time turnip grazier in Nebraska, likes to plant turnips behind oats and grazed green corn for finishing out heavy stocker cattle. He said the residual stubble of the previous crop has always provided enough supplemental dry matter for his stocker cattle to prevent problems. Other graziers will use an on-off method with pasture or corn stalks.

SIX CENTS PER POUND OF GAIN

Gompert figures that a pound of gain on stocker cattle with turnips costs only six cents based upon out-of-pocket expenses of $17 and a yield of 4.5 tons. This compares with 37 cents for grazed 70 bushel corn valued at $2.00 bushel and 23 cents for grazed alfalfa valued at $40 a ton.

He said larger stocker cattle (500 to 600 lbs) perform much better than small calves (350 - 400 lbs) on brassicas. This is because the high water content of brassicas requires a larger rumen to get enough dry matter intake. The larger steers will gain in excess of two pounds per day but the smaller calves will gain between nothing and one pound per day. He said thanks to this high moisture content, dry ewes and lambs do not need supplemental water sources when grazing brassicas.

Spring planted brassicas require a higher seeding rate than mid-summer seeded brassicas. Seeding rate for kale or rape is four pounds to the acre and two pounds per acre for turnips. All brassica crops require good soil drainage and soil pH between 5.3 and 5.8 for optimum growth. The seeds should be planted in rows six to eight inches apart and not more than one-half inch deep. Brassica seed can also be broadcast and incorporated into tilled seedbeds by cultipacking. Treflan is labeled for annual grass and weed control at 0.5 to 1.0 pint per acre. Once established brassicas are very competitive with weeds.

No-til plantings into sods are more successful for fall crops when planting can be delayed until after the spring flush of growth. Pastures should be grazed short prior to planting and a herbicide applied two weeks prior to planting. Some graziers have had luck with using a manure slurry to burn back the sod and then no-till the seeds through the slurry.

75 pounds of nitrogen should be applied at seeding and an additional 70 pounds between 60 to 80 days after seeding to increase yield and crude protein level of the brassica tops.

To reduce disease pressure, brassicas should not be grown on the same land for more than two consecutive years. "Forage Star" turnip and "Rangi" rape are better suited for summer stockpiling because of superior disease resistance.

GRAZING MANAGEMENT

Strip-grazing, whereby, the animals are given access to only a small portion of the crop at a time, is the preferred grazing method for brassicas. This greatly reduces losses due to trampling.

Rape is the brassica most easily managed for regrowth. Approximately six to ten inches of stubble should remain after grazing rape to promote rapid regrowth. The regrowth can be grazed in as few as four weeks after the first grazing.

When turnips are to be grazed twice, only the tops should be grazed during the first grazing. Turnip regrowth is initiated at the top of the plant, so this part of the plant should not be removed until the final grazing when the whole plant is consumed. Like rape, turnip regrowth can be grazed four weeks following the first grazing. In conclusion, brassica crops can solve a lot of problems for graziers in areas with hot summers and can also be used to extend grazing into the winter. Few other crops offer as much flexibility and value to a grazier seeking high animal performance.

FOR SUCCESS IN GROWING BRASSICAS

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.

(Thanks to Marvin Hall and Jerry Jung of Penn State University for help with this article.)

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