Pasture & Range: June 2008 Other Pasture & Range Articles

An increasingly common question asked by cooperators is "How many cows can I run if I don't fertilize anything"? Back in November of 2000, Jeff Ball and Dan Childs wrote a Ag News and Views article titled "Risky Business: Bermudagrass Fertilization". This article presented data which was collected over a 10-year time period at the Noble Foundation and showed that at 0 lbs/ac nitrogen (N) average dry matter (DM) yield was 2,168 lbs/ac and at a rate of 100 lbs/ac N yield was 4,756 lbs/ac DM or an average response rate of 26 lbs DM per pound of actual N applied.

Range in production for the 0 lbs/ac N treatment was 641 lbs/ac in 1969 with 23.1 inches of growing season rainfall (March-September) to 3,983 lbs/ac in 1994 with 33.83 inches of growing season rainfall. If we take this data and use it to answer the question of carrying capacity at 100 lbs/ac N and an average yield of 4,756 lbs/ac DM with 70 percent utilization, it would take approximately 3.3 acres per cow. At 0 lbs/ac N, an average yield of 2,168 lbs/ac DM and 70 percent utilization, the acres per cow go up to 7.2 acres per cow. Again, based upon the data, actual response is very moisture-dependent.

The numbers in the example above demonstrate that if you can't afford nitrogen fertilization, cow herd numbers will be forced to go down unless the difference is made up with hay or feed purchases from off the farm or ranch.

Legumes can provide an alternative to N fertilization, but they require a much higher degree of management to make a legume/grass system work compared to a grass/commercial N system. Depending on the species, legumes can fix anywhere from 50-200 lbs/ac N when grown in a pure stand. Grown in mixtures with grasses, nitrogen fixed will range around 20-80 lbs/ac dependent upon species and percentage of the legume making up the mixture. As a rule of thumb, target a percent legume composition in a grass of at least 30 percent in order to provide enough nitrogen to feed the grass.

To make legumes work, management must be focused on the legume and not the grass. Pastures will look different than they have in the past. Without the even distribution of fertilizer, pastures will look uneven and you may have more weeds. You must also understand the nitrogen transfer system in order to capture the full benefit of legumes.

Legumes do not freely give away the nitrogen that they fix. The majority of nitrogen transfer comes from decomposition of dead legume plant material. Therefore, the greatest benefit to a grass will be following a legume crop, such as growing cowpeas during summer, which are turned under and followed by winter pasture in the fall. Next would be grazing a legume/grass mixture and the decomposition of legume plant material through feces and urine. This increases the importance of rotational grazing, which creates a more even distribution of nutrients across pastures than you will have with continuous grazing. In bermudagrass hay systems, the challenge will be even harder since removing legumes with the hay crop will greatly enhance hay quality, but you will greatly diminish the benefits of nitrogen fixation.

For more information on the agronomics of legumes and their benefits, you are invited to attend a full day conference entitled Pros, Cons and Economics of Clovers and Legumes. This seminar is the first ever Texoma Pasture Conference and will take place at Heritage Hall in Ardmore on July 19, 2008.

Texoma Pasture Conference
Pros, Cons and Economics of Clovers and Legumes

Heritage Hall, Ardmore, Okla.
July 19, 2008
8:30 a.m.-5:00 p.m.

Speakers will include:
Joe Bouton, Ph.D. - Noble Foundation
Don Ball, Ph.D. - Auburn University
Garry Lacefield, Ph.D. - University of Kentucky Extension
Gerald Evers, Ph.D. - Texas A&M University
Dan Childs - Noble Foundation

Topics will include:

• how much can you afford to pay for fertilizer?;
• legume quality and animal performance;
• the nitrogen cycle and nitrogen movement to plants;
• forage legume yield and yield distribution;
• legume options for Texas and Oklahoma; and
• forage legume use.

This program features some of the nation's top specialists in forage legumes. Timely, practical information will be offered to stimulate your thinking about your forage production practices.

Preregister online at www.noble.org/agevents or call 580.224.6411 for additional information.

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