|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|Chapter 10: Fertilizer guidelines for specific crops|
During the wet season, well-managed tropical pastures can provide enough feed for normal growth of calves and beef cattle and for production of 1-2 gallons (3.75-7.5 liters) of milk daily per cow. Supplemental feeding with high-energy sources like maize, molasses, etc. will be needed for higher milk production or more rapid fattening. Fro. 2.5-5 460 kg cattle or 3.75-7.5 275 kg stock (or about the same number of dairy cattle) can be carried per hectare. Once the dry season sets in, both the amount and feed value of the pasture seriously declines, and even well-managed pastures can usually satisfy only the maintenance requirements of cattle (no growth or milk production). Under irrigation or well-distributed rainfall, tropical pastures should be able to produce about 550-1100 kg of live-weight gain per hectare yearly without supplemental feeding.
Tropical grasses like elephant (rapier), guinea, pangola, bermuda, pare, and star give excellent responses to fertilizer, especially N. However, if overall management of the pasture and animals is low, it's questionable whether chemical fertilizer would be costeffective.
N is the most important nutrient in terms of amount, and rates up to 300 kg/ha or more yearly may be profitable under good management and well-distributed rainfall (or irrigation). Aside from increasing the pasture yield, N also increases the protein content to varying degrees, depending on the amount applied, type of grass, rainfall, and stage of maturity at which the pasture is grazed.
To reduce leaching losses, N should be applied in several applications. In humid areas without a pronounced dry season' N is usually applied 4-6 times a year. In areas with a dry season, 3-4 applications should be made, all of thee during the wet season, unless irrigation is used. Work in Puerto Rico has shown that applying 110 kg/ha of N to recentlygrazed pastures 6-8 weeks before the start of the dry season will greatly increase the amount and nutritive value of the grass carried over into the dry season. With this method, grazing should be deferred immediately following the N application until the dry season begins. Guinea grass produce. an especially good standing hay with this method.
If urea is used, up to 30-35% of its N may be lost as ammonia gaff (refer to Chapter g); this may be partly offset by urea's typically lower price compared to other N sources; if urea is applied within a few hours before rainfall or irrigation, ammonia losses will be minimized.
Phosphorus: P can be applied once a year, since it won't leach. Rates of 60-90 kg/ha of P2O5 are common.
Potassium: Up to 220 kg/ha of K2O may be needed on low-X soils under intensive management and year-round grass growth. Grasses tend to take up K in excess of their needs, so it's a good idea to split the dosage into 2 or more applications to avoid this "luxury consumption".
Sulfur: A sulfur-bearing fertilizer should be included in the fertilizer program, especially on sandy soils under high rainfall. Ammonium sulfate, single superphosphate, and ammonium phosphate sulfate (16-20-0) are good 8 sources. It's a good idea to supply about 20 kg/ha of sulfur per year (60 kg. sulfate).
Calcium and Magnesium: Remember that urea or ammonium sources of N have an acid effect on the soil. Liming may be needed after a few years of continued N applications. Soils with a low exchange capacity (negative charge) will drop more quickly in pH. Lime can be broadcast over the pasture. Use dolomitic limestone, or supply magnesium in another for. to avoid deficiencies. Cattle are very sensitive to Mg deficiencies which can be caused by applying high rates of K without supplemental Mg. In cases where both the soil and the liming material are low in Mg, it may be necessary to apply about 100 kg/ha of magnesium oxide or 400 kg/ha of magnesium sulfate ((epsom salts) yearly in 2 applications.
Micronutrients: Deficiencies aren't likely, except in very leached or sandy soils or at pH's above 7.0 (except for molybdenum).
Value of "Self-fertilization" of Pastures by Cattle
Roughly 80% of the NPK and other nutrients in the feed are returned in the manure, which would seem to make fertilizers largely unnecessary for pastures. However, animals do a poor fob at uniformly distributing the manure over the pasture; research has shown that only about 15% of the pasture is actually covered per year under typical stocking rates. A good deal of the N is lost as ammonia gas or by leaching.
Grass-Legume Pastures in the Tropics
Temperate-zone legumes like alfalfa and most clovers aren't well adapted to tropical humidity or very acid soils. Unlike temperate-zone pastures, few tropical pastures contain legumes. Legumes can significantly improve the feed value of a pasture, because they're higher in protein than grasses; they also decline more slowly in feed value as they increase in height between grazings. Legumes can also supply all their own N as well as that needed by the grasses with which they're grown in association.
Relatively little research has been done with tropical pasture legumes, but things are improving. One problem is that most tropical legumes have trouble competing with the rapid growth rate of most tropical grasses and get shaded out. Some are sensitive to overgrazing or aren't very palatable. However, tropical kudzu (Pueraria phaseoloides), Centrosema pubescens, siratro (Siratro atropuroureus), and several others have been grown successfully in combination with tropical grasses like guinea, star, and molasses grass. Townsville stylo (Stylosanthes humilis) is a self-regenerating annual (it reseeds itself) that can be easily established and maintained with a variety of grasses. Leucaena (ipil-ipil, Leaucaena leucocephala) is a perennial tree/shrub that can be grown in rows in a pasture and used for browsing. (These and other pasture legumes are described in Appendix F; leucaena is also discussed in the agro-forestry section in Chapter 8.) Consult with a pasture specialist concerning recommended grass-legume mixes for your area.
Fertilizing Grass-Legume Pastures: Since the leguee fixes enough N for itself and the grass, no N fertilizer is needed. In fact, adding N will favor grass growth and eventually shade out the legume. Adequate P and K as well as sulfur are needed to maintain a good proportion of legume to grass. Compared to grasses, legumes are weak K extractors and are also susceptible to molybdenum and boron deficiencies.
USE THE "PACKAGE" APPROACH FOR PASTURE MANAGEMENT
It takes much more than just fertilizer for successful beef and milk production. Other practices like good grazing management, good stock, disease control, weed control, supplemental feeding, and worming are just as important. Some of these are summarized below.
NOTE: The following data is not designed to make you qualified to work with cattle but, rather, to give you some initial background in this area to facilitate further investigation and discussion cattle and pasture specialists in your country.
As grasses regrow after being grazed or cut, they decline in feed value as they nature, especially in protein. Tropical conditions favor rapid grass growth and maturity, and most grazed grasses may be unable to supply enough protein after only 4-5 weeks, even when fertilizer N is used.
Under low-management conditions, cattle are usually continuously confined to one pasture at a low stocking rate. The pasture's rapid growth outstrips the cattle's ability to consume the grass before it's become overly mature and low in quality. For example, a study in Trinidad showed that the crude protein content of pangola grass dropped from 15% 10 days after grazing began down to 4.2% 42 days later (dry-weight basis).
Rotation grazing consists of dividing up the pasture into 4-6 paddocks and putting all the cattle in one paddock at a time. Each paddock should be of a adze that allows the cattle to graze down the grass in 4-7 days before they are coved to the next one. About 3 weeks rest is needed between grazings to obtain sufficient regrowth. Longer periods may be needed during cooler weather and shorter periods during more rapid growth. Guinea grass should be grazed down to about 20 cm, and pare, elephant, and pangola down to 1015 cat N fertilizer can be applied after each grazing. Over-grazing will use up stored food reserves in the roots and weaken the stand.
Dry Season Feeding: Hay and Silage
Forage quantity and quality decline disastrously during the dry season. Cattle often lose a good part of their wet season weight gains during the dry months and may take 4-6 years to reach slaughter weight (360-550 kg). It's possible to reduce this to 2-3 years, largely through the use of hay or silage for supplemental dry-season feeding. Most low management cattle raisers in the tropics have too few animals per hectare to fully utilize all the lush wet season growth, but too many in terms of the scant amount of dry season forage. Making hay or silage out of the wet season surplus growth is one answer. Silage making is usually more feasible than hay making during the wet season. (About 2000 kg of water has to be evaporated from freshly cut grass in order to make 1000 kg of hay!) Peace Corps Volunteers in El Salvador helped to establish a successful silage program with small cattle-growers, using sorghum-sudangrass. Yields averaging around 100,000 kg/ha have been obtained from taking 3 cuttings during the 6-month rainy season. They have also made good-quality pangola, stargrass, and jaragua hay at the end of the rainy season.
Provide Minerals for Cattle
Except for salt, cobalt, iodine, and copper, livestock can usually get all their essential minerals from well managed pastures. Salt licks containing trace minerals should be supplied. Young cattle need about 20 grams of salt per da' and older ones about 30 grams. kidding 10 grams copper sulfate and 10 grams cobalt sulfate per 16 kg. of iodized salt will provide a satisfactory mineral mix for cattle grazing on fertilized pastures.
Control Weeds: Weeds compete for "pace, water, light, and nutrients with pastures; some may be poisonous as well. Broadlead weeds are the most common. Herbicides nay be needed, but first se sure the particular chemical is registered for use on pastures; follow label instructions closely to avoid injury to the pasture or contamination of meat and milk.
Keep Animals Healthy: Cattle raisers should follow the recommended vaccination schedule for their area; brucellosis, anthrax, blackleg, and others may be needed. Periodic worming is also essential, as well as tick control.
FOR FURTHER INFORMATION: Several international research institutes such as Winrock and CIAT are involved in research/extension efforts with pasture and cattle management in the Third World. See Appendix a for their addresses.