They include the type of nutrients applied, irrigation timing and application, the plant species composition of the meadow or pasture and grazing management. If appropriate irrigation management, species composition, or grazing management are lacking, additional fertility will not result in higher yields. This chapter describes situations in which economic response is attained through fertilization and when other factors may limit the response to fertilizer.
What Nutrients Do Irrigated Pastures Need?
The nutrients needed most for optimal plant production are nitrogen (N), phosphorus (P), and potassium (K), often called macronutrients. However, plants also need micronutrients such as sulfur, iron and zinc. On irrigated pasture in Nevada, the nutrients most likely to be limiting are N and P. Potassium is seldom limiting. A positive economic response is rarely seen from applying K or micronutrients in irrigated pasture.
Response of Irrigated Pasture and Meadows to Nitrogen
If meadows are properly irrigated and have the right species composition, a positive economic response is normally attained from the application of nitrogen. Plants species found in irrigated meadows and pastures do not respond the same to fertilization. The response can be minimal to dramatic. The following table shows the average yield response to nitrogen fertilizer of common meadow plant communities found in Nevada. The data is based on the result of over 2,500 plots established over a 15-year period in Nevada (Hackett 1973-1987).
Generally, a positive economic response to nitrogen fertilizer is obtained from nitrogen application of 50-100 lbs (actual N) on improved mixed grasses, unimproved timothy/clover, unimproved mixed grasses, and from lower applications (50 lbs N) on unimproved sedge and bluegrass. A soil sample should be taken before applying any fertilizer to determine the nutrient level already in the soil.
The economic response of N application depends upon the price of nitrogen and the value of the forage (for grazing or hay). The additional forage obtained must have a greater value than the cost of the fertilizer to have a positive economic response.
- Cost of Additional Fertilizer < Value of Additional Forage (Price x Quantity)
How Species Composition Change in Response to Nitrogen Fertilizer
The plant community found in irrigated pastures is a function of irrigation, fertility and grazing/harvesting. With the application of N fertilizer and proper irrigation, the plant community tends to change from sedge/rush components to mixed grasses. The following table shows the typical response of plant species to nitrogen fertilizer over time and a change from continuous to intermittent irrigation.
When applying nitrogen fertilizer to irrigated pastures and meadows that are composed primarily of rush, bluegrass, or sedge, the response may not be immediate. However, over time the plant community will change and become more responsive to fertilizer if some remnant desirable grasses are present.
Effects of Phosphorus on Irrigated Pasture and Hay Meadow Yields Phosphorus is often overlooked as a critical nutrient in fertility programs. If phosphorus is limiting, then a positive economic response will not be obtained from nitrogen until the phosphorus requirement of the plant is met. It is important to meet the phosphorus requirements of the plant in order to get the full benefit of N application.
Phosphorus is not mobile in the soil. Phosphorus can be built up in the soil over time when excess amounts are applied. A soil test should be taken and phosphorus applied as needed. In most cases the phosphorus is not lost when applied, however, it is expensive to apply when it is not needed. Furthermore, when the pH of the soil is high, some of the phosphorus can become tied up in the soil and become unavailable to the plants.
If phosphorus (P) levels in the soil are less than 5 parts per million (ppm), P is deficient and plant response to nitrogen will be small until the P requirements are met. If the P is between 5-15 ppm, it is considered marginal and applications should be made with nitrogen fertilizer. If phosphorus is over 15 ppm it is adequate for production on irrigated pastures and meadows.
Nitrogen and Phosphorus Applications The best economic response to nitrogen is generally between 50-100 lbs of actual nitrogen per acre. Applications greater than 100 lbs of N yield a productive response, but often are not worth the additional cost. Phosphorus should be applied on an as needed basis. Thirty to 50 lbs of phosphorus is required each year for production. If soil levels are low (>15 PPM) additional P needs to be added for optimal production.
Response to Fertilizer Under Continuous and Intermittent Irrigation Methods Plant response to fertilizer will be limited if proper irrigation techniques are not followed. If irrigation is applied season long (continuous irrigation) and taken off only to cut hay, then the soil oxygen levels will be low. Plants that can survive in these situations, such as rush and sedge, will dominate. As shown in the previous section, sedge and rush have limited response to fertilizer. Furthermore, continuous irrigation washes nitrogen fertilizer from the field and out of the root zones of the plants, which results in an economic loss.
In order to benefit from fertilizer application, irrigation must be changed from continuous to intermittent. Intermittent irrigation is the application of water when the plant needs it and in amounts to wet the soil to the rooting depth of the plant. The water is taken off once it reaches the rooting depth of the plant and applied again as needed (see Chapter 4 Irrigation Management).
Effects of Nitrogen on Dry Matter Yields on Continuous and Intermittent Irrigation
Initially as irrigation changes, yields will drop (see year 1 vs 2). As the soil becomes too dry for sedge and rush, production improved grass increase. Over time the pasture will become more productive. When irrigation practices change from continuous to intermittent and fertilizer is applied, plant composition will change more rapidly than in irrigation only and become more responsive to fertilization faster.
Fertilizer Application Under Grazing Situations
When a pasture will be grazed in the spring, fertilizer applications should be delayed. The most rapid part of the plant growth is in the cool season. During the spring, grass growth is often more rapid than livestock can consume. This results in the grass maturing and becoming "rank", or declining in palatability, protein and energy. The animal cannot keep up with the grass and keep it in the vegetative stage (see Beef Cattle Management). Applying fertilizer in the spring causes an even greater spurt of early growth, compounding this problem.
When applying nitrogen fertilizer in a grazing situation, apply it as soon after grazing as possible. Nitrogen is stored in the leaves of the plant and used by the plant as needed. When the plant is vegetative and grazed, the animal removes the nitrogen. Therefore, it is most beneficial to apply N immediately after grazing to allow time for plant response before the next grazing.
When to Apply Fertilizer
Nitrogen is mobile and can be leached out of the soil. Therefore, care must be taken not to apply too much N in one application. Production will be increased if N can be applied in 40-50 lb/acre per application. If soils are sandy, the full year's nitrogen should not be applied in one application. The full years N applied in one application will result in a loss of N below the roots of the plant. In areas that experience excess flooding, applications should be done in the spring after flood season. In other areas fall application work well. Because N is readily dissolved in water, unless managed well, there is a risk of N loss.
Phosphorus movement in the soil is very slow, therefore it can be applied in one application anytime of the year. However, applying phosphorus in increments with nitrogen has resulted in increased production. Phosphorus moves slowly in the soil, with most research showing that it moves one inch a year in the soil. Therefore, phosphorus applied this year may not be fully available for several years. Phosphorus can be applied in the fall or spring but fall is preferred in most cases.
Nutrient Requirements of Pasture Used for Hay vs. Grazing
Over the long run, fields that are grazed instead of used for hay require less fertilizer. Most nutrients (minerals) pass through the cow and are returned to the soil in manure and urine. Livestock return nutrients if they are on the field during excretion. If livestock lounge under a tree or around a water tank, up to 80% of the nutrients may be deposited in these areas, instead of on the field. The only way to increase efficiency of where the manure and urine is deposited is through more controlled grazing. Animals spend a greater amount of their time on the fields and deposit urine and manure more evenly when they are rotated around the pasture (rotationally grazed). Haying, on the other hand, removes the nutrients permanently. Over time, the fertility requirements of a properly grazed pasture can be kept in balance and the need for commercial fertilizer is reduced.
Fertilization as a Management Tool
Fertilization can be used as a management tool. For example, if you want to encourage legumes in your pasture, applying phosphorus without nitrogen will help stimulate the growth of legumes. Legumes such as alfalfa and clover require high phosphorus levels, yet can fix their own atmospheric nitrogen if they are inoculated with Rhizobium melioti (inoculated seed) when planted.
Nitrogen can also be used to manipulate production. The most rapid part of the plant growth cycle for most forages in Nevada (cool season forages) is April, May and June. Livestock grazing during these months often cannot keep up with the growth of the forage; hence the forage supply exceeds the rate of consumption and the forage declines in value as it matures. One way to help increase the production of forages season long is to delay the application of nitrogen until late June or early July. This will stimulate production during the months when it is usually at its lowest production. Phosphorus does not cause the same peak in production after application as nitrogen; thus the time of application of P is not as critical. Another example of using nitrogen to manipulate production is stockpiling forages for winter use. The most valuable forage is winter forage, because that is the most expensive time to feed livestock. Therefore, nitrogen can be applied in the late summer to increase production of forages late in the year that could be used as stockpiled forage for winter use. Enough time must be available in the growing season for plant response, and irrigation water must be available.
Economic Levels of Application
Although production can be increased with increased fertilization, that does not always equate to increased profit. Many ranchers are in equilibrium with the amount of hay they put up and the number of cows they have. Increasing fertility may increase hay production resulting in more hay than the cows can utilize. Much of the meadow hay is difficult to sell because it is low quality and put up in round bales. In this situation producers must assess their goal for increasing production. One option is to reduce the number of acres put up as hay (when the production per acre increases) and increase the number of acres grazed with livestock. Test strips of fertilizer can be valuable in determining potential yield increases and economic returns.
Conclusion
Soil Fertility is a critical part of optimal pasture production. Factors such as irrigation methods, plant species and grazing management can limit the response to fertilizer. Time should be allowed for plant growth between nitrogen application and grazing. Livestock should be managed so the maximum amount of nutrients is returned to the soil. If grazing is properly managed, the amount of commercial fertilizer applied can be reduced.
Additional Reading
- Blaylock, A.D., K. Belden and H.W. Hough. 1996. Guide to Wyoming Fertilizer Recommendations. B- 1045, University of Wyoming Cooperative Extension Service. College of Agriculture, Laramie, WY.
- Koch, D. 1995. Forage Research and Demonstrations Progress Reports 1991-1993. MP-84 University of Wyoming Cooperative Extension Service. College of Agriculture, Laramie, WY.
- Hackett, E.I., E.H. Jensen and C.D. Leedy. Planning a Fertilizer Program for Hay Meadows in Nevada. College of Agriculture, University of Nevada.
