IV. RESULTADOS Y DISCUSIÓN
4.3. Actitudes de ambivalencia moral, respecto del aborto provocado
By understanding your grass species’ regrowth require- ments (reserve energy versus live leaf area) and know- ing whether they are short- or long-shooted, you can assess their appropriateness for rotational versus con- tinuous stocking and for various types of defoliation management (more frequent and severe versus less frequent and lax). This information can help you make better decisions about the following:
• The type of grazing system appropriate for your pas- tures
• The timing of grazing
• Targets for residual stubble after grazing • Canopy height based on species of the pasture • Management of mixed pastures
• Predictions of forage regrowth Start with the following information:
• Are your species short-shooted or long-shooted? (table 5.1)
• Do they have a vernalization requirement? (table 5.1) • What is their seasonal pattern of shoot development?
(table 5.1)
• Do they depend more on energy reserves or live leaf area for regrowth? (table 5.2)
• Is the apical meristem elevated at the time of graz- ing? (your observation)
In this section, we’ll see how you can use this informa- tion in grazing management decisions.
TIMING OF GRAZING
For well-adapted pasture grasses, such as perennial ryegrass, orchardgrass, and tall fescue, the three- to four-leaf stage is an excellent time for grazing. At this time, the pasture growth rate has been high, nutritional value is starting to decline but is still excellent, and the plant is in positive energy balance for rapid regrowth. Research in the Pacific Northwest is limited for many taller-statured grasses such as smooth brome, timothy, and reed canarygrass. Consider grazing these species after at least the five-leaf stage.
Species Energy reserves Sunlight capture
Bluebunch wheatgrass x Crested wheatgrass x Kentucky bluegrass x Meadow brome x Meadow fescue x Orchardgrass x x Perennial r yegrass x x
Reed canar ygrass x
Smooth brome x
Tall fescue x x
Timothy x
Table 5.2. Comparison of the relative dependence on different sources of energy for regrowth in common pasture grasses.
TARGETS FOR RESIDUAL STUBBLE
Rotating grazing animals before they remove excessive leaf tissue allows for a mixture of younger and older leaves to fix carbon dioxide (CO2), promoting re- growth. For most pasture grasses, overgrazing into the stubble, i.e., the “bank account,” greatly reduces stand life and pasture productivity and lengthens the time needed for regrowth before regrazing. More often than not, pasture production and quality are reduced long before they should be because the stubble height rule was violated. The result is increasing weed competi- tion and bare soil areas.
The required stubble height for a particular species is related to the species’ relative reliance on energy re- serves or sunlight capture by residual leaf area for re- growth (table 5.2). Grasses that depend on stored reserves need relatively higher stubble for regrowth. Grasses that depend on sunlight capture by residual leaves may be a little more forgiving of shorter stubble heights, but they still require adequate leaf tissue to capture sunlight and form plant sugars quickly for re- growth and production. A good rule of thumb is to maintain a 3- to 4-inch stubble height for cool-season grass in irrigated pastures. Warm-season grasses re- quire twice the stubble height (6 to 8 inches) of cool- season grasses. Stubble is also important to protect the plant crown from hoof damage and from wind and water erosion.
Taprooted legumes store considerable amounts of en- ergy in their taproots. When grown in monocultures and allowed to store sufficient reserves between defo- liations, as is typical under hay management, they can be grazed to within 1 inch of the soil surface (although a number of problems could arise from consistently managing legume pastures in this manner). When growing these species in mixtures with grasses for grazing, you must take into account the impact of defo- liation on both the grasses and the legumes (see “Con- siderations for mixed pastures,” below).
Leaving a higher stubble or residual leaf area gener- ally allows for more frequent defoliation. By provid- ing for higher stubble heights in your pasture management plan, you will ensure stronger stands with fewer weeds, more rapid regrowth after grazing, more consistent forage quality, and more sustainable production.
A practical grazing example
Let’s look at an example of grazing management that takes into account plant regrowth patterns. We’ll assume we have a sprinkler-irrigated pasture in the Pacific Northwest (Lost Buck Ranch) and want to raise beef steers. We’ll purchase steers in January and background them until the established cool-season perennial pastures are ready to graze (mid-March to mid-April). On Lost Buck’s sandy soils, we will plant endophyte-free tall fescue and a grazing-type alfalfa. On the silty soils, we’ll plant or- chardgrass, perennial ryegrass, and annual rye- grass, plus red and white clovers. On the clayey soils, we’ll consider tall fescue again, or reed ca- narygrass or smooth brome mixed with Kentucky bluegrass and alsike, white, and strawberry clovers. Our pasture consists of several paddocks used in rotation. We use fencing to create the paddocks and control the steers.
The steers go from winter hay backgrounding to early-spring pasture on the sandy soils that dry out first. They will then move to the silty-soil orchard- grass-ryegrass pastures during spring flush, then onto the clayey-soil pastures during spring flush and into early summer. Then they will be rotated back to the tall fescue-alfalfa during July, as these species will have regrown well after resting. During the fall flush regrowth, the steers will return to the orchardgrass-ryegrass mix, where there should be plenty of high-quality, high-protein, digestible-fiber forage. Depending on the year, we may want to graze into early winter on the regrowth of the clayey pasture mixture, but we will make sure never to graze too low.
This is just one example of how cool-season grass and legume mixtures and rotations could be part of the Lost Buck Ranch management plan.
As we learn more about perennial warm-season grasses, these species could become an important part of the summer growth cycle. They could fill a niche when cool-season grasses become summer- dormant or have slower growth and reduced qual- ity.
CONSIDERATIONS FOR MIXED PASTURES To maintain productivity and desirable species propor- tions in a mixed pasture, grazing management—stub- ble height, residual leaf area, and frequency of defoliation—usually is determined by the species that is most sensitive to defoliation. For example, depend- ing on the time of the growing season, you could graze a mixed pasture of white clover and Kentucky blue- grass or perennial ryegrass every 2 to 3 weeks to a stubble height of 1 to 2 inches or even less. (Note that such close grazing is possible only with high fertility management to keep these short-shooted grasses ac- tively growing. Otherwise, the clover will quickly out- compete the grass in the summer, and the grass may not recover adequately in the fall.) A mixture of alfalfa and smooth brome, on the other hand, can be grazed only every 3 to 4 weeks to a stubble height of 3 to 4 inches. Frequency and severity of grazing for mix- tures of red clover with orchardgrass or tall fescue would fall between these extremes.
In overgrazed pastures, desirable grasses typically are severely grazed, allowing legumes or weeds to take over the stand. Because grass leaves are more vertical than legume leaves, they are easier to graze. Addition- ally, livestock are attracted by the higher sugar content in grasses. Thus, when grazing a grass-legume mixture, it is important to maintain the balance of forage species. PREDICTING REGROWTH
Your knowledge of grass growth patterns can help you predict how your pasture will respond to defoliation. First, you need to know whether your grass species are short-shooted or long-shooted and whether they have a vernalization requirement (table 5.1). Then, monitor meristem positions and activities. Is the meristem ele- vated? Is it vegetative or reproductive? As discussed above, when apical meristems are elevated, they may be removed during grazing, thus reducing the rate of regrowth. Once you know how each species responds to defoliation, and consider the time during the grow- ing season, you can better forecast and budget forage growth.
For example, smooth brome is a long-shooted species with a vernalization requirement. Smooth brome shoots emerge later in spring than those of other species such as Kentucky bluegrass, requiring live- stock to remain longer on winter hay rations or other feeds until the grass has adequate height and yield for grazing. Growth during the spring flush will be rapid and high-quality. Regrowth after grazing will be very
slow if apical meristems were removed, because re- growth from axillary buds is relatively slow. Thus, un- derstanding the limited regrowth potential of this species allows managers to avoid overgrazing. In contrast, Kentucky bluegrass is a short-shooted species with a vernalization requirement. Shoots emerge relatively early in spring, and pastures are quickly carpeted in green leaves. The growth rate in- creases with warmer temperatures, with the highest yields occurring during the spring flush. As summer temperatures increase and irrigation continues, re- growth may slow due to temperature stress. Nonethe- less, regrowth following defoliation may be faster than that of long-shooted species due to regrowth from api- cal and intercalary meristems.