5 January 2009

7 minute read

Introduction

Tame pastures are cultivated fields planted with introduced (non-native) grass and legume species or cultivars with the multiple purposes of providing livestock grazing forage to improve animal nutrition and health, balance forage supply and demand during low forage production, reduce soil erosion, improve water quality, improve soil quality and health, and provide food and cover for wildlife. Any one or all of these purposes are optimally achieved when the established pasture plant community is healthy and functioning to capture energy from the sun, and facilitate water and nutrient cycling.

Prescribed grazing is the management practice used on tame pastures to maintain healthy pasture plant communities and thus maintain a healthy forage base, achieve acceptable livestock production, and protect natural resources. Without grazing or mechanical harvest of plant material, dead stems and leaves (plant litter) build up and shade photo-synthetically active plant material reducing the capture of energy that drives the system.

However, grazing during the vegetative period when forage plants are most nutritious and digestible reduces the leaf area for energy capture. Careful management of the timing, intensity, and frequency of grazing during the vegetative growth stage is therefore important to allow recovery from grazing and maintain stand vigor. Timing is generally referred to as spring, summer, fall, or winter utilization. The intensity is usually determined by the stubble height of the grass or legume, and frequency is the number of times the pasture is grazed during and year and is regulated by the length of rest period between grazing.

Established forage plants have two general stages of development; vegetative, and reproductive. Plants in the vegetative stage have the greatest proportion of leaf material relative to stem material, at least for grasses. Being the predominant site of photo-synthesis, leaves have more (relative to stems) nitrogen compounds (including proteins), non-structural carbohydrates, and fat compounds that are readily digested. Stems being support structures have cells with thicker, lignified cell walls that provide rigidity but are difficult for digestive systems to break down into energy. Plants in the vegetative stage are functioning at their optimum photo-synthetic capacity, producing energy for root growth, development of buds, reproduction, and storage for future growth. As forage plants mature from the vegetative stage into the reproductive stages, forage quality decreases because energy is allocated to the production and dispersal of seeds. Forage species vary in their timing during the growing season of vegetative and reproductive stages and therefore the time at which forage value is highest. Forage species also vary in the quality of forage after they have reached maturity and therefore their value as a stockpile species for use in fall and winter.

With the exception of pastures in the annual crop phase of a crop rotation cycle, forage pastures are composed of perennial plants that regenerate each year from buds produced the prior year. These buds are on the plant crown at the soil surface (or just below), on stolons at the soil surface, or on rhizomes just below the soil surface. Dormant buds are activated to grow stems in the spring. Forage plants vary in the timing of active growth in the spring.

Corresponding to the two general stages of growth, forage grasses produce two distinct forms of vertical stems, vegetative and reproductive, both with the shoot apex at the stem tip. The apex has cells capable of cell division and therefore growth. The apex of non-reproductive, vegetative stems produce leaves. These stems are very short and consist of nodes where the leaves grow and non-elongated internodes. The apexes of vegetative stems are thus close to the ground and enclosed within a whorl of leaf sheaths where they are more likely to escape removal by grazing. When leaves are grazed above the apex, re-growth of new leaves from the apex is rapid. If enough leaf area remains after grazing, energy from photo-synthesis can fuel re-growth with little or no cost to the plant from stored energy. If the vegetative stem apex is removed by grazing, the plant can only resume vegetative growth by breaking dormancy of buds on the plant crown, stolons, or rhizomes. It may take a week or more to activate dormant buds and usually at the cost of stored reserves. When the plant allocates stored energy to recover from close grazing, less energy is available for root growth, reproduction, and the ability to recover from natural and competitive stresses. Repeated close grazing that removes the growing apex increases the time it takes the plant to activate buds with each removal. If continued, the plant eventually loses its ability to re-grow and dies. Grass species differ in the length of vegetative stems, the height above the ground of the growing apex, and the ability to recover if grazed too closely.

The other form of vertical stem forage grasses produce is the reproductive stem. When temperature or day length stimulates the plant to flower, the dormant internodes of the vegetative stems begin to elongate and elevate the apex above the leaves. The apex also differentiates into the reproductive inflorescence. Grazing after the transition to reproductive stages is likely to remove the growing apex because it is higher in the canopy causing the stem to die and requiring regeneration of new tillers. The timing of initiation for reproduction and the time it takes to complete this stage of growth varies among forage species.

After reproduction, forage plants generally go into a period of dormancy that enables them to endure periods of low soil moisture or freezing temperatures. Forage plants are capable of breaking summer dormancy to initiate vegetative growth in the fall if moisture and temperature conditions are suitable. Forage species vary in the timing and ability to break dormancy in the fall and spring.

There are a number of differences between forage grass and legume species. Grasses have fibrous root systems where as legumes are, for the most part, tap-rooted. The tap root of legumes not only absorbs water and nutrients from the soil, but also stores carbohydrates and proteins needed during re-growth after defoliation and for winter survival. Symbiotic rhizobia bacteria infect root hairs of legumes and make nitrogen available to the plants through nitrogen fixation. Nitrogen fixation is a high-energy demand process and the rhizobia get the energy from carbohydrates produced by the plants. Nitrogen fixation is higher for vigorous plants than plants under stress. Defoliation causes nitrogen fixation to stop temporarily, and frequent defoliation can result in sloughing of root nodules that contain the rhizobium.

Carbohydrate storage in legumes is greatest at or shortly after bloom. Frequent grazing reduces the ability of legumes to re-grow and survive winter. Defoliation during mid-October increases the chance of winter injury compared to mid-September defoliation. Similar to grasses, perennial legumes have root crowns that contain buds for new growth. Grazing tolerant legumes have many crown buds on low-set, broad crowns. Also similar to grasses, the apexes of the vegetative shoots are at the tip of the stem and the ends of stem branches. However, the terminal bud is at or near the top of the plant and nearly always removed by grazing. When the terminal bud is removed, re-growth comes from buds at the leaf axils, the nodes of grazed stems, or the crown buds, sometimes resulting is prostrate growth.

• Wheatgrasses, Wildryes and Brome Varieties
• All Other Grass Varieties
• Legume Varieties

December 2008