Grazing Management: Effects of Grazing on Plants

Table of Contents

1. Effects of Grazing on Plants
2. Other Grazing Management Recommendations
   • Animals as Grazers
   • Grazing Management Systems
   • Designing a Rotational Grazing System
   • Stocking Rate
   • Introducing Livestock to Pasture
   • Clipping Paddocks
   • Nutritional Supplement
3. Related Links

Effects of Grazing on Plants

Plants use water and carbon dioxide in the presence of sunlight to create carbohydrates and oxygen (a process called photosynthesis). Plants use carbohydrates as an energy source for growth. When plant growth slows and more carbohydrates are produced than needed for growth, the surplus carbohydrates are stored. These reserves are important for the plant's survival over winter and for initiation of plant growth in spring. Regrowth after grazing depends on energy being produced either by the remaining leaf area, or from the carbohydrate reserves.

The aim of grazing management is to:

• harvest the plants while leaving enough top growth and root reserves to support fast regrowth and
• allow the plants adequate time to recover from being grazed

Plant growth starts slowly in the spring or after close grazing. Carbohydrate reserves stored in stem bases, roots, rhizomes and stolons have to be mobilized before they can be used to fuel growth. In grasses, the dependence on reserves for regrowth usually lasts 2-7 days, while in legumes it can be 2-3 weeks. Once leaf area develops, growth quickens as the plant has an immediate source of energy from photosynthesis. After this vegetative period of fast growth is over, the plant becomes reproductive, growth slows and carbohydrate reserves are replenished. Grazed plants left with enough leaf area to continue photosynthesizing regrow at a quicker rate, as they are not dependent on carbohydrate reserves.

Plants frequently closely grazed can be damaged because they are unable to restore their carbohydrate reserves. With each defoliation, the plant's reserves are reduced and with time, depleted. In addition, any intensity of grazing causes plant root damage; root weight, length and vigour are reduced. The extent of the damage increases with the severity of the defoliation. Loss of roots slows plant growth as the plant cannot obtain sufficient water or minerals to support rapid growth. Severe overgrazing removes so much of the leaves that there is insufficient energy produced to keep the roots alive and the plant is unable to survive.

An important part of grazing management is to allow plants adequate time to recover from being grazed.

The amount of time needed for this changes during the pasture season. Plants grow at different rates during the grazing season due to changes in growing conditions.

The stage of growth grasses are grazed at influences how well they survive and regrow. Grasses are made up of individual tillers, each having a growing point (which controls the growth of the tiller and which may form a seed head), a stem, leaves, roots and dormant buds. In the vegetative growth stage the growing points are close to the ground - safe from being eaten - and produce new leaves after being grazed. Growing points continue to produce new leaves until the plant goes into the reproductive stage, or until the growing point is elevated by stem elongation.

With grasses such as Kentucky bluegrass, tall fescue and orchardgrass, stem elongation occurs as the plant goes into the reproductive phase. Forage production from these grasses is encouraged by frequent grazing. Grasses such as timothy, bromegrass and reed canarygrass elevate their growing point early in their growth and regrow slowly if grazed too early (before the boot stage). New growth must come from the dormant buds and the energy for that growth must come from the remaining leaf area or from reserve carbohydrates. These grasses are suited for controlled grazing, where they can be harvested quickly and given a rest period in which to recover and regrow.

Related Links

... on forages and pastures, visit Forages and Pastures (OMAFRA)
... on weed control, order Publication 75 Guide to Weed Control: Forage Crops
... on agronomy for field crops, order Pub. 811 Agronomy Guide for Field Crops: Chapter 3 Forages
... on field crop protection, order Publication 812, Field Crop Protection Guide
... on livestock, visit Livestock (OMAFRA)