Dry weight rank method

7 L. ‘t Mannetje Measuring Biomass of Grassland Vegetation

Measuring Biomass of Grassland Vegetation
L. ‘t Mannetje
Department of Plant Sciences, Wageningen University, Wageningen, The Netherlands

7

Introduction
Biomass of grassland vegetation refers to above-ground herbaceous material, commonly referred to as ‘dry matter (DM) yield’. Research workers and managers of grasslandvegetation are interested in this to determine the amount of available forage for animals or to measure the effects of management (e.g. fertilization, grazing, cutting) on the vegetation, whether the vegetation is for agricultural or amenity purposes. Vegetation biomass is important also for assessment of grassland or rangeland condition and for evaluation of new germplasm and cultivars. The economicvalue of grassland vegetation is for animal production, although grassy sports fields also have commercial value. Grassland vegetation for the feeding of animals is commonly referred to as ‘forage’ or ‘herbage’ and a distinction can be made between these two terms. ‘Herbage’ can be regarded as herbaceous biomass in general, e.g. as used in ecological studies of grassland vegetation, whereas ‘forage’refers to animal feed in keeping with the term ‘foraging’: the animal’s actions of selecting and ingesting herbaceous feed. The term ‘pasture’ is sometimes used for feed, but this is confusing, as ‘pasture’ also stands for a grazed grassland field. The agronomic value of standing forage is most directly related to its DM yield in (near) monospecific swards. In mixed swards the value of standingforage depends also on the botanical composition. In both monospecific and heterogeneous swards the value of measuring standing forage may be enhanced by measurements of nutritive value. Herbage or
CAB International 2000. Field and Laboratory Methods for Grassland and Animal Production Research (eds L. ’t Mannetje and R.M. Jones)

151

152

L. ‘t Mannetje

forage can be divided intobotanical species (Chapter 4), into groups of species (grasses, legumes, weeds or other species), or into standing green and dead material and litter. The quantity of grassland vegetation present at any one time can be used to calculate changes, such as herbage growth, utilization by grazing animals, or deterioration. Although the basic techniques of measuring the amount of vegetation present can beused for each of these purposes, the procedures and intensity of sampling will differ, depending on the objectives of the measurements. Most grassland used for agricultural purposes is stocked by animals for at least part of the time and in many cases year-long. Grazing affects grassland production because of defoliation, treading and fouling. Much of the grazing takes place in the growing seasonand that makes forage exceptional as an agricultural commodity: it is harvested whilst it grows and the harvestable product is at the same time the photosynthetic material that produces it. Harvesting takes place frequently or at least several times in a year. For these reasons measuring yield of forage is more difficult than that of other agricultural commodities. Many grassland areas, particularlyin the tropics, contain shrubs and trees. Some of these are edible, but all of them interact with the ground layer of the vegetation. They create shade, affect pasture DM yields, or, in some cases, alter the nitrogen economy of the habitat (Wilson et al., 1986) and the feeding value of the understorey herbage (Samarakoon et al., 1990; Belsky, 1994). Instead of DM yield, vegetation mass can alsobe expressed as organic matter (OM). OM yield has the advantage that it is true herbage yield without soil contamination, which often occurs in herbage samples as a result of mechanical cutting, raking up of grass or rain splash. Contamination increases weight and distorts chemical analyses. Where samples have to be used for mineral analysis, care needs to be taken to avoid contamination of the...