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Plant production can be determined using a variety of methods or combination of methods including estimating, harvesting, or by estimating and harvesting (double-sampling). When using the Estimating (by weight units) Method the relationship of weight to volume is not constant; therefore, production and composition determinations are based on weight estimates, not on comparison of relative volumes and does not require clipping of vegetation.
This method is based on a several factors: 1. A weight unit is established for each plant species occurring at the site, 2. A weight unit can consist of part of a plant, an entire plant, or a group of plants, and 3. The size and weight of a unit will vary according to the type of plant.
The weight unit method is the most rapid of the three production methods listed above. However, it is not as accurate as the harvesting method or the double sampling method due to the personal error caused by estimation of utilization levels. In addition, plants grow at different times of the year so when the sampling is conducted is important since sampling at different times of the year can change the results. It also requires adjusting for material that has not been produced yet or has been removed which is difficult. An advantage to using this method however, is that it does not require dstroying vegetion through harvesting.