Validation of the species composition index

• Published in: Agricultural systems Vol. 20; no. 4; pp. 269 - 279
• Main Authors: Mitchell, Ronald L., Fribourg, Henry A., McLaren, J.B.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1986; Elsevier
• Series: Agricultural Systems
• Subjects: botanical composition; forage crops; grazing experiments; indexes; pastures; plant community analysis; species; swards
• ISSN: 0308-521X; 1873-2267
• DOI: 10.1016/0308-521X(86)90117-4

This paper is a validation of the Species Composition Index (SCI). The SCI is a data-handling technique which expresses the botanical composition of a sward as a vector. Inclusion of these vectors allows the incorporation, in response models, of the dynamic changes in species composition of sods which occur over time. The SCI was developed originally with a data set which was obtained in a beef ( Bos taurus L.) steer 3-year experiment where several dependent variables (forage growth, consumption and animal performance) were measured concurrently with several environmental, plant and animal characteristics. The SCI was then validated with a different data set, obtained over another 3-year period, with the same species combined in different ways and more complex treatments. The SCI was related to the effects, if significant, of year, season, grazing pressure, forage quality, precipitation and air temperature on the dependent variables in the two data sets independently. The SCI was superior to the traditional classification variable ‘treatments’ in accounting for variation in the dependent variables in both data sets. When ‘treatments’ was entered into the model, coefficients of determination of 0·26, 0·43, 0·58 and 0·44 were obtained for forage growth and consumption, average daily gain and beef production with the first data set, and of 0·13, 0·40, 0·36 and 0·37 in the second data set, respectively; when SCI was used, coefficients increased to 0·40, 0·52, 0·68, 0·55, 0·42, 0·61, 0·58 and 0·58, respectively. Thus, the effectiveness of the SCI was similar in the two data sets and the SCI appears to be a useful tool for quantitatively describing dynamically changing sward compositions.