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The growth and development of individual grass tillers is frequently monitored in intensive studies of plant response to treatments or environmental parameters. Since growth may be recognized as an increase in tiller weight or tiller length over time, knowledge of the relationship between them is important if researchers are to select the measure most appropriate for their needs. To make these comparisons, the mean weight and length of tillers from perennial ryegrass (Loliumperenne L.), colonial bentgrass (Agrostis tenuis Sibth.), softchess (Bromus mollis L.), and annual fescue (Vulpiu Spp.) were measured on each of seven dates during the spring growing period, 1977. Sample sizes required to estimate mean tiller length were calculated on each date using Stein’s two-stage procedure. The relationship between mean tiller weight and mean tiller length over time was evaluated by least squares analysis of a linear regression model. Results of these analyses indicated that mean tiller length can be estimated satisfactorily from a substantially smaller sample size than can mean tiller weight. In addition, a strong linear relationship was observed between mean tiller weight and mean tiller length over time. The strength and linear nature of this relationship suggests that both measures will yield similar relative growth curves when sequential observations are plotted over time. Actual growth curves plotted for perennial ryegrass and colonial bentgrass support this supposition. Therefore, since tiller lengths require a small sample size to estimate each mean, both time and money can be saved by basing growth analysis on observations of tiller length rather than on tiller weight. Plant growth is often monitored to assess the effects of treatments on plant productivity and development in range and pasture studies. In studies of inter-specific plant competition and plantherbivore interactions, the distribution of plant growth during the growing season may be as important as the total amount of aerial biomass accumulated over the entire season. The study reported here grew out of a need to determine the relative growth curves of different grass species growing together in a mixed sward. High variability in plant distribution and plant size within the sward, the relatively small area available for study, and the need to measure growth for each species separately made the use of plot-based or plant-based techniques impractical. The intensive nature of the study suggested the use of a tiller-based technique. In intensive growth analyses, individual grass tillers often serve as the unit of observation for determining plant growth. Plant growth is generally measured quantitatively as an increase in weight or height over time. For example, Nelson et al. (1978) described growth rates in several populations of tall fescue (Festuc arundinacea, Schreb.) in mg/day and Beaty et al. (1978) reported Author is assistant professor, Rangeland Resources Program, Oregon State University, Corvallis 97331. This article is submitted as Technical Paper No. 4999. Oregon Agricultural Experiment Station, Corvallis, Oregon 9733 I. Manuscript received January 7, 1980. tiller weights in their study of tall fescue development while McCarty (1938), McCarty and Price (1942) and Joy et al. (1954) defined plant growth as an increase in height of individual plants. Since growth may be expressed in either units of weight or height, it is important to know the relative efficiency of using each unit and the relationship between them. The purpose of this paper is to evaluate (1) the relative efficiency of using tiller weight vs. tiller length as a measure of growth in perennial ryegrass and colonial bentgrass and (2) the strength of relationship between mean tiller length and mean tiller weight over time for perennial ryegrass (Lolium perenne L.), colonial bentgrass (Agrostis tenuis Sibth.), annual fescues (Vulpia spp.), and softchess (Bromus mollis L.). Materials and Methods This study was conducted on two contiguous plots, one fertilized with 34 kg/ ha of ammonium nitrate on May 1, 1977, and the other an unfertilized control plot. The study area was located approximately 1 km west of Corvallis, Oregon. Elevation is approximately 100 m. Annual precipitation is approximately 100 cm. Both plots supported a mixed stand of vegetation containing approximately 40% subterranean clover (Trfolium subterraneum L.), 26% perennial ryegrass, 18% colonial bentgrass, 5% annual grasses, and I 1% broadleaf forbs by cover on March 30, 1977. These two plots were repeatedly sampled on seven dates during the spring growing season (between May 5 and June 10) in 1977. Care was taken to disturb the plots as little as possible during sampling. Traffic on the plots was not observed to be a problem. For each sampling date, the first 25 tillers each of perennial ryegrass and colonial bentgrass together with the first 25 tillers of annual grass encountered by the pins of randomly placed ten-point frame (Sharrow and Tober 1979) were sampled. This sampling regime was designed to select both vegetative and reproductive tillers in proportion to their density in the population. In the initial sampling (May 5), all tillers were vegetative. As plant phenology progressed through the growing season, the ratio of vegetative/ reproductive tillers in the sample decreased. Virtually all reproductive tillers were encountered on May 5, May 5, May 27, and June 10 for annual fescues, perennials ryegrass, softchess, and colonial bentgrass, respectively. Sample tillers were clipped off at the soil surface, then taken to the laboratory where they were oven-dried at 500 C for 72 hours. Tiller length was then determined to the nearest millimeter by straightening the tiller along a meter stick. Length was measured from the base of the culm to the tip of the longest leaf. Oven-dry tillers were subsequently weighted to the nearest hundredth of a milligram on an analytical balance. The least squares relationships between mean tiller length (X) and mean tiller weight (Y) were determined using a simple linear regression procedure (Steel and Torrie 1960). The resulting regression equations of fertilized vs. unfertilized plots were compared for homogeneity of regression coefficients. No differences were evi354 JOURNAL OF RANGE MANAGEMENT 34(5), September 1981 dent. Therefore, data from fertilized and unfertilized plots were pooled for all further analysis. Table 2. Coefficient of variation (C.V.) and minimum sample size (n) required to estimate mean perennial ryegrass tiller length and tiller weight on seven dates. Results and Discussion The high r2 values observed for all grass species examined in this study indicate that the linear relationship between mean tiller lenght and tiller weight over time was strong (Table 1). Mean tiller length is a powerful predictor accounting for 65 to 94% of the observed variability in mean tiller weight. Standard errors of the estimate were moderately low, ranging from 9 to 28% of mean tiller weight. The lowest r2 (0.65) and highest Sy.x expressed as a percentage of the mean (28%) occurred in the annual fescue group. Table 1. Least squares estimates for simple linear regression between mean tiller length (mm) = X and mean tiller weight (mg) = Y of four grass species based upon n observations. Plant species Regression line n r2 G.x (mg) Perennial ryegrass Y = 0.60X 40.6 14 0.94* 30.0 Colonial bentgrass Y = 0.31X 6.9 14 0.94 8.3 Annual fescue Y = 0.13X 8.8 9 0.65* 10.2 Softchess Y = 0.24X 20.3 10 0.94* 5.7 ‘Significant at KO.01. Annual grasses can be difficult to identify in the vegetative stage even with the assistance of a vegetative key. While identification was not a problem with a distinctive species, such as softchess, it was a problem with relatively nondescript species such as the annual fescues. Even though all of the plants included in this group had similar tiller morphology, the inclusion of several species into a group, either intentionally or through misidentification, substantially weakened the length/ weight relationship. It thus appears the grouping of species for the purpose of estimating a common mean tiller length/weight relationship should be avoided even if the plants have similar morphologies. The strong linear relationship between mean tiller length and mean tiller weight indicates that these two units of measure will yield similar growth accumulation curves for the grasses studied with the possible exception of the annual fescues (Figs. I and 2). The growth accumulation curves based upon mean tiller weight and mean tiller length plotted in Figures 1 and 2, when compared for homogeneity of regression coefficients, revealed no meaningful differences (p>. 10). Therefore, proportional rates of growth calculated from either length increase or weight increase over time should be similar when expressed as a percentage of total growth observed. This concept is further substantiated by Scott’s (1961) work in which increase in leaf length and biomass gave similar proportional growth rates for three perennial tussock grasses. 0’ 1’0 20 I 30 I 40 I 50 I S’O Days Since 5 April 1977 Date C.V. (%)

Tiller Length vs. Tiller Weight: Applications to Plant Growth Studies