Prediction of Pasture Growth Rates from Climatic Variables

Feed planning enables ranchers to use feed resources efficiently and increase profits. Feed budgeting requires estimates of expected daily growth rate (DGR) of pasture. The purpose of this study was to estimate DGR and determine if weather variables and herbage mass could be used to predict DGR for an irrigated pasture dominated by tall fescue ( Festucu arundinucea Schreb.) and strawberry clover ( Trifolium fragiferum L.). A single‐probe capacitance meter was used to estimate herbage mass over 2 yr. Daily growth rate was calculated during the rest periods on 16 rotationally‐grazed paddocks. Maximum and minimum daily air and soil temperature, and solar irradiance were recorded by a California Irrigation Management Information System (CIMIS) weather station located within the pasture. Average daily weather values and the herbage mass at the beginning of the rest period were independent variables in regression with DGR. Solar irradiance was the variable most closely correlated with growth rate, and when combined with beginning herbage mass in multiple regression, predicted DGR with R 2 = 0.71 (S y.x = 9.7) in year 1 and R 2 = 0.68 (S y.x = 5.5) in year 2. Maximum air temperature with beginning herbage mass predicted DGR with R 2 = 0.67 (S y.x × = 10.4) for year 1 and R 2 = 0.67 (S y.x =5.6) for year 2. All weather variables were highly correlated with each other and had similar effects on growth rate. Daily growth rate increased with increasing beginning herbage mass up to 2500 lb dry matter (DM)/acre. While the climatic variables in this study were not strong predictors of DGR, they may be useful for adjusting long term DGR averages to actual weather conditions. Research Question Pasture feed budgeting, as practiced in New Zealand, requires an estimate of expected daily growth rate (DGR) of pasture for each month. This study estimated monthly DGR and the influence of solar irradiance, air temperature, soil temperature, and the amount of herbage per unit area of pasture (herbage mass) on DGR for an irrigated pasture in California's Sacramento Valley. Literature Summary Regional estimates of DGR are readily available to pasture managers in New Zealand but not in California. Daily growth rates will vary as solar irradiance and air temperature change. Herbage mass also influences DGR. Daily growth rates are often greatest within a herbage mass range of 1000 to 2500 lb DM/acre. Below that range leaf area is inadequate to capture solar irradiance efficiently and above that range shading and aging leaf tissue reduces photosynthetic capacity. Study Description Herbage mass was estimated during the rest periods on a rotationally grazed, irrigated pasture. Herbage mass was estimated weekly from May to September, biweekly from mid‐February to April and October to mid‐November, and monthly from mid‐November to mid‐February. Daily growth rate was determined from the change in herbage mass divided by the number of days. Solar irradiance, air temperature, and soil temperature were obtained from a weather station located on the irrigated pasture. Location: Sacramento Valley, California Pasture Mix: Tall fescue and strawberry clover Soil: Capay silty clay Pasture size: 120 acre divided into 16 equal paddocks Irrigation: Biweekly, April to October Fertilization: 300 lb/acre of ammonium phosphate‐sulfate (16‐20‐0‐15) in November and March Applied Questions What are the monthly average pasture growth rates for a typical Sacramento Valley irrigated pasture? Using data from this study, DGR for January through October can be estimated. Relying on data collected in other studies and the experience of the authors, November and December DGRs are estimated to be less than 10 lb/acre/d. urn:x-wiley:08908524:pag2jpa19930086:equation:pag2jpa19930086-math-0001 How do daily growth rates respond to seasonal changes in solar irradiance, temperature, and herbage mass? Daily growth rate increased with increasing solar irradiance until air temperatures exceeded optimum temperatures for growth of cool season pasture species. Herbage mass, solar irradiance, air temperature, and soil temperature were not strong predictors of DGR. Without a reliable predictive tool, DGR must be estimated at each location where feed budgeting is to be implemented. Several years of DGR estimation will provide an estimate of yearly variation in DGR. The authors have embarked on an irrigated pasture monitoring program that will estimate yearly variation in monthly DGR on six rotationally grazed irrigated pastures in northern California. Feed budgeting has the potential to increase harvest efficiency of irrigated pasture, resulting in improved water, fertilizer, and energy use efficiency.