Morphological Development of Alfalfa as a function of Growing Degree Days

Abstract

Growing degree days (GDD, base = 41 °F) may be useful for estimating alfalfa (Medicago sativa L.) morphological development (mean stage by weight, MSW) as a forage management tool. Our objective was to quantify the relationship between MSW and GDD of alfalfa in several environments and years. Data on MSW and GDD were collected from 22 growth cycles during five site-years. ‘Advantage’ alfalfa was sampled during spring and summer growth cycles during 1984 and 1985 at Ames, IA. ‘Cimarron’ alfalfa was sampled weekly during four growth cycles in 1989, seven cycles in 1990, and three cycles in 1992 at Stephenville, TX. Cimarron and ‘Florida-77’ were sampled during four growth cycles at Lubbock, TX, in 1992. Tests of homogeneity of intercepts and slopes resulted in equation groups for spring, summer, and fall for each location. Intercepts and slopes of the final spring, summer, and fall regression equations differed among locations, years, seasons, and cultivars. Morphological development rate was more rapid in summer than in spring or fall. Florida-77 developed more rapidly than Cimarron during the fall at Lubbock. Because the relationship between MSW and GDD differed among locations, years, season, and cultivars, GDD would not be a broadly applicable predictor of MSW. Research Question The concept of growing degree days (GDD) has been used in several crops to quantify crop development and as a management tool. Knowledge of the relationship between alfalfa morphological development and GDD may enable alfalfa producers to make better informed management decisions or schedule management practices more precisely. Before the concept can be applied to predicting alfalfa development, it must be determined if the relationship between GDD and alfalfa growth stage is stable across years and locations. Our objective was to determine if the relationship between mean stage by weight (MSW) and GDD was stable among years, locations, and cultivars. Literature Summary The Cornell mean stage system is a quantitative morphological index for describing alfalfa development and predicting forage quality. Northeastern and Midwestern studies have indicated the potential of the system as a tool for alfalfa management. Growing degree days (base temperature of 41 °F) are used in some alfalfa models to predict yield, morphological development, and forage quality. To enable robust equations for predicting alfalfa development (MSW), a constant relationship between GDD and MSW should exist. Previous reports indicate that this may not be so. Study Description This study used data collected during 22 growth cycles in five site-years. These were: Ames, IA. Spring and summer growth cycles during 1984 and 1985. The experiment was not irrigated. Alfalfa cultivar was ‘Advantage’. Four replicates were sampled in each cycle. Stephenville, TX. Spring, early summer, mid summer, and fall growth cycles in 1989; early spring, mid spring, late spring, early summer, mid summer, late summer, and fall growth cycles in 1990; spring, summer, and fall cycles in 1992. The experiment was irrigated. Alfalfa cultivar was ‘Cimarron’. Four replicates were sampled in each cycle. Lubbock, TX. Spring, early summer, mid summer, and fall growth cycles in 1992. The experiment was irrigated. Alfalfa cultivars were Cimarron and ‘Florida-77’. Two replicates were sampled in each cycle. At each harvest in each growth cycle MSW of alfalfa was determined according to the Cornell mean stage system. Linear regression was used to develop relationships between MSW and cumulative GDD. Applied Question What was the relationship between MSW and GDD and was this relationship the same in each year, growth cycle, location, and cultivar? In all instances, the relationship between MSW and GDD was linear; however, the equation parameters (intercept and slope) for each equation differed among seasons, locations, cultivars, and years (Fig. 1, 2, and 3). Regression analysis grouped equations into spring, summer, and fall cycles for each location. These data show that the relationship between GDD and MSW is not constant. Because this relationship is not stable among environments even for a single cultivar, a broadly applicable equation for predicting MSW from GDD may not be possible. Figure 1Open in figure viewerPowerPoint Relationship between mean stage by weight of alfalfa and cumulative growing degree days at Ames, IA; Stephenville (STPH), TX; and Lubbock (LBK; CM = Cimarron, FL 77 = Florida-77), TX, during the spring. Figure 2Open in figure viewerPowerPoint Relationship between mean stage by weight of alfalfa and cumulative growing degree days at Ames, IA; Stephenville (STPH), TX; and Lubbock (LBK; CM = Cimarron, FL 77 = Florida-77), TX, during the summer. Figure 3Open in figure viewerPowerPoint Relationship between mean stage by weight of alfalfa and cumulative growing degree days at Stephenville (STPH) and Lubbock (LBK; CM = Cimarron, FL 77 = Florida-77), TX, during the fall.