Codified Morphological Stage for Predicting Digestibility of Italian Ryegrass during the Spring Cycle

Abstract

A precise prediction of changes in digestibility of grasses during the growth cycle would allow targeting of harvests to desired levels of nutritive composition to meet specific animal requirements. In this study, regression analysis was used to formulate and test prediction equations for organic matter digestibility (OMD) of standing crops of Italian ryegrass ( Lolium multiflorum Lam.) with codified morphological stage, canopy age, growing degree days (GDD), and N fertilization as independent variables. The field study was conducted for 7 yr with six Italian ryegrass cultivars, diversified for ploidy and earliness, grown in the western Po Valley (Italy). Nitrogen fertilizer was applied at various rates as top dressing. Sequential harvesting from early vegetative to full flowering provided herbage samples varying in age and stage of development. The 0–400 classification system that was used to evaluate the morphological stage included four primary stages: vegetative, elongation, inflorescence emergence, and flowering. The OMD decreased from 928 to 576 g kg −1 OM, as the herbage aged. All cultivars responded in the same way during the whole growth cycle. Morphological stage was a better predictor of OMD than GDD or age in days, with a higher r 2 (0.88) and a lower root mean square error (28.0 g kg −1 OM). Nitrogen fertilization slightly decreased digestibility. The developed equation tested on an independent set of farm scale data showed that this codified morphological stage system can provide a rapid and easy estimate of Italian ryegrass digestibility in the field.