INFLUENCE OF GROWTH STAGE AND SEASON ON THE ENERGY VALUE OF FRESH HERBAGE .2. RELATIONSHIPS BETWEEN DIGESTIBILITY AND METABOLIZABLE ENERGY CONTENT AND VARIOUS LABORATORY MEASUREMENTS

This paper gives details of a study which has examined the effect of herbage growth type and season on the relationships of digestible organic matter in the dry matter (DOMD) and metabolizable energy (ME) contents measured in vivo to various laboratory measurements including three in vitro estimates of DOMD based on rumen fluid pepsin (RFP), pepsin cellulase (PC) or neutral detergent cellulase (NCD). Seventy herbages harvested over two years and comprising 32 spring primary growths, 14 summer regrowths and 24 autumn growths were used. For DOMD in vivo, significantly different regression relationships were required for spring- and autumn-harvested herbage. Equations based on PC were: Spring herbage: DOMD (g kg-1 DM) = 436 + 0.501 PC (g kg-1 DM) R2BAR = 76.7%; r.s.d. = 25.4 Autumn herbage: DOMD (g kg-1 DM) = 594 + 0.195 PC (g kg-1 DM) R2BAR = 15.5%; r.s.d. = 23.1 For autumn herbages there were no significant relationships with ME. As with DOMD in vivo the PC method was shown to provide the most accurate relationship with ME concentration of spring herbage. The relationship was: ME (MJ kg-1 DM) = 6.16 + 0.0094 PC (g kg-1 DM) R2BAR = 69.8%; r.s.d. = 0-57 For both DOMD and ME the relationships for spring herbage based on PC were the only ones which did not differ significantly from those of an earlier study. It is concluded that even where significant relationships for summer and autumn herbages were developed, the predictors accounted for substantially less of the variability than in spring herbages. This appeared to be largely due to reduced range in the DOMD and ME content in summer and autumn herbages.