NUTRITIONAL AND ENVIRONMENTAL IMPLICATIONS OF STARCH DEGRADABILITY AND NITROGEN FERTILIZATION OF ORCHARDGRASS SILAGE IN TOTAL MIXED RATIONS FED TO LACTATING COWS

Abstract

The objectives of this research were to determine the effect of starch and nitrogen (N) availability on microbial protein production and N efficiency, ruminal N efficiency and ammonia, and to assess forage fertilization and grain selection decisions. Diets were incubated in vitro batch culture, and fed in a 6 x 6 Latin square in vivo digestion trial. Total mixed rations (TMR) contained 50:50 forage:concentrate (dry matter (DM) basis) of second-cutting orchardgrass silage fertilized with 200 (OG200) or 400 (OG400) pounds per acre N, plus concentrate mixes using high to low rumen available starches: barley, corn, and milo. TMR crude protein (CP) was 17% and 18% for in vivo, and 20% and 21% for in vitro OG200 and OG400 diets, respectively. Synchronous diets were low:low or high:high rumen starch availability:diet N (corn or milo withOG200, and barley with OG400). No effects on ruminal microbial protein synthesis and flow, N flow, or milk production were observed. DM, organic matter (OM) (P<0.01), N, and neutral detergent fiber (NDF) (P<0.02) total digestibilities increased with synchronous diets. N digestibility was depressed in diets of low:high rumen starch availability:diet N, due to increased hindgut fermentation adding microbial protein to the feces (P<0.001). All OG400 diets had higher fecal N percentage (P<0.001). OG400 had higher ruminal ammonia both in vitro and in vivo (P<0.05), and higher total in vivo volatile fatty acid (VFA) concentration (P<0.001), but rumen pH was stable due to increased recycling of urea. Orchardgrass fertilized at high N can be digested as well as lower N fertilized forages when combined with a rapidly available ruminal starch such as barley, and decrease outputs of fecal DM by up to 401.5 and N by nearly 22 kilograms per year per cow. Crop fertilization and grain selection decisions affect forage composition, rumen fermentation, ration digestibility, and fecal DM and N output.