Changes in Carbohydrate and Nitrogen Fractions and Digestibility of Forages: Method of Sample Processing

NUTRITIONAL evaluation of fresh for-ages is often difficult because of problems involved in sampling and preparation of the fresh plant material. Thus, the tendency in forage research has been to analyze oven-dried plant material to determine the chemical composition of the fresh plant. Couchman (1959a, b) and Melvin (1963) reported that temperature affects the chemical composition of stored forage. Their data suggest that changes in chemical composition may result from one or more of the following: (1) loss of components, such as ammonia and carbon dioxide; (2) degradation of carbohydrates; (3) enrichment of nonvolatile fractions; and (4) formation of protein-carbohydrate artifacts not originally present in the forage. The objective of this study was to determine the effects of sample handling and drying on the carbohydrate and nitrogen fractions and digestibility of fresh and ensiled forages. Materials and Methods Experimental plots of Pioneer 931 (Pio 931) and Rudy Patrick (RP-30F) forage sorghums, high-sugar corn (Cargill HS-50) and silage corn (Cargill 1089) were harvested with a field chopper at 70, 100, 130, 160 and 190 days post-emergence. At each harvest, 15 300-g samples of each chopped forage were taken and assigned at random to three treatments (freeze-ground, oven-dried at 65 C and oven-dried at 100 C). Samples to be freeze ground were mixed thoroughly, placed in a polyethylene bag and immediately immersed in liquid nitrogen. The frozen samples were stored at —28 C until ground in a Waring Blender with liquid nitrogen and dry ice. This material was stored at —28 C until analyzed in the freeze-ground state. Samples oven-dried at 65 C were dried in a forced-draft oven at 100 C for 2 hr., then 65 C for 46 hours. For the higher drying temperature, samples were dried at 100 C for 48 hours. All oven-dried samples were ground in a Wiley Mill (2-mm screen) and stored in sealed glass jars at room temperature until analyzed.