ABSTRACT
Summary Autumn harvest of alfalfa (Medicago sativa L.) offers a potential for obtaining additional forage production during a particular season. Little is known about the quality or changes in the chemical composition of this forage. The objective of this research was to examine changes in chemical composition, dry-matter intake (DMI), and apparent digestibility of alfalfa during autumn in Wisconsin. An established stand of Vernal alfalfa, early bud to midbud, was harvested using a flail harvester on 5 and 20 October and 5 and 18 November 1979. The forage was dried at 55°C for 72 to 96 hours in burlap bags. A total collection digestion trial was conducted utilizing four castrated Saanen goats. Apparent dry-matter digestibility and DMI were determined for each forage. Each sample was analyzed for cell-wall constituents (GWG), nitrogen (N), total nonstructural carbohydrates (TNG), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). The concentrations of neutral detergent fiber (NDF) and acid detergent fiber (ADF) in the forage declined by 3.4% and 5.8%, respectively, between 5 October and 5 November, and the cellulose concentration decreased 6.7% during the same period. The reduction in concentration of CWC probably represents a dilutive response to the 79.6% increase in total nonstructural carbohydrates (TNC) between 5 October and 5 November. Cellulose, ADF, and NDF concentrations increased significantly between 5 and 18 November. A 4.8% increase in in-vitro dry-matter disappearance (IVDMD) accompanied the initial increase in TNC, but a 4.2% reduction in IVDMD occurred between 5 and 18 November. Total nitrogen concentration averaged 4.0%, 3.7%, 3.3% and 3.1% of dry matter at each successive harvest date, respectively. Dry-matter intake (g/kg0.75) increased 9.2% from 5 October to 5 November and then decreased 4.7% by 18 November. Digestible dry-matter intake (DDMI) increased from 51.7 g/kg0.75 on 5 October to a high of 56.1 g/kg0.75 on 5 November and
593then decreased slightly by the last harvest date. Decreases of 5.8% and 8.2% in the apparent digestibilities of NDF and ADF, respectively, occurred between 5 October and 5 November. This reduction in fiber digestibility may reflect an increase in rate of passage resulting from the increase in DMI. The apparent digestibility of cellulose also decreased 5.0% during this period. The apparent digestibility of cellulose, ADF, and NDF increased between 5 and 18 November. Apparent digestibility of nitrogen decreased slightly during autumn. These data indicate that the feeding value of alfalfa, as well as the ensiling potential, may increase initially during autumn due to increasing TNG concentrations during that period.
