Effects of Natural and Simulated Rainfall On Indicators of Alfalfa Silage Fermentation.

The frustrations of forage producers attempting to conserve high-quality alfalfa (Medicago sativa L.) silage during periods of unstable or inclement weather are widely known. Our objectives were: i) to assess the effects of simulated or natural rainfall on indicators of ensilability, such as pH, buffering capacity, water-soluble carbohydrates (WSC), and also starch; and ii) to use these data as inputs to calculate the threshold moisture concentration that would prohibit a clostridially dominated fermentation (MAX). Rainfall events were applied to wilting forages by both simulated and natural methods over four independent forage harvests. Generally, simulated rainfall was applied to forages that were relatively wet at the time of application, and then were dried to final moisture endpoints under near ideal conditions within a constant temperature/humidity environmental chamber. Under these conditions, indicators of ensilability changed only marginally as a result of treatment. When forages were subjected to natural rainfall events followed by prolonged exposure under field conditions, indicators of ensilability were much less desirable. In one study in which alfalfa received 49.3 mm of natural rainfall over a prolonged (8-d) field-exposure period, fresh pH increased (P < 0.001) from 6.48 to 7.43. Furthermore, there also were sharp reductions in buffering capacity (410 vs. 337 meq/kg DM; P < 0.001), WSC (61.3 vs. 29.0 g/kg; P < 0.001), starch (22.8 vs. 4.5 g/kg; P < 0.001), and MAX (627 vs. 594 g/kg; P < 0.001). Based on these experiments, the potential for good fermentation is affected only minimally by single rainfall events applied to relatively wet forages, provided these events are followed by rapid dehydration; however, attaining acceptable silage fermentations with forages subjected to prolonged field exposure under poor drying conditions is likely to be far more problematic.