Silage Fermentation End Products and Microbial Populations: Their Relationships to Silage Quality and Animal Productivity

Understanding the roles that various microbes play in silage fermentation and the factors that affect their growth can help us understand why silages ferment in various ways. In many, but not all cases, the fermentation that a crop undergoes can be explained by how microbes interact with factors such as moisture content, buffering capacity, and sugar content. However, management factors such as silo packing speed, silage pack density, type of additive used, chop length, silo management during storage, and silo management during feed-out can also affect the silage fermentation. In some cases fermentation analyses can qualitatively explain poor silage nutritive value or low intakes, but they cannot be used to balance diets for cattle. Thus, they should always be used in conjunction with other standard chemical analyses (i.e. ADF, NDF, CP, RDP/RUP, NE L , NDF digestibility, etc.). A large portion of this review was patterned after a paper by Kung and Shaver (2001) and an invited presentation by Muck and Kung (2002). Lactic Acid Bacteria and Their End Products Lactic acid bacteria (LAB) are responsible for converting water soluble sugars to various end products. The two types of LAB include heterolactic acid bacteria (HAL) and homolactic acid bacteria (HOL). The former make lactic and acetic acids, ethanol and CO2 (representing a loss of energy and dry matter) whereas the later make only lactic acid when fermenting 6-carbon sugars like glucose and fructose. Fermentation from HOL is more efficient than from HAL but under practical conditions sometimes fermentation from the latter may be desirable (specifically when aerobic stability of silage is a problem). Standing crops average about 10 4 to 10 6 colony forming units (CFU) of LAB per gram of wet forage. Most of these organisms are “wild” HAL. Frost, drought conditions, and exposure to high heat and UV rays from the sun may lower their numbers. The population on hay crops usually increases while the crop is drying in the field. During the early stages of ensiling, populations of total LAB may reach 10 9 to 10 10 CFU/g of forage but after months of storage, their numbers usually decrease to 10 4 to 10 6 CFU/g. HOL have traditionally been the microbes of choice for silage inoculants because they can rapidly produce lactic acid and minimize DM losses. Lactic acid is primarily responsible for lowering the pH of the forage crop during ensiling, but it has weak antifungal activity. Some strains of lactic acid bacteria from silage inoculants may provide a “probiotic” effect in the rumen and have the ability to improve fiber digestion by possibly altering ruminal fermentations (Weinberg et al., 2007). In addition, several strains of lactic acid bacteria have been identified with the ability to express ferulic acid esterase activity and also have the potential to increase fiber digestibility (Nsereko et al., 2007). When utilizing microbial inoculants, even distribution of the microbes throughout the silo mass is important. In drier silages (≥ 40% DM), application