U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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J. Dairy Sci. 96 :2521–2535; http://dx.doi.org/ 10.3168/jds.2012-6145


Unstable weather, poor drying conditions, and unpredictable rainfall events often place valuable hay crops at risk. Recent research with large round bales composed of alfalfa (Medicago sativa L.) and orchard grass (Dactylis glomerata L.) has shown that these large-bale packages are particularly sensitive to spontaneous heating and dry matter (DM) losses, as well as other undesirable changes with respect to forage fiber, protein, and energy density. Various formulations of organic acids have been marketed as preservatives, normally for use on hays that are not desiccated adequately in the field to facilitate safe bale storage. Our objectives for this study were to (1) evaluate the efficacy of applying a commercial (buffered) propionic acid-based preservative at 3 rates (0, 0.6, and 1.0% of wet-bale weight) to hays baled at 3 moisture concentrations (19.6, 23.8, and 27.4%) on the subsequent storage characteristics and post-storage nutritive value of alfalfa-orchard grass forages packaged in large rectangular (285-kg) bales, and then (2) evaluate the in vivo digestibility of these hays in growing lambs. Over a 73-d storage period, the preservative was effective at limiting spontaneous heating in these hays, and a clear effect of application rate was observed for the wettest (27.4%) bales. For drier hays, both acid-application rates (1.0 and 0.6%) yielded comparable reductions in heating degree days >30°C relative to untreated controls. Reductions in spontaneous heating could not be associated with improved recovery of forage DM after storage. In this study, most changes in nutritive value during storage were related to measures of spontaneous heating in simple linear regression relationships; this suggests that the modest advantages in nutritive value resulting from acid treatment were largely associated with perturbations of normal heating patterns during bale storage. Although somewhat erratic, apparent digestibilities of both DM (Y = −0.0080x + 55.6; R2 = 0.45) and organic matter (Y = −0.0085x + 55.5; R2 = 0.53) evaluated in growing lambs were also directly related to heating degree days in simple linear relationships. Based on these data, applying propionic acid-based preservatives to large rectangular bales is likely to provide good insurance against spontaneous heating during storage, as well as modest benefits with respect to nutritive value and digestibility.