Supervisors: Weinberg Zwi, Arieli Amichai

Silage making is based on anaerobic lactic acid fermentation of forage crops. Silages are exposed to air during preparation, storage, and feedout. Practically, it is almost impossible to achieve a complete avoidence of aerobic exposure in silage making process. During aerobic exposure of silages, aerobic microbial activity results in losses of nutritive value of silages. Along with abiotic factors, aerobic stability of silages depends upon crop composition and additives used. The objective of this study was to determine the changes and losses during aerobic exposure of silages of two wheat cultivars, Bet Hashita (BH) and Galil, harvested at flowering and milk stage of maturity, ensiled as direct-cut (DC) and wilted and inoculated with homofermentative lactic acid bacteria (LAB) (Lactobacillus plantarum). In addition, the relationship between wheat silage composition and aerobic stability was evaluated. Another objective of the study was to determine the changes and losses in sorghum and corn silages during aerobic exposure and to evaluate whether any of these silages contain factors contributing to aerobic stability by mutually spraying the aqueous extracts of these silages. We hypothesized that important intrinsic factors of wheat silage such as stage of maturity, type of silage (DC and wilted), and silage composition (DM, watersoluble carbohydrates [WSC], lactic acid, volatile fatty acids [VFA], etc.) act both separately and interactively to affect the aerobic stability of silages. In sorghum and corn silages, it was hypothesized that sorghum silages contain certain intrinsic stabilizing agents responsible for its aerobic stability and that can be exploited and used in aerobically suceptible silages by spraying aqueous extract of sorghum silages. Silages were prepared in 1.5 L anaerobic glass jars and after 4-6 months of storage at room temperature (26�20C), silages were opened and subjected to aerobic stability test lasting 7 days. Chemical (pH, residual WSC, lactic acid, VFA, CO2 production, DM and NDF digestibility), temperature change, numbers of yeasts and molds, and sensory (visual and olfactory) parameters served as indicators to study the changes during aerobic exposure of silages. In cultivar BH, flowering DC (287.0 g/ kg DM) and wilted silages (362.2 g/ kg DM) had ensiling DM losses of 11.5% and 8.0% respectively, whereas milk DC (398.4 g/kg DM) and wilted silages (427.0 g/ kg DM) had only 3.7% and 4.7%, respectively, DM losses. Flowering DC and wilted silage were stable during aerobic exposure. Milk DC silages were stable but wilted silages slightly tended to spoil for 7 day of aerobic exposure (AE), having higher number of yeasts and molds and increased CO2 production. In cultivar Galil, flowering DC silages (199.2 g/ kg DM) had 11.8% ensiling DM losses but were stable during aerobic exposure. Inoculation resulted in aerobic deterioration, having 149.6 g/ kg DM CO2 production, 12.7 % DM losses and heating to 32OC during 4 day of AE. Flowering wilted silages were unstable during 7 day of AE, inoculation did not result further deterioration. Milk DC silages (299.2 g/ kg DM) had ensiling losses of 8.9% DM and were unstable after 7 day of AE, showing 67.2 g/ kg DM CO2 level, 8.8% DM losses and 31OC temperature. Inoculation further increased deterioration resulting in 82.3 g/kg DM CO2 production. Milk wilted silages (437.3 g/ kg DM) had ensiling losses of 1.7% DM and showed higher level of spoilage both on day 4 (34.0 g/ kg DM CO2 and 4.5% DM losses) and 7 (133.2 g/ kg DM CO2, and 12.9% DM losses) of AE and inoculation further increased deterioration. It is concluded that in wheat silages, cultivar, stage of maturity at harvest, wilting, DM content affect the ensiling fermentation, silage composition, and aerobic stability. Stepwise regression analysis indicated that levels of acetic acid had a marked effect on wheat silage aerobic stability. Sorghum silages (305.6 g/ kg DM) had higher ensiling losses of 11.0% DM whereas corn silages (319.4 g/ kg DM) had only 0.9% DM. Sorghum silages were more stable during aerobic exposure (7.4 g/kg DM CO2 after 7 days), but corn silages were unstable after day 4 (38.3 g/ kg DM CO2) and 7 (47.7 g/ kg DM CO2) of AE. The aqueous extracts from corn and sorghum, silages contained 13.5 and 28.1 mg/ml polyphenols, respectively. As expected, spraying of corn silage extracts on sorghum silages further enhanced the aerobic deterioration of sorghum silages (15.6 g/ kg DM CO2 and 3.9% DM losses on day 7). Unexpectantly, spraying of sorghum silages extract on corn silages did not improve the aerobic stability of corn silage, instead, it further enchanced deterioration (101.0 g/ kg DM CO2 on day 7). Although sorghum silages were more stable than corn silage and their extract contained higher concentrations of polyphenols, spraying of sorghum silages extract did not improve the aerobic stability of corn silage. Further study is needed to explore the factors or compounds behind the aerobic stability of sorghum silages and to find a suitable way to exploit and utilize such compounds for the aerobic stability improvement of aerobically suceptible silages like corn. Key words: Silage, wheat, corn, sorghum, DM losses, aerobic stability