Abstract
OBJECTIVE: To study the contribution of plant enzyme and microbial activities on protein degradation in silage, this study evaluated the nitrogen transformation dynamics during ensiling of non- and irradiated alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.). METHODS: Alfalfa and red clover silages were prepared and equally divided into two groups. One group was exposed to γ-irradiation at a recommended dosage (25Gky). Therefore, four types of silages were produced: (1) non-irradiated alfalfa silage; (2) irradiated alfalfa silage; (3) non-irradiated red clover silage; and (4) irradiated red clover silage. These silages were opened for fermentation quality and nitrogen components analyses after 1, 4, 8 and 30 days, respectively. RESULTS: The γ-irradiation successfully suppressed microbial activity, indicated by high pH and no apparent increases in fermentation end products in irradiated silages. All nitrogen components, except for peptide-N, increased throughout the ensiling process. Proteolysis less occurred in red clover silages compared with alfalfa silages, indicated by smaller (P<0.05) increment in peptide-N and free amino acid N (FAA-N) during early stage of ensiling. The γ-irradiation treatment increased (P<0.05) peptide-N and FAA-N in alfalfa silage at day 1, whereas not in red clover silage; these two nitrogen components were higher (P<0.05) between day 4 and day 30 in non-irradiated silages than the irradiated silages. The ammonia nitrogen (NH3-N) and non-protein nitrogen (NPN) were highest in non-irradiated alfalfa silage and lowest in irradiated red clover silage after ensiling. CONCLUSION: The result of this study indicate that red clover and alfalfa are two forages varying in their nitrogen transformation patterns, especially during early stages of ensiling. Microbial activity plays a certain role in the proteolysis and seems little affected by the presence of PPO in red clover compared with alfalfa.