Yeast in forage crops and silage aerobic stability at 15 Swedish dairy farms

Sammanfattning: This study investigates the role of yeast in green crop and its impact on the aerobic stability of silage. Fresh crop was collected from 15 farms in southern and middle parts of Sweden during the summer 2014; samples from the primary harvest was collected from eight farms and samples from the first regrowth harvest was collected from seven farms. The grass was ensiled in 1.7 l glass silos. After three months of ensiling, silos were opened and silages were stored aerobically for 10 days. Samples from both harvests were ensiled in completely airtight silos, but samples from the second harvest were also ensiled in slightly ventilated silos. Chemical analyses and yeast counts were performed for fresh crop and silage. During the ensiling period, weight loss was registered for all silos and during the aerobic stability test, the temperature in the silage was measured every second hour throughout a ten day period. Yeast species present in the fresh crop were identified with DNA sequencing. Data of management applied in silage making was collected from the 15 farms and processed in relation to silage quality. Dry matter (DM) content of the fresh crops ranged from 22.2 to 52.8 %, metabolizable energy content from 9.2 to 11.6 MJ/kg DM, crude protein (CP) content from 110 to 190 g/kg DM and water soluble carbohydrate content (WSC) ranged from 38 to 118 g/kg DM. Yeast counts in the fresh crop averaged log 5.4 colony forming units/g (min log 4.6; max log 5.9) with no difference between first and second harvest. No correlation was found between fresh crop and silage yeast counts. Among the yeast isolates, 16 different yeast species were identified with Rhodosporidium babjeave and Rhodotorula glutenis being the two most common. In the air tight silos, yeast count was higher in the first harvest than in the second harvest. Among management factors a tendency was observed for higher yeast counts in the fresh crop when manure was used instead of inorganic fertilizer. Silage from the second harvest had shorter aerobic storage stability compared to silage from the first harvest. Also silages with high DM content had shorter aerobic stability than silages with low DM content. Comparing the air-tight and slightly ventilated silos, the number of yeasts were higher in ventilated than in air- tight silos after the ensiling period. Weight loss was 3.1% of the initial DM in air-tight silos and 3.4% in ventilated silos during the ensiling period. Content of WSC decreased during ensiling by 81% of the initial WSC content in both air-tight and ventilated silos (s.d. 12.59% air-tight; s.d. 13.97% ventilated). After aerobic storage the weight loss was 15.3% of the initial DM (s.d. 8.86%) in the air-tight silos and 28.9% of the initial DM (s.d. 14.99%) in the ventilated silos. Loss of WSC content after the aerobic stability test was 32.6% (s.d. 33.11%) in air-tight silos compared to WSC content at the opening of the silo and 51.6% (s.d. 36.44%) in ventilated silos. The results showed that the greatest weight loss occurred during the first week of ensiling. The decrease in WSC content, yeast counts and weight loss were lower in air-tight silos compared to ventilated silos and silage in air-tight silos also had longer aerobic stability than silage in ventilated silos. Higher DM content also resulted in shorter aerobic stability. High numbers of yeast in the silage at silo opening was highly correlated to shorter aerobic stability.