Effect of combined ensiling of sorghum and soybean with or without molasses and lactobacilli on silage quality and in vitro rumen fermentation

Two sorghum (Sorghum bicolor (L.) Moench) varieties (SG₁ & SG₂), with the former showing higher grain and total DM yield, but also increased tannin contents compared to the latter, and one soybean (Glycine max (L.) Merr.) variety (INCASOY-35) were sown, harvested (at pasty grain state), chopped and ensiled (CIAP, Cuba). Silages were made in following combinations: either SG₁ or SG₂ combined with soybean in two proportions (0.4 and 0.6). All silages were prepared with or without molasses (3.5% of fresh material) and Lactobacillus sp. as inoculant (3 × 10⁵ colony forming units/g). Silage quality parameters included pH, ammonia, lactate, acetate, butyrate and water soluble carbohydrates content. Further, both fresh and ensiled materials were incubated in vitro with buffered rumen fluid to study the fermentation characteristics. Silage of a good quality could be produced with both sorghum varieties alone, but combined silages showed improved quality compared to soybean silage (p<0.05 for all quality characteristics). Addition of molasses and bacterial inoculant further improved silage quality (p<0.05 for all quality characteristics). In vitro incubation (24 h) of ensiled material resulted in lower acetate and higher propionate proportion compared to fresh forages. However, ensiling without molasses and inoculant reduced in vitro short chain fatty acid production and hence the apparent rumen degradability of organic mater as well as the fermentation rate. As expected, a higher proportion of sorghum increased the molar propionate proportion and the fractional fermentation rate, whereas ammonia (mmol/L) concentrations were reduced. SG₁ silages produced higher molar propionate proportions, lower acetate proportions and ammonia concentrations.