91 Ex Vivo Fermentation of Corn and hay by Rumen Bacteria from Sheep and Cattle

Data from cattle and sheep studies are often utilized interchangeably to make inferences regarding ruminant nutrition and physiology, but few studies have directly compared rumen microbiological responses. The objective of this study was to evaluate species differences in ex vivo fermentation of concentrates and forages. Rumen fluid was obtained via oral intubation from Holstein steers (n = 3; age: 212 ± 8 d) and Polypay ram lambs (n = 3; age; 34 ± d) fed a 100% tall fescue (TF) hay diet ad libitum. Mixed bacterial cell suspensions in buffered media contained the following feed substrate treatments (3% wt/vol): TF hay (HY), 2:1 TF hay-to-corn (HC), 2:1 corn-to-TF hay (CH), and corn (CN). Samples were collected after 2, 4, 8, and 24 h of incubation (39°C) for analysis of pH and lactate. Data were analyzed by PROC MIXED in SAS (v. 9.4, SAS Inst. Inc., Cary, NC). The pH differed by species (P = 0.004), treatment (P = 0.005), and time (P < 0.001), with a species by treatment by time interaction (P = 0.001). The pH at 2 h was 6.61 ± 0.02 regardless of species or treatment, and pH declined by 24 h for both steer and ram rumen cell suspensions in all treatments (P < 0.001). Final pH at 24 h did not differ between species for HY (6.15 ± 0.03), but for all treatments including corn, pH was lower in steer rumen cell suspensions (HC: steer – 5.86, ram – 6.06; CH: steer – 5.59, ram – 5.98; CN: steer – 5.26, ram – 5.91 ± 0.05; P ≤ 0.004). Lactate accumulation differed by species (P = 0.038), treatment (P = 0.048), and time (P < 0.0001), with a species by treatment by time interaction (P < 0.001). Lactate at 8 h did not differ between species for HY (steer:3.06, ram: 9.22 ±.84 mmolּ L-1), but was greater in steer than in ram cell suspensions fermenting HC (steer: 12.64; ram: 6.39 ±.84 mmolּ L-1), CH (steer: 13.44; ram: 3.21 ±.84 mmolּ L-1), and CN (steer:3.63; ram:.10 ±.84 mmolּ L-1; P ≤ 0.023). By 24 h, lactate accumulation was minimal for HY, HC, and CH (≤ 1.50 mmolּ L-1) and did not differ between species (P ≥ 0.657). However, lactate at 24 h was greater in steer rumen cell suspensions fermenting CN (steer: 25.78; ram: 182 ± 1.84 mmolּ L-1; P < 0.001). These results indicate that the rumen microbial community of sheep and cattle may differ both in composition and function resulting in different capacities to utilize common dietary feed ingredients.