Glycerin, the main biodiesel production by-product, has the potential to partially replace corn as an energy source for cattle. The objective of this study was to evaluate the effects of partially replacing dry ground corn (DGC) with glycerin on ruminal fermentation using a dual-flow continuous culture system. Six fermenters were used in a replicated 3x3 latin square arrangement with 3 periods of 10 d each, with 7 d for diet adaptation and 3-d for sample collections. Three dietary inclusion levels of glycerin were tested (0, 15, and 30% on DM basis). All diets contained 75% concentrate and were formulated to meet NRC (2000) recommendations. Fermenters were fed 72 g of DM/d equally divided in 2 meals per day. Liquid and solid dilution rates were adjusted to 11 and 5.5%/h, respectively. On d 8, 9, and 10, samples of digesta effluent were collected for ruminal NH₃-N and VFA analyses. Nutrient flow and digestibility, N balance, and microbial growth were also measured. Data were analyzed using the MIXED procedure in SAS. Ruminal traits are presented in Table 1. Concentrations of total VFA, propionate, and NH₃-N increased linearly (P < 0.05) and concentrations of acetate, butyrate, iso-valerate, and BCVFA, as well as the acetate: propionate ratio decreased linearly (P < 0.05) as glycerin replaced DGC. These results suggest that partially replacing DGC with glycerin may change ruminal fermentation, increasing total VFA, propionate, and NH₃-N concentrations, which may increase energy status and microbial yield in beef cattle. Table 1. Ruminal traits

Item	Glycerin (%)			SEM	P-value
0	15	30	L	Q
NH3-N, mg/100 mL	8.7	10.6	17.6	1.22	<0.01	NS
Total VFA, mM	113.0	122.9	124.8	1.76	<0.01	NS
Acetate, % of total VFA	59.0	49.0	40.1	2.13	<0.01	NS
Propionate, % of total VFA	22.3	35.4	47.8	2.54	<0.01	NS
Butyrate, % of total VFA	15.7	13.4	10.1	0.68	<0.01	NS
Valerate, % of total VFA	1.18	0.96	1.02	0.05	NS	NS
Isovalerate, % of total VFA	1.43	0.97	0.58	0.10	<0.01	NS
Acetate:propionate	2.65	1.41	0.84	0.19	<0.01	NS
BCVFA, % of total VFA	2.98	2.62	2.03	0.31	<0.01	NS