Evaluating starch degradability (ShD) in combination with starch level can be used as a better predictor for a diet to have a high milk fat depression potential. Adding sugar to a ration is common, but little is known about its effects in conjunction with a high or low ShD diet. The objective of this study was to determine the effects of adding lactose (L) or sucrose (S) to high or low ShD diets on fatty acid (FA) outflow and fermentation in continuous culture. Treatments included 2 ShD levels, high (HDS) and low (LDS), 4 levels of sugar, no sugar (N), lactose (L), sucrose (S), and a combination of L and S (C). Diets were formulated to contain 30% starch, 70 or 90% 7h ShD, and 5 (N), 7 (L), 7 (S) or 9% sugar (C). Fermenters were randomly assigned in a 2x4 factorial design and ran for 4, 10 d periods. Data were analyzed using the MIXED procedure of SAS with repeated measures in a model including ShD and sugar as fixed and fermenter and period as random. Preplanned contrasts were utilized to compare N vs all, S vs L, and, S and L vs C. Dry matter, OM, and ADF apparent digestibility (AD) were unaffected by ShD or sugar, but NDF AD differed with S and L vs C (P = 0.02). Outflow of the saturated FA C12, 20, 22, and 24 were all reduced by HDS (P < 0.01). Outflow of trans-11 18:1 tended to be greater with LDS, and trans-12 18:1 was greater with HDS (P = 0.08 and P < 0.01). Other FA outflows are listed in Table 1. Methane output decreased with sugar addition (P = 0.03) and S vs L differed (P < 0.01). Ammonia was greater with sugar addition (P < 0.01) and all contrasts were significant. These results suggest that ShD with sugar can alter production of FA isomers, AD, and ammonia. Table 1. Outflow of major fatty acids and isomers

Item, mg/d		Sugar Addition				SEM	Contrasts, P-value
		N	L	S	C		N vs. All	S vs. L	S, L vs. C
18:1	LDS	361.2	378.6	343.5	319.6	29.54	0.12	0.13	0.39
	HDS	351.9	368.4	369.9	358.9
18:2	LDS	573.3	655.8	576.5	536.2	52.82	0.25	0.84	0.10
	HDS	653.8	736.8	720.8	686.6
18:3	LDS	89.3	96.2	84.9	79.3	6.51	0.37	0.87	<0.01
	HDS	69.7	96.6	92.2	89.3
18:1 t-12	LDS	9.2	8.1	9.3	8.2	1.42	0.54	0.74	0.90
	HDS	11.2	12.1	10.4	10.7
18:2 c-9,t-11	LDS	7.6	2.0	1.4	1.4	1.80	0.31	0.15	0.07
	HDS	2.7	1.6	1.0	1.0
18:2 t-10,c-12	LDS	14.7	13.7	11.5	8.2	2.06	0.09	0.13	0.57
	HDS	12.4	15.5	12.4	13.0