Previous studies demonstrated that crude glycerin (Gly) was an excellent energy source and can partially replace corn grain in ruminant diets. The aim of this study was to evaluate the effect of supplemental protein sources associated with Gly on digestibility of OM, NDF and CP of Nellore bulls in tropical pasture during the rainy season. Five Nellore bulls (448.2 ± 14.8 kg) were used in a 5 × 5 Latin square design (5 periods and 5 treatments). The pastures were divided into 5 paddocks of 0.25 ha each, consisting of Brachiaria brizantha ‘Marandu’. Crude glycerin was used in all supplements to replace (15% of DM) corn. The supplements were: T1 - Gly with urea, T2 - Gly with soybean meal, T3 - Gly with cottonseed meal, and T4 - Gly with gluten, and the control received mineral mixture ad libitum. Animals were individually supplemented at the rate of 300 g·100 kg⁻¹ of BW, daily, at 1000 h. To estimate feed intake, chromium oxide was used as external marker, and indigestible neutral detergent fiber (iNDF) was used as internal marker (forage intake). Fecal grab samples were collected on d 15–17 in different times. Fecal samples were composited on a wet-basis by period for each bull. Fecal and ingredients samples were analyzed for OM, NDF and CP contents. Data were analyzed using the PROC MIXED of the SAS. There was no effect of Gly associated protein sources on digestibility of OM and NDF (Table 1). However, there was effect on digestibility of CP (P = 0.04). Crude glycerin showed be an good energy source and can partially replace corn grain (15% of DM) in ruminant diets. Independently of protein source utilized, there was an increase of 21.3% (P = 0.04) on digestibility of CP for animals supplemented when compared with control group. Urea associated with Gly can be effective for improving CP digestibility of Nellore bulls in tropical pasture. Table 1. Effect of supplemental nitrogen sources associated with glycerin on nutrients digestibility of Nellore bulls

Item	Dietary treatments					SEM	P-value
Control	T1	T2	T3	T4
OM, %	54.87	62.50	60.41	59.62	59.99	2.72	0.39
NDF, %	60.58	64.56	62.05	60.07	62.05	2.11	0.45
CP, %	55.38b	70.55a	73.00a	69.78a	68.01a	3.76	0.04