The objective of this study was to assess the level of Salmonella shedding and exposure in a fattening unit when protected sodium butyrate was added to the diet of the pigs. The study was carried out in a commercial Salmonella-infected fattening unit (8 pens, 110 pigs). Feed with 70% sodium butyrate protected with vegetable fat (3 kg/t) was administrated to animals from 4 randomly selected pens throughout the whole fattening period (4 mo) (TG). Pigs from the remaining 4 pens were fed with regular diet without additive (CG). Individual serum and fecal samples were collected at 30, 60, and 90 d of fattening period and at slaughter. Bacteriology on fecal samples was performed following the ISO 6579:2002 protocol. Serum samples were analyzed by means of an indirect ELISA and 3 cutoff values were used (OD% ≥ 10, ≥ 20 and ≥ 40). Chi-squared analyses were performed to compare microbiological and serological results between groups at different time periods. A difference was considered significant when the one-tail P-value was ≤ 0.05. In addition, a repeated measures analysis was used to estimate differences in mean OD%, after taking into account sampling times and the interaction treatment × time. The levels of shedding were significantly lower for TG when compared with CG for all samplings but the second one (60 d). No significant differences between groups were observed when cut-off values of OD% ≥ 10 or %OD ≥ 20 were used. However, when OD% ≥ 40 was considered, significant differences in seroprevalence were observed for the sampling just before slaughter (CG 89.6% vs. TG 48%). Overall, a lower mean OD% value was observed for samplings at 60 and 90 d, and at slaughter in the TG (71, 66, and 46%, respectively) compared with the CG (88, 87, and 83%, respectively). The results indicate that the use of protected sodium butyrate at 3 kg/t may reduce the shedding of Salmonella spp. under this farm conditions, and therefore the risk of contact of the animals with this pathogen, as suggested by serological results. Table 1. Number of positive Salmonella animals/no. of total animals (% positive Salmonella)

Treatment	30 d	60 d	90 d	Slaughter
CG	19/29 (65.5%)	5/21 (23.8%)	7/21 (33.3%)	36/48 (75%)
TG	12/28 (42.8%)	2/21 (9.5%)	1/22 (4.5%)	29/50 (58%)
P (one-tail)	0.04	0.2	0.01	0.05