mycols <- '(x2,x5)'
f <- paste0('min',mycols)
df %>% rowwise() %>% mutate_(min = f)
df
# A tibble: 10 × 6
#      x1    x2    x3    x4    x5   min
#   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#1      2     3     0     1     1     1
#2      0     2     1     0     1     1
#3      0    NA     0    NA    NA    NA
#4     NA     5     1     3     1     1
#5      0     3     3     0     3     3
#6      1     2     0     0     4     2
#7      1    NA    NA    NA    NA    NA
#8     NA    NA    NA     0     3    NA
#9      0     4     0     0     3     3
#10     1     5     1     1     1     1