Full-time farmers and particularly dairy farmers who plan to expand their production have voiced concerns about their physical and mental work ability in recent studies. The objectives of this study were to characterize the work ability of dairy farmers and to identify demographic groups at risk of disability and in greatest need of interventions to promote work ability. We conducted a postal survey using the Work Ability Index (WAI) questionnaire. The WAI of 399 dairy farmers (245 female and 154 male) was analyzed (response rate 41.5%). The mean WAI score was 36.0 among female and 39.0 among male respondents (scale: 7 = worst to 49 = best). The WAI decreased with age. The WAI was systematically better among males compared to females in all age groups, and the difference was greatest among those over 45 years of age. About one-fourth of females and one-tenth of males over 45 years of age were at an imminent risk of disability (poor WAI). The WAI of farmers in our study was similar to farmers in previous studies where entrepreneurs and salaried workers had better WAI compared to farmers. This study indicates that interventions are needed among older dairy farmers, particularly females, to help them improve their work ability. The first question (of seven) in the WAI questionnaire correlated well with the complete questionnaire-based WAI. The first question could be used in surveys as a condensed version of the WAI, if the same correlation is found in future studies. Based on this study, we recommend using the Work Ability Index questionnaire for assessing the health of those working in agriculture.