Grouping previously unknown bucks is a stressor with negative effects on reproduction.

Grouping previously unknown animals produces social stress, which might have negative effects on reproduction. The aims of the experiment were to determine if grouping unknown bucks (1) triggers a stress response and produces changes in body weight; (2) affects scrotal circumference, testosterone concentration, and semen quality; and (3) has differential effects between resident and introduced bucks. One group of nine Saanen bucks was transported and introduced (introduced bucks, group IG) to a group of eight Saanen bucks (resident bucks, group RG). On day 0, cortisol concentration and rectal temperature were determined, and from day -7 to day 29, body weight, scrotal circumference, and testosterone concentration were recorded and semen quality was determined. The stress response was different between groups: on several moments on day 0, resident bucks had greater cortisol concentration (P < 0.0001), while introduced bucks had higher rectal temperature (P = 0.02). Body weight decreased similarly in both groups from day -7 to day 2 (P < 0.0001), but on day 29, IG bucks were lighter than RG bucks (P = 0.05). Also, the reproductive response differed between groups: introduced bucks had lower scrotal circumference (P < 0.01), lower testosterone concentration (P = 0.02), and lower percentage of motile spermatozoa in the ejaculate (P = 0.05). It was concluded that grouping unknown bucks was stressful and negatively affected the reproduction, being more serious for the introduced than the resident bucks.