Technical note: Evolution of exit velocity in suckling Brahman calves.

The purpose of this study was to assess changes in exit velocity (EV) of Brahman calves from 21 d of age (DOA) to 56 d postweaning (231.30 ± 1.23 DOA). Spring-born calves (n = 308) from 2006 to 2008 were sired by 18 bulls. Exit velocity (m/s) was determined as the rate of speed of a calf traversing 1.83 m after being released from a working chute. Temperament score was determined as the average of EV and pen score 28 d before and at weaning (2006: 173 ± 2 DOA; 2007: 174 ± 2 DOA; 2008: 163 ± 2 DOA). Pen score was determined by separating calves into groups of 3 to 5 animals and scoring their reactivity to a human observer on a scale of 1 (calm, docile, approachable) to 5 (aggressive, volatile, crazy). The number of calves that switched temperament group was determined by ranking calves based on their EV at 21 to 24 DOA, 90 DOA, weaning, and at 56 d postweaning. The GLIMMIX procedure of SAS was used to analyze EV with DOA, birth year and sex as fixed effects, and sire and calf nested within sire included as random effects. Temperament classification (calm, intermediate, temperamental) was modeled as a fixed effect, and the linear regression of traits on DOA investigated. Spearman rank order correlations were determined between EV at 21 to 24 DOA, 90 DOA, weaning, and 56 d postweaning and correlations decreased as the number of days between EV measurements increased. Differences in EV were observed between the 2006 (2.23 ± 0.057 m/s) compared with the 2007 (1.90 ± 0.059 m/s) and 2008 (1.83 ± 0.057 m/s) calves (P < 0.001), but the 2007 and the 2008 calves did not differ (P = 0.75). The random effect of sire approached significance (P = 0.07) and accounted for some of the variation observed. Exit velocity increased as days of age increased (P < 0.001). Exit velocity of temperamental calves increased at a faster rate with age (P < 0.001; estimate of slope = 0.005 ± 0.0004 m/s daily) compared with intermediate (slope = 0.003 ± 0.0005 m/s daily; P < 0.001) and calm calves (slope = 0.0007 ± 0.0005 m/s daily; P < 0.001). Exit velocity is a useful and viable indicator of temperament classification. Results suggest that temperamental calves increase their EV at a faster rate and may be identified before weaning, which may enhance the ability of producers to select against temperamental animals.