PSXI-9 Using growth implants in suckling beef calves enhances their growth performance and hematological responses.

Abstract This study evaluated growth performance and hematological parameters during two consecutive years in beef calves under two different implant treatments. Each year, 90 crossbred beef calves (121 ± 19 kg body weight (BW) and 74 d of age; Year 1 and 137 ± 17 kg BW and 73 d of age; Year 2) were stratified by birth BW and age. Then, animals were randomly assigned to 1 of 3 treatments (30 per treatment/year) in a randomized complete block design: Synovex C (SC; 100 mg progesterone and 10 mg estradiol benzoate), Synovex One Grower (SG; 150 mg trenbolone acetate and 21 mg estradiol benzoate), or no implant (CON). On the day of treatment implantation (day 0) and during weaning (WN), calves received a vaccination protocol that included Bovi-Shield Gold One Shot® and Ultrabac® 7. During that period, calves were managed in a grazing system along with their dams in a single pasture. After WN, calves were placed in a single pen and given ad libitum access to a total mixed ration until 60-d post-weaning (d-60PWN). Body weight for performance and blood samples for complete blood count were collected on d 0, WN, d 14-d post-weaning (d-14PWN) and d-60PWN. A treatment × day interaction was observed for leukocytes (P = 0.03), with SG having lower concentrations at WN compared with SC while CON calves were intermediate. No other differences in treatment were detected for blood parameters (P ≥ 0.05). In Year 1, hematocrit (P = 0.01), mean corpuscular volume (P = 0.0001), and mean corpuscular hemoglobin concentrations were higher than in Year 2 (P < 0.0001). Average daily gain (ADG) was greater from d 0 to WN (P = 0.02) and from d 0 to d-60PWN (P < 0.0001) for SG and SC calves compared to CON calves. However, from WN to d-60-PWN, there was a difference in ADG (P = 0.02), with SG calves outperforming the CON group, while SC calves did not differ from both groups. Differences in BW were noted on d 14, with the SC group showing greater BW (P = 0.04) than the CON, while the SG group was intermediate. At WN and d-60PWN, the SG and SC calves were heavier (P = 0.0004) than the CON calves. In conclusion, calves implanted with SG appear to experience less stress during weaning, as indicated by lower leukocyte counts. The fluctuations in hematological values observed during the first year compared to the second year may be influenced by environmental, animal, and management factors. Additionally, the implantation of suckling beef calves improved growth performance pre- and post-weaning, with the SG group showing a notably stronger performance response.