Impacto de la inclusión de productos derivados de levadura en sustitución de la monensina sódica en la performance y la respuesta al estrés en terneros destetados precozmente alimentados a corral

Early weaning in beef cattle is commonly used to improve cows’ body condition score and pregnancy rate. However, at such a young age, nutritional management of post-weaned calves could be challenging given the great nutritional requirements. One way to improve pregnancy rate together with weaning weight is a dry-lot feeding system, where the inclusion of feed additives in the diet plays a key role in improving growth, feed efficiency, stress response and health. The objective of this study was to evaluate the impact of substituting sodium monensin as a feed additive with a natural yeast derived product (Celmanax™; Church & Dwight Co., Inc.; Princeton, NJ) on performance and stress response in early weaned calves in a dry-lot feeding system. Thirty-two male castrated Hereford calves, born in spring 2024, were weaned at 71 ± 10 days of age and 88 ± 12 kg body weight (BW). Calves were stratified by BW and randomly assigned to one of the two treatments: 1) total mixed ration with 17% whole grain oats as a source of fiber (TMR without long-size fiber) with the inclusion of sodium monensin (50 mg/kg) as a feed additive (MON; n = 16); and 2) the same TMR, replacing sodium monensin with a natural yeast derived product as a feed additive top dressed (7 g/animal/day) (CEL; n = 16). Calves were placed in individual semi-roofed pens equipped with a feeder and a water trough, and received dietary treatments for 73 days, including a 17-day adapting period. Feed was offered in 2 equal meals a day, adjusting the amount offered every two days to ensure ad libitum intake. Dry matter intake (DMI, kgDM/d) was measured daily as the difference between feed offered and refusals; body weight was recorded every 14 days without fasting, and feed efficiency (FE, kgDM:kgBW) was calculated based on DMI and average daily gain (ADG, kgBW). Two blood samples were taken from the jugular vein, at the beginning and at the end of the experimental period, for plasma cortisol level determination. During the same weeks, fecal samples were collected on three consecutive days to estimate total tract apparent digestibility of dry matter (DM) and organic matter (OM), using acid-insoluble ash as an internal marker. The data was analyzed in a completely randomized design. The inclusion of CEL as a substitute for MON, allowed to achieve the same DMI and ADG, reaching the same final body weight (P ≥ 0.316); and resulted in a better stress response, or better adaptation, due to a lower plasma cortisol level at day 73 (MON: 1.7238 μg/dL and CEL: 1.3225 μg/dL; P = 0.049). However, the FE achieved in MON was better (3.57 vs 3.80 CEL, P = 0.038), accompanied by greater digestibility, of both DM and OM (P < 0.0001), and lower water intake (P = 0.041). Despite this, the substitution of sodium monensin with a natural feed additive is considered feasible since the performance of calves was not affected and an apparent better stress response was achieved, even when calves were fed with a highly concentrated TMR without long size fiber in a dry-lot feeding system.