Efectos de formas alternativas de transición corral-pasto durante primavera en terneros Hereford manejados a corral (ADT) durante invierno

The trial was conducted to evaluate the effect of transition method (gradual vs. abrupt) when calves are moved from winter confinement to spring pasture on performance and grazing behavior. Thirty Hereford heifers with an average weight of 337.5 ± 32.6 kg were used, originating from the experimental herd of EEMAC born in the spring of 2020, fed in confinement during winter with a total mixed ration without long fiber. The trial lasted for 41 days, from November 16th to December 27th, 2021.

Calves, grouped by diet type in the pen, were assigned to two treatments based on the feeding strategy at the exit from the pen to pasture in spring: gradual transition system (STG) and abrupt transition system (STA). During the first 6 days of the trial, STA animals remained confined with the total mixed ration as the sole feed, while STG animals also stayed in the pen receiving the same ration, but additionally went out to graze a pasture with a forage allocation of 6 kg DM/100 kg live weight, starting with 2 hours of grazing on the first day and increasing by 2 hours daily. From day 7 onwards, both treatments left the pen to graze the same pasture as their sole feed in spring. Management was on a weekly strip grazing basis with equal forage allocation.

Variables studied during the trial were average daily gain (ADG) estimated by regression from live weight recorded every 7 days without prior fasting, ruminal pH, and ingestive behavior. ADG for the 41-day evaluation period was not affected by treatment (0.629 vs. 0.581 kg/day for STG vs. STA respectively, P=0.196). Both treatments showed weight loss in the first week of grazing (-0.700 kg/day vs. -2.87 kg/day, p<0.01 for STG and STA, respectively), which reversed from the third week of grazing onwards, resulting in no differences between treatments in ADG (1.16 kg/day vs. 1.11 kg/day, P=0.504, for STG and STA, respectively).

No significant differences in ruminal pH were observed due to treatment on day 1, whereas on day 7, pH was lower for STA (p<0.05). From day 14 onwards, no differences were observed due to treatment or day of measurement.

During weeks of grazing together in the pasture, there were no differences in ingestive behavior between treatments. Grazing was the main activity occupying most of the time. Over time, there was a decrease in resting time and an increase in rumination time.

These results indicate a 14-day transition phase, characterized by a negative residual effect on ADG after leaving confinement for pasture, occurring regardless of the transition method.