None of the calves developed diseases severe enough to be a reason for exclusion from the study. The descriptive values of the duration of the calves' behaviors are reported in Table 3. During the experiment, some behaviors were identified as belonging to some specific timings. In general, resting and ruminating were the two main behaviors performed by the calves throughout the day. The ingestion behavior was observed mostly during the day. The calves spent 8.4% of their time eating solid food in the late morning and 9.9% in the late afternoon after the administration of the two meals (6.00 a.m. and 5.00 p.m.). As regards the resting behavior, both sternal and lateral recumbency appeared predominantly during the night when the calves just lay down inside the pen. Sternal recumbency was observed in the late morning (38.6%) and early afternoon (40.5%), especially in the evening (53.1%) and during the night (45%). On the other hand, standing resting was barely observed in the evening (5.9%) and night (2.1%), as the calves preferred to rest in the recumbent position. The most common ruminating behavior was sternal ruminating, occurring mainly during the night (22.9%), where the animals remained in sternal recumbency. The environmental interaction of the calves was mostly represented by olfactory investigation and licking. Olfactory investigation toward the other pen mates was observed mainly during the late morning (6.4%) and late afternoon (9.7%). Generally, this behavior occurred at the end of the meal as did licking. The negative interaction behavior in which calves spent the most time was cross-sucking, usually performed in the early morning (1.3%), before the first meal, and in the late afternoon (1.8%) after the evening meal. The other negative behaviors were not strictly linked to a specific time slot but were displayed throughout the day. A few stereotypies were also observed in all the time slots considered.

The mixed-model analysis (Table 4) shows the incidence of the main effects (diet, sex, and timing) and their interactions for each target behavior (Table 5). Diet-influenced (P < 0.05) behaviors include eating liquid, running, cross-sucking, licking, olfactory investigation, and mutual grooming. Sex influenced (P < 0.05) eating liquid, sternal recumbency, lateral recumbency, sternal ruminating, moving, running, licking, and sexual behavior. Timing influenced (P < 0.05) all behaviors.

Figures 2–6 report the most relevant interactions among the main effects that were significant (P < 0.05). The inclusion of 3% and 6% of HSC significantly increased the time spent running and licking (Figure 2) and decreased the amount of time carrying out olfactory investigation and cross-sucking (Figure 3). Both sexes expressed more time cross-sucking during the early morning and late afternoon, but mostly in the latter after the second milk administration. Male calves reduced the time spent on cross-sucking as the HSC inclusion increased in the diet. For female calves, this reduction only happened for the T3 group. Meanwhile, the T6 and CTR groups were not influenced by the diet (Figure 3). Females spent significantly greater time than males eating liquid, resting, ruminating in sternal recumbency, moving, running, and licking, whereas males spent longer duration in lateral recumbency and sexual behavior. The CTR group spent more time eating solids in the late morning than T3 and T6 (Figure 4). Calves that received the diet with the HSC inclusion finished all the solid feed right after the first meal, whereas the CTR group always had some leftovers in the feeder, and they tended to eat more slowly. The diet did not influence the positive interaction behavior, but sex and timing did (Figure 5). This behavior was observed, in particular, during the late afternoon. For this time period, male calves decreased the time of positive interaction inside the pen when the HSC was included in the diet, whereas the same effect was not noticed in the female calves, which spent the same amount of time interacting positively regardless of the diet. During this trial, both male and female calves showed a different time budget for movement behavior (Figure 6). Even though both female and male calves were more active during the late afternoon, the HSC inclusion had opposite effects on the different sexes. Female calves of T3 and T6 spent more time (P < 0.05) on movement than those in the CTR group. On the contrary, male calves decreased (P < 0.05) their time spent on movement when the HSC inclusion increased in the diet. Although sexual behavior was the least noticed of the positive interaction behaviors, there was a statistical difference between sexes (Figure 7), showing that male calves expressed more sexual behavior than females, mostly in the late afternoon.