Feed Intake and Growth Performance
During the milk-feeding period, calves that were offered hay consumed a numerically lower DMI (96.2 g/d) compared to calves fed concentrate only (121.8, P = 0.14). Similarly, it has been reported that calves are likely to consume more concentrate than hay when provided as the sole feed [24, 25] or offered separately as free choice diet [26]. Xiao et al. [25] showed that calves that fed hay only consumed a negligible amount (6 g/d) of hay before weaning, which was lower than calves consuming concentrate (34.1 g/d). Castells et al. [26] claimed that when calves were offered concentrates along with different forages (straw, hay, and silages, respectively) ad libitum and separately, the consumption of forage was only 4 to 15% of total solid intake, while concentrate accounted for 85 to 96% before weaning. Previously, studies on effect of the flavor of feeds demonstrated that cattle are highly sensitive and have a strong preference for sweet tastes [2, 27], which might partially explain the bias towards higher consumption of DM in calves offered concentrate as opposed to hay in the current study. On the other hand, given that newborn calves have a small and nonfunctional rumen [28], the bulkier forage is more likely to fill the rumen rather than the highly fermented concentrate, which might limit the forage consumption and resulting in further intake of hay before weaning.
Numerous studies have claimed that concentrate, high in rapidly fermentable carbohydrates, have a greater ability to improve the rumen papillae development than hay [29, 30]. Therefore, compared with offering hay as solid feed, feeding concentrate during the pre-weaning period is more likely to increase BW and ADG as a result of greater nutrient absorption in rumen [31, 32]. In contrast to previous studies, calves offered concentrate had similar BW and ADG to calves offered hay in current study. It was noteworthy that the DMI was only 121.8 and 96.2 g/d before weaning for calves offered concentrate and hay, respectively, which was quite low when compared with other studies [30, 33]. Hill et al. [33] fed their calves a total of 120 L of milk (3.9 L/d) and 850 g/d of concentrate starter during the milk-feeding period. Castells et al. [30] observed that calves consumed 214 L (3.8 L/d) of milk and 600 g/d of solid feed. The lower DMI of solid feed in current study was probably due to relatively high levels of milk (376 L in total, 6.7 L/d) fed to calves, which met the daily nutritional and energy requirements hence limiting the solid feed consumption [34], thus, the effect of different feed sources on BW and ADG might have been masked.
Temperature, fecal and cough score
Temperature and days with fever did not differ across treatments. Furthermore, similar body temperatures were recorded in pre-weaning and post-weaning. These findings are in line with Jahanimoghadam et al. [35] who did not notice any detectable effects of feeding forage on body temperature. In similar with current study, most studies claimed that early inclusion of forage in the diet at various levels did not affect the fecal consistency and cough score [36–38]. However, a few studies reported that increasing dietary fiber intake decreased calves fecal score [39] and cough score [35], which might be related to fewer abnormal behaviors when forage is supplemented in calves diet [40, 41]. Although controversy remains on whether different solid feed affect fecal score or not, however, it has been well documented that time significantly influenced fecal score. In agreement with previous studies [42, 43], we documented that the fecal score increased from birth to week 3 before declining and reaching nadir before weaning. This is probably because calf’s passive immunity obtained from colostrum gradually decreased and its own immune system was not yet well developed [44], and thus reduced the calf’s resistance to disease during this stage.
Feed Sorting
Given that the feed particles of concentrate are mainly observed in short and fine fractions and the feed particles of hay are mostly distributed in long and medium fractions, the long and fine particle fractions of TMR to greater extent represent hay and concentrate, respectively [6]. After transitioning to a mixed diet, differences between treatments were observed in feed sorting. Calves fed concentrate early were in favor of selecting short and fine particle fractions of novel TMR, which were primarily concentrate, while those fed hay preferred long particle fractions, which were mainly hay. These results indicate that calves begin to sort the feed immediately following transition to a mixed diet, and they prefer the feed component they were initially familiar with. In agreement with current study, previous studies also revealed that feed sorting behavior was observed in both milk-feeding [45] and weaned calves [46], suggesting that feeding and sorting behaviors are established early in life even though the brain and digestive system are not fully developed at this stage. It was not surprising to find that calves were in favor of feed components which they had been exposed to earlier. Previous evidence demonstrated that early feed experience with different feed flavor [15] or physical shapes [47] could influence the feeding behavior in ruminants later in life. On the other hand, food neophobia which is defined as avoidance of unfamiliar foods, and helps animals to avoid toxic foods [48] is well known in animals [49] and humans [50]. Recently, Costa et al. [51] observed that food neophobia to unfamiliar feed in calves was initially present, whereby calves were hesitant and reluctant to taste a novel feed and consumed only small amounts. Therefore, a period of neophobia might encourage calves to sort against the novel feed component for which they have not been exposed to early in life. Similar to our results, Miller-Cushon et al. [24] showed in their study that the early feed experiences would affect the feed selection in later stages, calves previously exposed to concentrate sorted against long particle fractions (56%), while those exposed to hay sorted against fine particle fractions (89%). A similar trend was noticed in the current study for calves offered concentrate and hay sorting against long and fine particle to the extent of 97.5 and 95.9%, respectively. A more severe sorting behavior was found in Miller-Cushon’s study compared to this study, which was probably because of the differences in TMR. It is well documented that ration characteristics greatly influence feed sorting [52]. The higher DM content (90.7 vs. 56.0%) [12, 13] and higher forage particle size (Long particle fraction: 67.9 vs. 32.0%) [6] in ration of Miller-Cuhson’s study than ours might aggravate the degree of feed sorting. Besides, the area occupied by feed might also affect ability of cattle to sort. While cows are more capable to pushing feed far and wide on an open and flat space, it is more difficult for them to increase sorting behavior in a more enclosed feed bunk [52]. One rectangular bin (diameter = 80.6 cm) was used in the study done by Miller-Cushon et al. [24], while we offered feed in two separate circular buckets (diameter = 25.0 cm) for each calf in the current study. Hence, less feeding space might have limited the extent of sorting. However, no empirical data of calves are available to support this point of view; further research is encouraged to investigate the effect of feeding space on feed sorting.
In contrast to a previous study [24], no carryover effects on feed sorting were found in this study. After feeding on TMR for a long period, treatment effects seemed to disappear during d 190 to 196. All calves began to sort for short and fine particle fractions, and sort against long and medium particle fractions. To our knowledge, this is the first study to investigate the effect of early feed experience on feed sorting beyond 6 month of heifer life. It is possible that dietary experience would eventually override previous feed experience. Given that only a small sample size (n = 4) was used in the previous study [24], it was worth noting that a greater number of replicates per treatment were used in the current study (n = 20 calves/treatment during d 57 to 70; n = 10 pairs/treatment during d 190 to 196), which gives us more statistical power to support the long-term observations made. However, our results should also be interpreted with caution because calves were grouped in pairs after d 70. Group-feeding system might increase the feed sorting behavior because of the increased competition and social pressure [52]. Leonardi and Armentano [53] found that cows fed in a group free-stall setting had a more severe extent of feed sorting than cows individually fed in a tie-stall setting. Hosseinkhani et al. [54] reported no differences in the extent of feed sorting when comparing cows fed in a non-competitive (1 cow/bin) to those fed in a competitive group (2 cows/bin). These results suggest that the effect of social environment on feed sorting is still a controversial area of study and to the best of our knowledge, there are no calf-based studies in this area. Hence, further research is encouraged to determine whether social environment has an effect on the extent of feed sorting in calves.
Rumen Fermentation and the incidence of SARA
It was not surprising to observe that pre-weaning calves fed concentrate had a lower rumen pH and a higher total VFA concentration than calves fed hay early. In line with the current study, previous studies claimed that concentrate, high in rapidly fermentable carbohydrates, stimulated the rumen fermentation and produced more VFA, especially propionate and butyrate [55]. It is well documented that rumen total VFA has a linear negative correlation with rumen fluid pH [40], indicating that higher VFA concentrations might result in lower pH value. Inclusion of forage in calf diet increases the ruminating and chewing activity [56], which further enhances saliva production that can neutralize VFA, and consequently improve the buffering effect resulting in higher rumen pH [1, 57]. When all calves were transitioned their respective pre-weaning feed to the same TMR, a variation occurred between treatments, whereby the rumen pH increased in CON group and decreased in HAY group from d 56 to 58, respectively. These changes could be attributed to the consumption of hay and concentrate by CON and HAY groups, respectively when TMR (containing 43% of hay and 57% of concentrate) were introduced to calves post-weaning.
Previous studies demonstrate that feed sorting (for fine particles and against long particles) was often observed in dairy cows [52], which resulted in greater consumption of fermentable carbohydrates and lesser consumption of physical fiber than intended [6], and this type of sorting was associated with reduced rumen pH [7] and the risk of SARA [8]. As stated above, calves were found to sort for the different feed particle fraction when TMR was offered. However, in contrast to our hypothesis, the treatment difference in feed sorting did not bring about any differences in rumen fermentation. After changing the diet to same TMR, no treatment effect on rumen pH was found at d 58, 63 and 70 of age. This could be partially ascribed to the limited sorting behavior observed in the current study, which might result in small differences in the intake of concentrate or physical fiber between treatments. However, no empirical data is available to support this hypothesis; further research is needed to further explore the relationship between feed sorting and rumen pH in calves. Furthermore, the lack of difference in rumen pH might relate to the method of feed presentation. In our previous study, Xiao et al. [25] observed concurrent differences in feed preference and rumen pH, when concentrate and hay were offered separately after transitioning to new feed to allow calves to leisurely choose between feed components. Whereas, total mixed ration was used to test the effect of early feed experience in the current study, given that the main purpose of feeding TMR is to encourage the consumption of a balanced ration and prevent feed selection, sorting could be more easily accomplished in a free-choice diet as opposed to a mixed diet [3], which might minimize the effects. Although no differences were found in rumen pH, it was worth noting that a numerically higher number of calves might suffer from SARA (pH < 5.8) at d 58 and 63, probably due to the effect of feed sorting. In line with the current study, DeVries et al. [7] continuously tested the rumen pH and demonstrated that most cows selecting against long particles had the lowest maximum rumen pH, which might aggravate rumen acidosis. Feed digestion and fermentation in rumen is a dynamic process; the single time point rumen fluid sampling in the current study may mask the effect of different patterns of sorting. Continuous rumen pH measurements might allow for precise and accurate detection of duration and severity of SARA [22, 58]. For example, Yang et al. [58] claimed that the time that pH less than 5.8 (the threshold for SARA) almost doubled (7.9 vs. 4.1 hours/day) in cows fed finely rolled diet rather than coarsely diet even when mean pH (6.1 vs. 6.0) was not significantly different. Therefore, increasing sampling time points or utilizing an indwelling probe system to continuously measure rumen pH is encouraged.
After a long-term (6 months) transition to TMR, all calves sorted for fine particle fractions and against long particle fractions. Possibly because of the similar sorting pattern between treatments, calves fed concentrate early had similar rumen pH and VFA concentration at d 190, and no calf had rumen pH below 5.8. These results suggest that early feed exposure may have the capacity to regulate feed sorting in the short term, but long term the effects fade away, and rumen fermentation could not be affected even when a diverse variation in feed sorting occurred in both CON and HAY groups. These outcomes showed that the characteristics of the feed itself may play a much more important role in feed sorting and rumen fermentation than early feed experiences.