The first aim of the present study was to investigate the effects of a complete TMR ration for healthy calves focusing on their growth performances, intake, and fecal characteristics. All of the parameters considered in the present investigation have a direct effect on calves’ intake, growing, health, and welfare.
3.1. Feed Intake
Regarding milk intake, no differences were found among the calves, who all finished their meals without leaving any waste. Additionally, milk intake observed at different timepoints was consistent with that observed in other studies [
24]. As expected, TMR intake (TMRI,
Table 3) increased as the animals grew older and the weaning time approached (
Figure 1). Indeed, during the first days of the trial, the average value was very low (
Figure 1), and only after 27 days did it reach and exceed 100 g. After this period, TMRI gradually increased as weaning approached (
Figure 1). A similar trend was observed by Welk et al. [
25] in calves fed with forage-based TMR. Some authors [
26] have observed that early TMR administration to young livestock results in better ration consumption, reducing sorting against forage and for concentrates. According to Groen et al. [
27], feeding heifers with a TMR from a young age not only has immediate behavioral effects but may also have longer-term benefits. For example, Xiao et al. [
28] reported that early-life feed experience may affect the development of feeding behavior in female Chinese Holstein calves. Moreover, their study reported that early exposure to various feed sources and presentations may affect calves’ preferences, basically solving feed sorting problems.
Furthermore, calves, as lactating animals, have a digestive system similar to monogastric animals such as horses. As a result, a significant portion of the solid feed they consume bypasses the rumen. This characteristic has a limited effect, making it appropriate to exclusively provide calf feed. However, when concentrate feed is their sole source of sustenance, the hay is separated, leading to considerably lower fiber intake compared to more appealing feed options [
29]. Our research suggests that incorporating a total mixed ration (TMR) could improve overall fiber intake compared to separate feeding methods. Therefore, we recommend implementing a slightly earlier weaning process and facilitating a gradual transition from monogastric to ruminant feeding.
Similarly, Miller-Cushon and DeVries [
30] previously reported that calves exposed solely to concentrate tend to select short grain particles, while those exposed only to hay tend to select longer forage particles. Another example of this was reported by Greter et al. [
26], who observed that heifers previously fed a top-dressed ration maintained similar feeding patterns (whereby they were slug-feeding their concentrate) after they were switched to a silage-based TMR for a period of 7 weeks.
3.2. Fecal Output
Table 4 shows the results related to the characteristics of the feces (pH and chemical analysis). Fecal pH is a crucial parameter for evaluating the digestive health of dairy calves since it can provide insights into the fermentation activity in the rumen and hindgut and allow for adjustments to be made to the diet accordingly. In the present study, fecal pH increased with animal growth from 5.40 on day 7 to 7.06 on day 42. This rise was justified by the increase in TMRI due to animal growth. Similar trends were observed by Kodithuwakku et al. [
31] in female Holstein calves ranging from 7 to 49 days of age who were fed a solid diet from their first week of life. Therefore, monitoring fecal pH in dairy calves is an essential method for assessing their digestive health. Khorrami et al. [
32] suggested that the fecal pH of dairy calves should be maintained within the physiological range of 6.0 to 7.0. In our study, values within this range were observed after the weaning period (6.23 at 63 days; 6.42 at 77 days). Regarding the color of the calves’ feces (
Table 4), some variability among calves was observed only in the samples taken at 7 days of life, which may be due to differences in milk digestion capacity. However, all samples showed a brown coloration after day 7. Overall, there were no statistically significant differences in fecal color between the observations in younger and older calves (
p = 0.35). No undigested feed particles were macroscopically visible during sample manipulation, processing, and analysis.
Regarding fecal consistency (
Table 5), there was a clear prevalence of solid feces (71–78%) up to 63 days of life. At 77 days, solid feces were 56% and liquid feces were 44%. This may be due to the adaptation of the digestive tract to solid feed during weaning. However, there was no statistically significant difference in consistency between younger and older calves (
p = 0.75).
In the present study, DM and CP content in feces (
Table 4) decreased from 7 to 77 days of age (DM: from 20.1 to 14.0%; CP: 45.0 to 18.5%). Regarding fiber fractions, aNDFom, ADF, and ADL increased from the start to the end of the trial (e.g., aNDFom ranging from 11.2 at 7 days to 45.1 at 77 days). Fecal starch levels decreased during the same period from 2.3% to 1.1%. Lastly, the average ash content fluctuated, showing a minimum value of 6.51% at 63 days and a maximum value of 9.77% at 42 days of age. These results should be interpreted with caution, however, as the limited number of studies available on calves’ feces make it difficult to draw general conclusions.
3.3. Growth Performances
As depicted in
Figure 2, the average birth weight of the calves that partook in the investigation was 37.14 ± 2.72 kg (range 95%: 34.82–39.46). This result is consistent with the scientific literature on specialized dairy breeds calves, including Holstein calves [
33,
34,
35]. According to Renaud et al. [
36], calves with a low body weight at birth have a greater risk of dying within 21 days and during the entire growing period [
37,
38]. Conversely, a higher body weight at birth has been associated with a lower incidence of respiratory disease and diarrhea in the two weeks following arrival on farm after transportation [
39]. Rot et al. [
40] also reported that greater body weight at birth was one of the most consistent predictors of future mortality and morbidity at calf-rearing facilities. In the present study, the average weight at 77 days (post-weaning period) was 84.72 ± 8.90 kg (
Figure 2). Therefore, the average ADG during the trial was 0.618 ± 0.04 kg. The calf weight values in our study, which were measured at specific timepoints, were higher than those reported by Seifzadeh et al. [
41] for replacement Holstein calves.
Looking across the different timepoints considered in the present study (
Figure 2), the average weight gain in the first week was 3.50 ± 1.91 kg. During the pre-weaning periods (between 7 and 42 days of life), the average weight gain was 17.39 ± 3.67 kg; during the weaning phase (from 42 to 63 days of life), the average weight gain was 15.17 ± 3.74 kg, and in the post-weaning phase (from 63 to 77 days of life), the average weight gain was 11.52 ± 4.04 kg. The average ADG observed during the trial was 0.618 ± 0.125 kg. This result was similar to that observed by other researchers, such as that observed by Thomas et al. (0.61 kg/d) [
42]. In the present study, ADG was 0.50 ± 0.27 kg during the first week, 0.49 ± 0.10 kg during the pre-weaning period, 0.49 ± 0.18 kg during the weaning phase, and 0.82 ± 0.29 kg during the post-weaning period. The ADGs observed are consistent with the range (from 0.3 to 1.3 kg/d) observed by Soberon et al. [
43] for Holstein calves.
3.4. Preliminary Development of a Prediction Model: Practical Applications and Limitations
The data collected in the present investigation was plotted on a graph with a corresponding polynomial regression curve. The estimation equation was obtained using the method described in the statistical analysis section, which allows for the estimation of calf weight and feed intake at a specific age under similar conditions to those of the present study.
The equation obtained shows high R
2 (0.98 for solid feed intake and 0.99 for calf weight;
Table 6), indicating a good ability to predict solid feed intake and body weight. High accuracy (R
2 > 0.9) of equations is the most important measure of goodness of fit and represents a model’s ability to predict actual values [
44,
45]. A similar result (R
2 > 80) was reported by Guinguina et al. [
46] for the prediction of feed intake and feed efficiency in lactating dairy cows using digesta marker techniques. Furthermore, the R
2 values obtained in the present study were higher compared to those reported by Silva et al. [
45], who reported a R
2 value lower than 0.75 for the equation used to predict starter feed intake in pre-weaned dairy calves. It is important to highlight that the findings of Silva et al.’s study [
45] revolve around a model crafted using data from numerous studies conducted across various farms. Their study specifically centered around a distinct feeding regimen that aimed to model the intake of starter feed. This distinction is significant, given that the composition of this starter feed diverges in terms of its compositional attributes from the feed regimen we employed in our investigation. Furthermore, it is worth noting that although Silva et al.’s study involved a larger number of animals, their primary focus was on male subjects. Additionally, their study encompassed animals of different genetic types (crossbreeds), meaning that it was solely concerned with Holstein breeds. This diversity in both gender and breed poses a challenge for making direct comparisons, as the characteristics of the subjects are not entirely analogous. This intricate interplay of varying factors underscores the complexity of drawing direct parallels between their findings and our study results.
The model’s predictive efficacy could represent a useful tool for farmers and on-farm advisors, particularly in its application to the management of upcoming heifers. For this reason, the equation developed in the present study could be readily integrated into a decision support tool to help dairy calf management support farmers to select the best heifers to include in future breeding stock. Indeed, the use of these equations can enhance farm management, improving the welfare of reared dairy calves by allowing for the early identification of discrepancies in feed intake or growth.
Regarding the use of TMR in calves, feeding management during weaning and pre-weaning is essential to promote optimal growth, animal welfare, and future production [
47,
48]. Indeed, colostrum and milk administration are crucial for the growth and survival of calves [
49]. However, the early supplementation of solid feed is crucial to stimulate the development of a functional rumen [
50]. In addition, early intake of solid feed before weaning enhances calf growth, also increasing their chances of survival [
24,
51] and, consequently, their stayability [
52]. Calf welfare may also benefit from this situation; in fact, calves consuming solid feed earlier are less likely to experience hunger in the first few days after weaning [
53]. For this reason, usually, starter feeds have low fiber and high grain and legume contents to improve palatability [
54]. However, a concentrate-rich diet is associated with some negative effects on calf health, such as impaired rumination and salivation, which in turn affect the digestive processes in the rumen and the overall development and health of the gastrointestinal tract [
4]. Therefore, early forage provision (hays in particular) is accepted and largely recommended [
55]. Calves with limited access to fiber may show rumen under-development, the presence of rumen plaques, and hyperkeratinisation, as well as abomasal lesions (ulcers), mainly in the pyloric region [
56]. Moreover, hays could potentially be used to offset feed costs [
57], although it should be remembered that calves have low ruminal capacity and particular nutrient requirements, so forage feeding should be carefully managed [
58]. According to Groen et al. [
27], feeding TMR provides a useful approach to promoting balanced nutrient intake within the day, also reducing feedbunk competition in animals raised in groups. Furthermore, TMR could be used to increase the use of feed produced on farm, contain feed costs, and maximize farm efficiency [
59].
Overall, an interesting practical implication of this study is its relevance to the management of “surplus” calves within a farm, encompassing both economic considerations (animal potential and use of TMR) and welfare concerns (proper health and development of the future milking herd). In dairy herds, many calves are born every year without the need to replace the milking herd, and these are commonly referred to as surplus animals [
60]. The majority of surplus calves are males [
59]. However, as reported by Bolton and von Keyserlingk [
60], the increasing utilization of sexed semen, combined with the growing demand for crossbreeding, has led to an increasing percentage of these excess calves being female. All these calves are sold at an early age for different purposes, including for veal or dairy–beef crossbred production [
61]. As reported by Berry and Ring [
62], certain dairy farmers tend to rear their surplus progeny after weaning. Selling these animals can provide a valuable cash injection during periods of financial strain. Using the data obtained in the present study, farmers could produce heavier (and consequently better-conformed) calves [
63], resulting in heavier and higher-value primal carcass cuts if they are male or in heifers with better body conformation. These possibilities could be particularly interesting for heifer rearing, since studies [
64,
65,
66] report a strong relationship between conformation traits and lifetime production efficiency.
We hope that this study can help towards the enhancement of farm management and dairy calf welfare through the early identification of feed intake or growth discrepancies. The early provision of solid feed supplementation aims to foster functional rumen development and improved growth and survival rates. The proposed equation can support decision-making, guiding both optimal heifer selection for future breeding stock and the management of surplus calves, potentially yielding economically beneficial outcomes. Overall, our findings show that administering hay-based TMR to Holstein Friesian heifer calves results in unaffected growth rates, feed intake, and fecal characteristics, providing valuable insights for calf rearing practices.
However, some limitations need to be acknowledged. First of all, it is obvious that the productivity of a dairy herd does not solely depend on calf management; instead, it is the result of a complex number of factors (e.g., culling policy, breed, lactation characteristics, milking management, veterinary input and herd health, farmer attitude, etc.). Therefore, our study specifically focused on calves’ projected potential. In addition, this preliminary model was developed on a single experimental group at one farm (female Italian Holstein calves); therefore, its generalizability remains to be assessed in multi-farm studies and meta-analyses and is yet to be externally validated. Recognizing these limitations is crucial to properly contextualize the study and the research questions that need to be investigated in order to gain a more comprehensive understanding of the impact of hay-based Total Mixed Rations (TMR) on calf development.