1. Introduction
Bovine milk in Italy is mostly intended for cheese manufacture and mostly comes from intensive Holstein farms. Intensive farming systems are characterized by the use of specialized dairy breeds, free stall barns, milking parlor or robotic milking, use of total mixed ration and no access to pasture. More than in extensive low-input production systems, female calves for replacement are an essential resource for intensive commercial dairy farms; however, the management in early stage of life of future cows is often suboptimal and there is room to improve care given to the young stock. Calves reared under suboptimal conditions may face several issues, which may regard growth performance, welfare, and health. For example, an impaired feed intake at the early age of calves could affect productive and reproductive aspects of the future lactating cows. Moreover, some aspects of cows’ performance are primarily related to the age at first calving, which in turn is influenced by body weight (BW) and growth rate [
1].
Appropriate feeding strategies and plans adopted in the early life of replacement calves can be translated into a reduction of the age at first insemination, ideally around 15 months of age, and age at first calving, ideally around 24 months of age [
1,
2,
3,
4,
5,
6]. The availability of repeated measurements of the individual BW and growth rate is informative and useful to farmers in order to make informed decisions. Monitoring the BW of future dairy cows is useful for the optimization of insemination plans, for a better herd fertility in the long term, for the improvement of herd productivity in terms of milk yield, and for the reduction of metabolic diseases at the onset of first lactation [
7].
In a recent study, the effects of BW at birth, BW at weaning, and average daily gain on reproductive performance of Holstein heifers have been evaluated in a commercial herd [
8]. The number of services required for a positive diagnosis of pregnancy were greater (
n = 2.42) in animals with low BW at birth than those with medium (
n = 2.13) or high BW at birth (
n = 2.05), where low and high indicated BW at birth <36 and >39 kg, respectively. The percentage of heifers diagnosed as pregnant after first insemination was lower in animals with low BW at birth than those with high BW at birth. Therefore, it has been demonstrated that calves heavier at birth and at weaning calved earlier than lighter calves [
8]. Another study has reported a positive correlation between lifetime milk yield and BW of Holstein cows [
9]. According to literature, the genetic selection pursued for decades has led to a progressive deterioration of fertility in Holsteins worldwide [
10]; in fact productivity and overall fertility are genetically divergent traits. The correlation between the index for milk yield at 305 days of lactation and the index for fertility was −0.56 in a recent study on Holstein cattle [
11]. Nevertheless, very limited information is usually available from commercial farms due to practical difficulties in recording repeated information on calves. The aim of the present study was to evaluate the effect of BW and average daily gain (ADG, kg/d) in the early life of female Holstein calves on age at first calving and number of inseminations per pregnancy.
3. Results and Discussion
Descriptive statistics of the investigated traits are in
Table 2 and agree with results of published studies. In fact, according to the age, BW of Holstein calves should range from 59 to 61 kg (1 months of age), 80 to 86 kg (2 months of age), 102 to 111 kg (3 months of age), 125 to 136 kg (4 months of age), 147 to 161 kg (5 months of age), 169 to 185 kg (6 months of age), 191 to 210 kg (7 months of age) and 213 to 235 kg (8 months of age) [
12].
In a recent study on 57,868 data of Holstein calves, mean and standard deviation of BW
0 were 41.48 and 4.82 kg, and minimum and maximum values were 20 and 60 kg, respectively [
13]. Mean ADG observed in each phase (
Table 2) is comparable to findings available in the literature; in particular, the recommended ADG in dairy cattle in the first 30 days of life should vary from 0.68 to 0.82 kg/d in order to reach an optimal BW and body conformation at first calving [
14]. The average age at BW
1, BW
2 and BW
3 was 1.99 ± 0.29, 3.25 ± 0.38 and 5.77 ± 0.73 mo, respectively, and the observed minimum and maximum age were 1.19 and 2.83 months for BW
1, 2.53 and 4.54 months for BW
2, and 4.10 and 7.94 months for BW
3. All the investigated traits were normally distributed, except for NS (
Figure 2), which showed a positive skewness, as also reported by [
15].
The correlations of fertility with BW and ADG traits are summarized in
Table 3, and those between BW and ADG traits are presented in
Table 4. The NS and AFC were significantly correlated (0.68), suggesting that heifers that needed less inseminations to get pregnant tended to calve earlier compared to cows that needed more inseminations. Despite being weak, the negative correlations (
p < 0.01) of AFC with BW
0 and BW
1 highlighted that heavier animals tended to calve earlier than those with lower BW
0 and BW
1. In addition, the NS was weakly negatively related to BW
3 (
p < 0.05), suggesting that heavier animals need less inseminations to get pregnant than lighter animals. Findings are supported by [
16], who found negative genetic (−0.34) and phenotypic (−0.21) correlation between AFC and yearling weight in Brown Swiss cattle. All the BW traits were significantly positively correlated to each other (
p < 0.001), except for the association between BW
0 and BW
3 (
Table 4). The strongest association was estimated between BW
1 and BW
2 (0.52). Overall, correlations between the four ADG traits were significant (
p < 0.001) and moderate (
Table 4), with the strongest calculated between ADG
II and ADG
I-III (0.51).
The negative association of ADG
III with ADG
I and ADG
II suggested that animals with higher growth rate in the first weeks of age tended to show a slower growth rate afterwards. Despite this, ADG
I-III showed a positive correlation with the other three ADG (
p < 0.001), and it was negatively correlated (
p < 0.001) with BW
0. The BW
3 was significantly positively correlated with ADG
II, ADG
III, and ADG
I-III. These findings agree with previous research in Holstein cattle; in fact, a favorable positive genetic correlation between fertility traits and BW
0 has been observed in this specialized dairy breed [
13]. In particular, BW
0 was favorably genetically correlated with days open (−0.11), weaning weight (0.79), and weight at calving (0.50) [
13]. Potentially, it would be possible to work on low-heritable fertility traits by selection on the heifer BW, a phenotype available in the very early life of the future lactating cow and with exploitable heritability.
Results of the analysis of variance are summarized in
Table 5. The fixed effect of year-season of birth was significant for both AFC and NS in all models (
p < 0.01). By definition, fertility is a complex trait affected by several factors (e.g., feeding, season, genetics, and management) and their interactions; therefore, we expected a low coefficient of determination of the models (
Table 5). The least squares means of NS and AFC are presented in
Table 6. Significantly lower NS was detected in Class 3 of BW
3 compared to Classes 1 and 2, meaning that heavier calves were those with more favorable (lower) NS (
n = 1.32) compared to the others (
Table 6). Also, significantly greater (i.e., worse) AFC was estimated in Class 1 of BW
0 compared to Class 2, and in Class 1 of BW
1 compared to Classes 2 and 3 (
Table 6), meaning that lighter calves at birth had greater AFC compared to calves that were born heavier, and animals with low BW
1 had worse AFC (around +6 mo) compared to animals with medium or high BW
1.
A straightforward comparison of such findings with the literature was difficult due to a different definition of the phenotypes and to different observing periods considered. Despite this, results of the present study are generally in agreement with previous research. For instance, [
15] investigated fertility and growth performances in Holstein heifers and reported that greater BW was associated with lower (favorable) AFC. Similarly, it has been observed that animals that failed to conceive at 15 months of age were the lightest at 9 months of age [
17]. One could argue that lower AFC may lead to lower milk yield, because the development of the mammary gland is not yet completed and part of the energy available is still intended for cow growth [
1]; this explains why it is important to find the optimum between fertility and growth of the animals in order to identify the best compromise and the best economic return. For example, different specific nutritional strategies can be adopted to speed up cow growth and anticipate puberty, age at first insemination, and AFC. Obviously, increasing the nutritional input/energy content to overcome the requirement needed for maintenance in early life is translated into a greater ADG. Delayed puberty is observed in presence of ADG < 0.75 kg/d, the optimal for dairy heifers. On the contrary, nutritional deficiency and/or severe health issues in early life of calves are responsible for undesired consequences in the medium to long term [
18]. In cattle, puberty depends mostly on age and BW and usually takes place 6 weeks before the target breeding age; farmers should go for an AFC between 23 and 25 mo, i.e., the point that minimizes losses due to the non-productive period and that maximizes the cow fertility. In fact, according to [
18], the conception rate of Holstein cows is maximum (57%) between 15 and 16 months of age with a decline afterwards.