Respiratory system pathologies that lead to pneumonia constitute a chronic endemic disease in dairy calves in intensive systems worldwide (Buczinski et al., 2021). This disease is one of the main causes of livestock losses in dairy farming when considering production performance (Windeyer et al., 2014; Jourquin et al., 2023), animal welfare (Barry et al., 2020), the use of antimicrobials (Bokma et al., 2020) and consequently antimicrobial resistance (Zhang et al., 2022). This disease is a significant problem in dairy operations where calves are raised artificially, as it causes high morbidity and mortality rates (Dubrovsky et al., 2019), increases farm costs related to treatment and prevention (Hyde et al., 2022), and potentially affects the future performance of animals (Dubrovsky et al., 2020; Buczinski et al., 2021).

Likewise, gastrointestinal tract diseases manifested in diarrhea are frequent in preweaning dairy calves and are the main cause of death in the pre-weaning period (Urie et al., 2018). Diarrhea in the preweaning period is associated with adverse effects on subsequent reproductive performance (Aghakeshmiri et al., 2017), increased antimicrobial use (Zhang et al., 2022), and long-lasting economic impact, including a reduction in average daily gain (ADG) (Anderson et al., 2003), an increased services per conception, and a reduction of around 300 kg in milk production during the first lactation (Abuelo et al., 2021).

Previous research on calfhood bovine respiratory and enteric diseases has been linked to reduced ADG and future cow performance. These studies have been carried out in cooler environments with comfortable calf sheds. Diarrhea and pneumonia can vary by climatic conditions, herd size, and farm management practices (Cho & Yoon, 2014; Maier et al., 2019). Therefore, it is pertinent to determine the impact of preweaning pneumonia and diarrhea on preweaning weight gain and subsequent fertility in heat-stressed calves because calves experiencing thermal stress during their development reduce their feed intake (López et al., 2018), increase energy needs, weaken their immune function (Gupta et al., 2023) and increases their susceptibility to diseases (Bagath et al., 2019).

The present study investigates the null hypothesis that preweaning growth rate and reproductive metrics among artificially-reared Holstein calves do not differ between calves suffering diarrhea or pneumonia from healthy ones. This study aimed to determine the impacts of pneumonia and diarrhea from birth to weaning on preweaning ADG of Holstein calves raised in a commercial dairy farm in a hot environment. Additionally, the study examined the effects of birth weight on calf health and the occurrence of diarrhea and pneumonia from birth to weaning on the reproductive performance of Holstein heifers.

Overall, 58.2% of the enrolled calves suffered from a disease, and 4.5% had concurrent diseases. Among the 7,809 calves in the present study, 1,680 (21.5%) had diarrhea, and 2,511 (32.2%) had pneumonia. Mean ± Standard Deviation birth weight was 37.6 ± 2.8 kg and ranged from 34.0 kg to 47.0 kg. The median (50th percentile for the set of values) time to diarrhea and pneumonia was 8.0 and 31.0 d, respectively.

The growing performance of calves from birth to weaning according to their health status is presented in Table 1. When evaluating the effects of diarrhea, pneumonia, or the concurrence of these conditions on preweaning ADG, it was found that calves suffering from these diseases had lower (p<0.01) preweaning weight gain than their healthy counterparts. Calves presenting both diseases before weaning had a lower ADG than calves that had only one disease. Likewise, weaning weight was greater (p<0.01) in healthy calves than in their peers with diarrhea, pneumonia, or both illnesses. Simultaneous presence of these medical conditions was associated with lower (p<0.01) weaning weight than calves suffering a single illness.

The reproductive performance of heifers based on diarrhea and pneumonia occurrence is presented in Table 2. Days to first insemination were eight days shorter (p<0.01) for healthy heifers than those with diarrhea. Likewise, days to first insemination were seven days shorter (p<0.01) for healthy heifers than for those with pneumonia. Heifers with co-morbidity of these diseases required 13 more (p<0.01) days for their first service.

Calves suffering from diarrhea, pneumonia, or the concurrence of these diseases had a greater (p<0.01) number of services per pregnancy than healthy calves before weaning. Days to pregnancy were lowest (p<0.01) in heifers without diarrhea or pneumonia and highest in those who suffered diarrhea comorbid with pneumonia. The occurrence of diarrhea or pneumonia in the preweaning period reduced (p<0.01) the pregnancy rate at first service by 10 percentage points compared to healthy heifers. Co-occurrences of diarrhea and pneumonia in the preweaning period decreased (p<0.01) pregnancy rate at first service by 22 percentage points compared to heifers suffering from single diseases.

The overall pregnancy rate was lower (p<0.01) in heifers having diarrhea, pneumonia, or comorbid conditions than in healthy heifers in the preweaning period. Days to calving were 9 days shorter (p<0.01) in healthy calves than heifers suffering diarrhea or pneumonia in the preweaning period. A comorbid condition was associated with 15 more (p<0.01) days to first freshening than heifers with only diarrhea or pneumonia.

Lighter calves at calving were more likely (p<0.05) to present diarrhea than heavier calves (Figure 1). However, birth weight did not affect the occurrence of pneumonia. The survival analysis for age when a calf was diagnosed with diarrhea or pneumonia (Figure 2) shows more reduced (p<0.01) time for calves suffering diarrhea (50th percentile = 8.0 days; SEM = 0.08) in the first 60 days of life, compared to pneumonia (50th percentile = 31.0 days; SEM = 0.68). After 25 days of age, the probability of diarrhea declined sharply, whereas pneumonia was more evenly distributed during the preweaning period.

The present study showed that the occurrence of diarrhea or pneumonia in the preweaning period resulted in lower daily weight gain, which is consistent with previous studies in dairy calves where abnormal fecal consistency (Schinwald et al., 2022), diarrhea (Pardon et al., 2013) or pneumonia (Bringhenti et al., 2021; Tomazi et al., 2023) decreased ADG. As expected, calves with co-occurrence enteric and respiratory disease events during the preweaning period gained less weight than calves with no disease events or a single infection. Still, results showed no association between underweight and comorbidity. Therefore, these data indicate that birth weight status is not a determinant of comorbidity of diarrhea and pneumonia.

Disease status is a significant factor influencing the growth rate of calves. Preweaning illness impacts the available nutrients in multiple ways. For instance, satisfactory nutrition is necessary for cells, including the innate response to an invading pathogen, to perform their functions optimally (Calder et al., 2020; Venter et al., 2020). The nutritional requirement further increases during infection as immune cells use direct and indirect approaches to combat those pathogens (Lee et al., 2020). For example, consuming enough nutrients is essential for optimal antibody production (Hamidianshirazi et al., 2022). Additionally, diarrhea contributes to low calf weight gain by causing nutrient malabsorption or nutrient loss from the diet via diarrhea (Smith, 2009), reduced feed intake (Morrison et al., 2019), and decreased feed conversion efficiency (Hessman et al., 2009; Khan et al., 2011). Additionally, it could be that slow-growing calves are at higher risk of diarrhea or pneumonia before weaning.

The preweaning ADG in the present study was much lower than that reported by other researchers on Holstein calves (Rosenberger et al., 2017; Lucei et al., 2021). The prolonged heat stress during the year in the present study potentially could explain the lower ADG in the study population (López et al., 2018). Low birth weight was associated with a higher incidence of diarrhea from birth to weaning. These results align with those of Fagundes et al. (2016), who observed that birth weight ≥ 40 kg in crossbred Holstein calves reduced the risk of diarrhea. Furthermore, it has been observed that higher weight in the first week of life is associated with less diarrhea (Windeyer et al., 2014). Low birth weight results from in-utero nutritional insufficiency, and such exposure early in life may have long-term implications for immunocompetence (Osorio et al., 2013). Therefore, this variable would help develop preventive measures and assess the prognosis of enteric infections for individual calves. Birth weight did not affect the occurrence of preweaning pneumonia, consistent with observations of Curtis et al. (2016) and Glover et al. (2019), who have reported negligible associations between birthweight and incidence of this respiratory disease.

The peak incidence of diarrhea in the present study was at eight days of age, which agrees with previous studies (Foster et al., 2009; Windeyer et al., 2014; Berber et al., 2021). Peak incidence of pneumonia in the present study was at 33 days, which is in line with previous studies where this respiratory tract disease becomes a critical health problem in replacement heifers older than 30 days of age (McGuirk, 2008).

In the present study, the age at first breeding was shorter in healthy heifers than heifers diagnosed with diarrhea, pneumonia, and co-occurrence of these diseases before weaning. The decision on at what age to start breeding heifers is based chiefly on the puberal weight of heifers. Healthy heifers in the present study grew faster than sick counterparts and, therefore, the association between diarrhea and pneumonia occurrence is probably because the absence of diseases from birth to weaning enhanced growth rate of heifers from weaning to puberty, which allowed heifers to attain earlier the appropriate weight for first service. The adverse effects of respiratory infection on ADG following movement to group housing can be observed up to approximately nine months of age. Average daily gain results in a 14.3 kg decrease in body weight (BW) for calves with respiratory infections at one year of life (Stanton et al., 2012). Although a calf's life returns to normal after pneumonia recovery, lung damage often remains (Hermeyer et al., 2012), indicating how critical healthy lungs are to an animal’s subsequent fertility. In the case of diarrhea, the long-lasting effect of this disease is less clear because when multiple pathogens damage the intestines, the damaged lining tissue grows back, and the intestines’ function returns to normal fairly quickly when the appropriate antimicrobials are used (Constable, 2009).

It has long been suggested that heifers be inseminated to calve between 23 and 24 months (Ettema and Santos, 2004; Boulton et al., 2015) because decreasing the age at first calving (AFC) reduces rearing costs and extends the herd life. In the current study, the mean AFC for healthy heifers was shorter than in heifers diagnosed with diarrhea, pneumonia, or comorbidity of these diseases during calfhood, which is in line with observations of Teixeira et al. (2017) where heifers suffering lung consolidation at 60 d of life had a higher AFC. This response could result from a greater weight gain in healthy heifers, as AFC is linked to ADG before puberty (Raeth-Knight et al., 2009; Stefańska et al., 2021). Healthy calves could have shown a better ADG not only because they did not suffer any disease but also because they were born with a higher birth weight, and this fact could condition that they reach a greater weight at weaning and consequently improve the fertility indexes.

Furthermore, early episodes of pneumonia or diarrhea could cause carry-over effects on reproductive outcome, as reported by Heinrichs et al. (2005), who showed that AFC was not only affected by ADG but also health problems during the first two months of life. Likewise, Warnick et al. (1994) found that heifers with respiratory tract infections in the first three months of life calved three months later than their healthy herd mates. Age at first calving is a crucial factor for dairy farmers because it significantly affects the lifetime productivity of heifers (Le Cozler et al., 2008). Therefore, avoiding diseases before weaning is a good start to life in heifers because it paves the way for increased productivity in the future.

It is worth mentioning that previous economic analyses indicate that calving at 24 months is more advantageous than calving at older ages because of reduced replacement heifer costs and earlier heifer productivity (Boulton et al., 2017). The median age at calving observed in heifers suffering from pneumonia in our study (22.6 months) indicates that this goal was achieved regardless of pneumonia history.

In agreement with Abuelo et al. (2021), services per pregnancy were increased in heifers with a history of preweaning diarrhea. This response can be explained by the reduced weight gain of the diseased calves, as poorly grown calves require more services to conceive (Wathes et al., 2008). Furthermore, the pregnancy rate was higher in healthy heifers early in life, which is in line with previous studies in which the likelihood of pregnancy decreased in heifers with a history of diarrhea (Aghakeshmiri et al., 2017; Abuelo et al., 2021). Furthermore, the results of the present study are in line with previous studies in which heifers with a history of calfhood diseases have lower reproductive efficiency, particularly in the key reproductive metric of pregnancy rate (Teixeira et al., 2017; Abuelo et al., 2021). These data suggest that the health problems early in life had a long-term carryover effect on heifers’ fertility, and periods of calfhood disease compromise development with long-term adverse consequences on reproductive outcomes. Therefore, some of the economic losses attributed to preweaning episodes of pneumonia or diarrhea are due to suboptimal fertility.

This study provides evidence that Holstein calves in a hot environment that suffer from diarrhea, pneumonia, or co-occurrence of both diseases before weaning presented lower preweaning weight gain, which was reflected in decelerating the occurrence of the first service and increasing the age at pregnancy and calving. Furthermore, episodes of these diseases early in life were detrimental to pregnancy rate. Since initial low birth weight affected the occurrence of diarrhea, the need to selectively treat these light calves to prevent this condition should be emphasized. Furthermore, strategies to prevent diarrhea and pneumonia before weaning might be required to improve the reproductive performance of Holstein heifers in intensive production systems in warm environments.