The present study has shown that the crude morbidity and mortality rate of calves in the study areas is 13.2 cases per 100 calf months and 3.8 cases per 100 calf months at risk, respectively. The present study also revealed that the cumulative incidence of all-cause morbidity and mortality was 40.29% and 12.85%, respectively. With the exception of one study [15], there are methodological variations in the estimation of the morbidity or mortality rate in earlier studies in Ethiopia. The current morbidity and mortality rate is comparable to the crude morbidity and mortality rate reported by Hordofa et al. [15], which was 13.81 cases/100 calf months and 4.12 cases / 100 calf months, respectively. The cumulative incidence of morbidity in the current study is substantially lower than the range of 50.12 to 66.7% reported from different parts of the country [15–18]. It exceeds the cumulative incidence of 29.3% to 34.1% found in other studies [11, 19], nevertheless. Similarly, the cumulative mortality incidence found in the current study is higher than that reported by Megersa et al. [19] and Tora et al. [11], which corresponds to 9.3% and 8.64%, respectively, but lower than the incidence ranges reported in other investigations (20.04-30.7%) [15–18, 20]. Compared to studies outside Ethiopia, the current cumulative mortality incidence is higher than estimates from Europe, which vary between 1 and 9% [8, 21, 22], but within the range (13.0-30.0%) reported from other African countries [23]. Variations in morbidity and mortality rates between studies can be attributed to a range of calf- and herd-level risk factors, agroecology, case definition, age of the calves, study design, sample size, and study methodology [24].
The current study assessed a wide range of potential risk factors for calf morbidity and mortality. Accordingly, multivariable Cox regression analysis revealed that calf age, calf sex, calving difficulties, whether dairy farming is a primary or secondary source of income, and dairy farmers' educational level were risk factors for calf morbidity, while calf age, calving difficulties, the time calves were separated from their dams, and educational status were risk factors for calf mortality.
In comparison to calves under 3 months old, calves older than 3 months had a 78% (HR = 0.22) reduced risk of morbidity and an 86% (HR = 0.14) lower risk of mortality. In keeping with our findings, studies from Ethiopia and elsewhere have also demonstrated that calf morbidity and mortality rates are much higher in the early life of calves, particularly during the first month of a calf’s life [5, 8, 16, 17, 21]. In general, the relatively higher risk of morbidity and mortality in young calves found in previous and current studies implies that dairy farmers need to pay due attention and provide calves with the best possible health care at an early age.
The current study found that female calves had a 43% (HR = 0.57) reduced risk of morbidity than male calves. Although the difference was not statistically significant, female calves also had a lower mortality rate than male calves. This is probably related to the better management and health care services provided for female calves in the farms because female calves are considered future replacement stocks on farms and are of greater economic significance. Male calves often get less attention when it comes to feeding, medical care and other things. Although data from all farms are lacking, some dairy farmers we interviewed responded that raising male calves is not profitable and therefore they usually sell them at a young age for slaughter or fattening. As a result, they ignore concerns about feeding and other management issues, potentially leading to higher morbidity and mortality rates. Therefore, there is a need to raise dairy farmers’ awareness in order to change their attitude and reduce morbidity and mortality rates in male calves. Similar findings have also been reported by other studies conducted in Ethiopia and abroad [19, 25, 26].
Calves born to dams with dystocia had a 2.4 and 9.3 times higher risk of morbidity and mortality, respectively, compared to normally born calves. This result is in line with what has been described in previous studies [15, 21, 27, 28]. Due to the high possibility of contamination during delivery and delayed suckling or decreased colostrum intake, assisted delivery increases the risk of disease and mortality [28]. Furthermore, due to stress during delivery, adrenocorticotropic hormone is released. This hormone stimulates the adrenal cortex to produce and secrete more cortisol, leading to immunosuppression and increasing calves’ vulnerability to numerous pathogens [29]. Dystocia can also affect the vitality of new-born calves. According to Campler et al. [30] calves with low vitality are less inclined to stand up and suckle, which prevents the passive transfer of maternal antibodies.
In the current research area, dairy farming was the primary business for 62.86% of the farmers but it was only a sideline for 37.1% of the farmers. The risk of morbidity was 1.7 times higher on dairy farms where dairy farming is the secondary source of income for the farmer. This is related to the fact that farmers devote less time caring for the calves since they spend much of their time engaged in other activities that would give them a better income.
While some dairy farms at the current research sites leave newborns with their mothers for a few hours to suckle as much colostrum as possible, some dairy farms take calves from their mothers shortly after birth and offer colostrum six hours later. The risk of mortality was 60% (HR = 0.40) lower in calves that were allowed to stay with their mothers and consumed colostrum immediately than in calves that were immediately removed from their mothers and bottle-fed colostrum with some delay. The most likely explanation for this is that the calves, which were left with their mothers after birth, were able to ingest sufficient colostrum to protect them adequately from infectious agents. In contrast, calves that are taken away from their mothers and bottle-fed may not get enough colostrum at the right time. Failure to receive timely passive transfer of maternal antibodies may be the main reason for a higher risk of morbidity and mortality in these calves. It is recommended that calves be fed 10% to 12% of their body weight with high quality colostrum (3–4 L for a Holstein calf) within 1 to 2 hours after birth to successfully transfer passive immunity [31]. Furthermore, studies have shown that providing calves with 2 to 3 L of colostrum 5 to 6 hours after the first feeding optimizes the transfer of passive immunity, reduces morbidity and improves average daily weight gain in dairy calves [32].
In the current study, the educational level of the farmers had a significant (p<0.05) influence on the morbidity and mortality rates of the calves. The risk of morbidity was shown to be 74% (HR = 0.26) and 70% (HR = 0.30) lower in farms owned by farmers who completed secondary and college education, respectively as to compared to farms owned by farmers without a formal education. The risk of mortality was also decreased on the farms of farmers who completed their secondary school and college education by 92% (HR = 0.08) and 87% (HR = 0.13), respectively. This is most likely related to more effective calf management practices used on farms run by farmers with a higher level of education. This is evidenced by the fact that in comparison to calves from farmers without formal education or with only primary school, all farms of farmers with college degree and 91% of farms of farmers with secondary education provided the new-born calves with colostrum immediately after birth, separated them from their dams only after ingestion of colostrum and housed them in cleaned stalls.
The most prevalent disease syndrome and the main factor in calf deaths in the current study was diarrhea, followed by pneumonia. It accounted for 71.3% of all morbidity and 62.1% of all mortalities. Similar to the current data, other authors in Ethiopia have identified that diarrhea and pneumonia are the two most significant health issues affecting calves [11, 15–18, 33]. In studies conducted outside Ethiopia, the two disease conditions were likewise included as the primary and secondary causes of calf morbidity and mortality [34, 35]. The incidence of diarrhea was more pronounced in very young calves, possibly due to the sensitivity of newborn calves to diarrhea-causing agents and inadequate transfer of passive immunity as a result of not providing enough colostrum in a timely manner [31, 32]. Poor hygiene management of feeding equipment, the calving environment, and the calf pen can also contribute to the occurrence of diarrhea as a cause of calf morbidity and mortality [7]. Diarrhea is one of the major cause of economic losses for cattle farmers worldwide due to high morbidity and mortality in calves, especially in the first few weeks of life. In order to manage diarrhea, and its impact on the calf’s growth and performance in the future, it is crucial to determine the root causes. Therefore, future calf morbidity and mortality studies in the country should take into account identifying the underlying causes of diarrhea. The present study also found that 20.7% of calf deaths were due to unknown causes. Because the study was based solely on observation and physical examination, we could not identify the specific causes. Therefore, future studies must apply specific and highly sensitive diagnostic methods in order to identify the primary infectious and noninfectious causes of calf deaths on current dairy farms and elsewhere.