Mortality rate in dairy calves in Chile and the association with waste milk feeding, a pilot study

Raising healthy calves is the foundation for a sustainable and profitable dairy herd. Dam nutrition during the last trimester, calving supervision/assistance, maternity facilities hygiene, umbilical care of calves, colostrum management, and calf nutrition are important factors that impact calf development and health (Lorenz et al., 2011). Calf mortality of 5% or less has been suggested as target for calfrearing operations (Lanuza, 2006). In Chile, however, there is lack of knowledge about calf losses and contributing factors. This retrospective pilot investigation aims to generate information about calf mortality in dairy herds and to explore factors that may increase or reduce calf death in Chile.

A pilot retrospective cohort study was conducted to estimate the mortality rate of dairy calves during the first 90 days of rearing. The data were gathered from five dairy farms calving year-round from September 2018 to August 2020. Farms are located in the regions of Biobio (n=3), Valparaiso (n=1), and O’Higgins (n=1). Data recorded included calves’ date of birth, date of death, cause of death, rearing unit type (single/multiple), and calves’ feeding type (milk replacer/waste milk). The date of birth of each calf was categorized according to corresponding season of the year in the southern hemisphere. For this, calves born between September 21st and December 20th, December 21st and March 20th, March 21st and June 20th, and June 21st to September 20th were categorized as born in “spring”, “summer”, “autumn”, and “winter”, respectively. Mortality rate during the first 90 days of rearing was estimated dividing the number of recorded deaths by the number of days at risk that each calf contributed. A generalized linear mixed model (GLMM) with Poisson link function was used to estimate the association between mortality rate and recorded predictors. The herd of origin was included in the model as a random effect. Mortality risk (i.e. the number of deaths over the population of calves at risk during 90 days of rearing) was approximated from the estimated herd adjusted mortality rate (i.e. number of cases per calf-day at risk) using the exponential formula (Rothman et al., 2008).

Throughout the period assessed, 4204 calving events were recorded in the five dairy farms (A to E). Calving occurred year-round with 30% (1259/4204) of all calves born in summer, 26% (1075/4204) in spring, 24% (992/4204) in autumn, and 21% (878/4204) in winter. During the first 90 days of rearing, 610 deaths were recorded. These deaths occurred at a median age of 19 days (1st quartile 9 days – 3 rd quartile 45 days), which indicates that the critical period of survival is during their first three weeks of life. The two most common causes of death recorded were pneumonia (21/610) and diarrhea (17/610). However, most death causes were unrecorded (536/610). The farm adjusted mortality rate was 0.048 deaths per calf-month, which is equivalent to 13.3 cases (95% CI 8.5 – 20.6) per 100 calves reared over a 90-day period. However, there was a significant difference in mortality among dairy farms that ranged from 6.1 to 25.4 cases per 100 calves at risk over a 90-day period in herd B and A, respectively.

Multivariable GLMM results showed that mortality rate was three times higher in farms feeding calves with waste milk compared to farms using milk replacer instead (Table 1). In addition, mortality rate was higher during winter and spring compared to that of summer and appeared to have increased over the years with higher mortality rate in 2020. Feeding waste milk to calves can pose health risks. Consumption of high pathogen loads through waste milk may cause disease directly. Also, the presence of antimicrobial residues in milk may impair gut bacteria development in calves (Pereira et al., 2018). However, it is still unclear if these changes can cause or predispose calves to disease occurrence and death. Finally, the presence antimicrobial residues in milk contributes to the selection of resistant bacterial strains within the calf’s gut that can jeopardize future antimicrobial therapies not only in food producing animals, but also in people.

Waste milk feeding appears to increase mortality of dairy calves, threatening the profitability and sustainability of dairy herds. The results of this retrospective pilot study provide preliminary evidence of the effect that waste milk feeding may have on the survival of dairy calves during the first 90 days of rearing. These findings justify conducting further research in dairy farms that may confirm the associations observed in this pilot study.

Abstract presented at the XLV Annual Congress Sochipa, Chile, 2020.