Dairy calves' mortality survey and associated management practices in smallholding, pasture-based herds in southern Brazil

Our retrospective, cross-sectional study aimed to identify the mortality rate of preweaned heifer calves and associated herd-management factors in family-run dairy holdings in the state of Rio Grande do Sul, Brazil. A survey was made available through an online platform to all the municipalities (n = 494) with a state official extension office (Emater), allowing for any milk producer in RS raising the calves on-farm to take part in the study. A total of 1451 farmers responded to the survey between October 2014 and December 2015. Yearly total mortality, stillbirths and post-natal mortality to weaning were calculated from responses. The association of herd characteristics and practices with herd mortality was evaluated using multilevel multivariable logistic regression models. Herd mortality was modelled as a two vector binded variable that included the number of failures (n = female calves born alive but dead before weaning) and successes (n = weaned female calves) on-farm in the past 12 months. Models included region as random effect. The herds had 13 lactating cows (median, range 1-130). Pure breed herds, especially Holstein, predominated (81 %). Milk production was 190 L/d (median, range 7-4000). Total mortality rate was 8.5 % (1065/12563, median 0.0 %, Q(1) - Q(3) 0.0-14.3 %/farm), and stillbirths 1.7 % (207/12563; median 0.0 %, Q(1) - Q(3) 0.0 - 0 %/farm). Post-natal mortality rate was 6.9 % (858/12356, median 0.0 %, Q(1) - Q(3) 0.0-11.1 %/farm). In 89.2 % of the farms, no stillborn calf was reported, and in only 2.8 % there was more than one case. In 67 % of farms, no death calf born alive was reported; in 33 %, an average of 2 deaths/farm (range 1-12 deaths/farm) was reported. In 16 % of the farms, mortality was >= 20 %, and in 4.3 % it was >= 50 %. The main reported causes of death were diarrhoea and unknown causes. Higher mortality was associated with herds with mixed breeds (OR = 1.3, CI = 1.09-1.59), performing unattended calvings (OR = 1.2; IC = 1.04-1.40), leaving the calf for long periods with the cow (OR = 1.21, CI = 1.00-1.45), and housing various calves in a pen (OR = 1.4, CI = 1.20-1.60). Furthermore, reduced mortality was associated with medium size herds (i.e. 21-40 lactating cows; OR = 0.82, CI = 0.69-0.97). Although the mortality rate observed is conservatively lower or equal to other international reports, it is higher than could be desired. The factors identified as associated to mortality are understood as proxies for the poor quality of management of the practices adopted. Thus, reduction of mortality is at hand without representing major infrastructural changes.