It could be due to the greater management needed to prepare the milk replacers (compared with feeding of maternal milk).
In this sense, the buckets and other fomites could contribute to the indirect transmission among calves if they are not deeply cleaned and disinfected, which is common in Ecuadorian farms. Other possible explanation would be related tothe lower nutritional value of poor quality milk replacers (frequently used in Ecuador) compare with maternal milk. The only references found about the role of milk replacers as risk factors were related to the bovine spongiform encephalopathy