Effect on the gut microbiome of feeding waste milk from cows to calves

Waste milk (WM) is a by-product of the dairy industry that cannot be sold for human consumption, and a convenient alternative for the farmer is using it as calf food. Still, its nutritionally unbalanced composition and the presence of pathogens, toxins, and antimicrobial agents can lead to intestinal problems with possible lasting effects. We investigated the short-term and medium-term consequences of feeding a pooled, standardized WM from cows with chronic bacterial mastitis on intestinal health, weight gain, and hindgut microbiota in an 8-week feeding trial. After 3 days of colostrum, 6 calves received WM, and 6 received normal milk (NM) for 2 weeks. Then, all calves received a weaning diet of milk whey and starter feed for further 6 weeks. WM calves had a significantly higher incidence of diarrhea than NM calves in the first two weeks of the trial (odds ratio 6.54) and significantly lower body weight until the end of the trial. The 16S rDNA marker gene analysis revealed a sharp decrease in alpha-diversity of the hindgut microbiota in WM calves. Alpha-diversity continued to decrease significantly even after two weeks of weaning diet and then recovered slowly with time, but minor differences were still present at trial completion. Concerning beta-diversity, several taxonomic groups were significantly different in NM versus WM at all time points, from the genus level to the phylum level. Among others, Negativicutes, Lachnospiraceae, Ruminococcaceae, Allisonella, Fusicatenibacter, Prevotella, Acetatifactor, and Shuttleworthia were higher in NM during the trial, while Alphaproteobacteria, Deltaproteobacteria, Catenibacterium, Veillonella, and Rhodococcus were higher in WM. Remarkably, after six weeks under the weaning diet, the phylum Saccharibacteria was still significantly higher in NM. In conclusion, feeding pre-weaned calves with WM from cows with chronic mastitis compromised intestinal health, weight gain, and hindgut microbiota diversity. Significant taxonomic changes up to the phylum level persisted in the gut microbiota even after 6 weeks of weaning diet, suggesting that this practice may lead to prolonged consequences on the calf intestinal microbiota, possibly interfering with the calf health and nutrient utilization efficiency in the longer term.

废弃乳（Waste milk, WM）是乳制品工业的副产品，无法供人类食用，农户常将其作为犊牛饲料的便捷替代方案。然而，其营养组成失衡，且含有病原体、毒素与抗菌剂，可能引发肠道问题并带来持久不良影响。
本研究通过为期8周的饲喂试验，探究了饲喂慢性细菌性乳腺炎奶牛所产混合标准化废弃乳对犊牛肠道健康、体重增长以及后肠道微生物群的短期与中期影响。在饲喂3天初乳后，6头犊牛接受WM饲喂，另6头犊牛饲喂常乳（Normal milk, NM），持续2周。随后，所有犊牛更换为乳清与开食料组成的断奶日粮，继续饲喂6周。
试验前两周，WM组犊牛的腹泻发生率显著高于NM组（优势比为6.54），且直至试验结束，其体重均显著低于对照组犊牛。16S核糖体DNA（16S rDNA）标记基因分析结果显示，WM组犊牛的后肠道微生物群α多样性出现显著下降。即便在更换断奶日粮两周后，WM组的α多样性仍持续显著下降，随后随时间缓慢恢复，但直至试验结束仍存在细微差异。
就β多样性而言，在从属水平到门水平的所有分类层级上，NM组与WM组的多个分类类群均存在显著差异。试验期间，NM组中奈氏球菌纲（Negativicutes）、毛螺菌科（Lachnospiraceae）、瘤胃球菌科（Ruminococcaceae），以及Allisonella、Fusicatenibacter、普雷沃氏菌属（Prevotella）、Acetatifactor与Shuttleworthia菌属的相对丰度更高；而WM组中α-变形菌纲（Alphaproteobacteria）、δ-变形菌纲（Deltaproteobacteria）、Catenibacterium菌属、韦荣球菌属（Veillonella）与红球菌属（Rhodococcus）的相对丰度更高。尤为值得关注的是，在饲喂断奶日粮6周后，NM组的糖细菌门（Saccharibacteria）相对丰度仍显著高于WM组。
综上，给断奶前犊牛饲喂慢性乳腺炎奶牛所产的废弃乳，会损害其肠道健康、体重增长以及后肠道微生物群多样性。即便在更换断奶日粮6周后，肠道微生物群仍存在直至门水平的显著分类学变化，这表明该饲喂方式可能对犊牛肠道微生物群造成长期影响，长远来看或干扰犊牛健康与营养利用效率。