Effects of bulk tank milk, waste milk, and pasteurized waste milk on the nutrient utilization, gastrointestinal tract development, and antimicrobial resistance to Escherichia coli in preweaning dairy calves.

Waste milk (WM) may be colostrum and milk derived from cows undergoing treatment for several ailments, including clinical mastitis, foot and reproductive diseases etc. Additionally, WM may have elevated SCC, rendering it unsuitable for commercial use. Yet despite this adversity, the use of WM in suckling programs is worldwide a common practice, since producers perceive the use thereof as an economical feed alternative which can effectively replace calf nutrition derived from bulk tank milk (BTM) or milk replacer. From their standpoint, this approach mitigates the necessity for specific waste treatment systems, which would otherwise be required to manage microbial contamination and dispose of drug residues safely in the environment. However, despite its utilization as a feed, concerns persist of antibiotic residues on nutrient digestion of the solid diet but also highlighted the susceptibility of bacterial phyla to these residues. These alterations have been shown to be substantial enough to affect the development of the gastrointestinal tract (GIT), particularly the rumen. Despite these observations, scientific studies evaluating such effects remain limited and therefore the objective of the study cited was to assess the impact of utilizing BTM, WM, and pasteurized waste milk (PWM) on nutrient digestibility, ruminal and caecal fermentation, organ development, and antimicrobial resistance of faecal Escherichia coli in dairy calves.

The calves in the experiment were grouped according to body weight, serum protein levels, and breed composition. Three treatments were included: BTM, WM from cows under antibiotic treatment, and PWM (WM submitted to HTST pasteurization). A total of 63 calves were used, of which 18 animals (n = 6 per treatment) were evaluated in the period of 4 to 30 days, and 45 (n = 15 per treatment) from 4 to 60 days. During the experimental period, a daily intake of 6 L of milk was divided into two equal meals, with ad libitum access to water and starter. Milk and feed intakes were recorded daily. Apparent total-tract digestibility and nitrogen balance were conducted from 25 to 29 days of age and from 53 to 57 days of age respectively. The calves were slaughtered at 30 and 60 days of age for the assessment of ruminal and caecal fermentation and GIT development. Antimicrobial susceptibility testing was conducted at 1, 30, and 60 days of age (n = 15 per treatment). In the statistical analysis a linear mixed-effects model was used for continuous outcomes and a generalized linear model for single measurements.

Treatments WM and PWM had lower rumen pH, higher ruminal acetate concentration, larger reticulorumen and liver, and a higher prevalence of faecal-resistant E. coli compared with BTM at both 30 and 60 days. Up to 60 days, both BTM and WM treatments exhibited higher digestibility of ether extract and gross energy compared with the PWM, whereas WM and PWM treatments showed increased nitrogen intake and retention compared with the BTM treatment.

In conclusion, the results showed that pasteurizing WM has adverse effects on nutrient digestibility of calves, potentially compromising their performance, whereas the changes in ruminal fermentation patterns suggest a potential effect on rumen development. The use of WM for calf feeding may contribute to the gradual selection of resistant faecal E. coli in the GIT over time, posing a potential threat to the effectiveness of disease treatments within the herd. This should be considered a warning sign in the quest to limit AMR.