Current Progressive Dairy digital edition

Case study: Prolonged antibiotic use shows negative effects to digestibility and performance in calves

Xavier Suarez for Progressive Dairyman Published on 24 November 2017

Current regulations, which went into effect in January 2017, require a prescription to feed calves certain antibiotics in milk replacer. Prior to the change in regulations, some calf growers fed certain antibiotics in milk replacer for the entire pre-weaning period.

Recent studies have shown that prolonged feeding of antibiotics can have negative effects on overall calf performance.



Prophylactic or preventative use of antimicrobials can help to improve health and reduce mortality; however, a study conducted at Washington State University observed that feeding neomycin and oxytetracycline, better known as neo-terramycin or NT, to calves with adequate passive immunity transfer reduced starter intake, weight gain and increased diarrhea.

Additionally, this antibiotic combination not only targets pathogenic bacteria but can kill both good and bad bacteria in the digestive tract, changing the microbiome of the gut.

To see if these changes in the gut microbiome affected feed digestion, a trial was conducted in 2016 looking into the effects of feeding NT on feed digestion immediately post-weaning. There were three treatments:

  • 1.5 pounds of milk replacer per day with NT
  • 1.5 pounds of milk replacer per day without NT
  • Up to 2.5 pounds of milk replacer powder per day without NT

Calves being fed NT were on the following sequence for the 42-day trial:

  • Repeated sequence of 14 days on NT followed by one day on non-medicated for the first 30 days

  • The trial finished with calves being on NT for 12 days.

The high-milk-replacer feeding treatment was included because feeding more milk replacer has been shown to reduce pre-weaning starter intake and negatively impact feed digestion.


Calf performance

Post-weaning dry matter digestibility of calves on NT was lower than calves fed the same amount of milk in spite of pre-weaning starter intake being similar between treatments. The dry matter digestibility of calves on NT was similar to calves fed more milk, even when calves on NT consumed roughly twice the amount of starter pre-weaning.

Fiber-digesting micro-organisms take longer than starch- and sugar-digesting micro-organisms to get established in the rumen. The reduction seen in fiber digestion for calves fed more milk or NT points to a less mature rumen microbiome.

The loss in digestibility observed for calves on NT and calves fed more milk replacer negatively affected growth, as hip-width change over the eight weeks was lower for both treatments when compared to calves fed 1.5 pounds of non-medicated milk replacer.

Reducing post-weaning digestion can increase stress, which puts more pressure on the immune system. In field observations, it is common to see in feeding programs where NT was used over the entire pre-weaning period that additional medical treatments post-weaning were needed to maintain good calf health.

If digestion is reduced, the calf may not be able to meet nutrient needs, weakening the immune system and making the calf more susceptible to sickness. This study supports the previously mentioned Washington State University findings where feeding NT to healthy calves negatively affected performance.


The reduction of exposure to pathogens may be a better approach to reducing bacterial infections than the prophylactic use of antibiotics.

The observations presented herein are informational in nature and should not be viewed as medical advice. Please consult a licensed veterinarian regarding animal treatment.  end mark

Xavier Suarez is a calf and heifer specialist with Provimi. Email Zavier Suarez.

References omitted but are available upon request. Click here to email an editor.