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Different lifetime dietary strategies affect carcass characteristics and rumen function in Holstein steers

Helen Golder , Josh Rehberger, Alexandra Smith, Elliot Block, Rod Polkinghorne, Holly Cuthbertson, Michael Campbell, Veronika Vicic, Garth Tarr , Jane Quinn, Jesrie Tong, Sam Rowe, Ian Lean

Abstract

Context How to improve integration of male dairy calves into red meat supply chains. Aims To evaluate the effects of two diets differing primarily in starch, antimicrobial content, and milk replacer volume on (i) short- and long-term rumen adaptation; and (ii) lifetime production in Holstein steers. Methods Holstein males, 3-7 days old (n = 72; 36 steers/treatment; 6 per replicate) were randomised to Control (CON; 6 L milk replacer/calf.day; 38.2% of DM lifetime dietary starch; 50 ppm monensin, 20 ppm flavophospholipol) or Treatment (TRT; 4 L milk replacer/calf.day; 47.5% of DM lifetime starch diets with yeast products) strategies. Calves were fed milk replacer twice daily for 42 days, but different pre-starter (days 0-24), starter (days 25-99), and finisher diets (days 100-452). Ruminal fluid was collected at 104, 200, and 438 days old (14 days pre-slaughter) from 24 steers (2 per replicate). Fermentation, production, and carcass measures were analysed by mixed models; ruminal bacterial genera were centre log transformed and subjected to redundancy analysis. Key results The CON had higher risk of subclinical ruminal acidosis at day 438, than TRT (P < 0.001). Liver abnormalities were 17.1% (CON) and 31.3% (TRT). Controls had greater fermentation with 138.4 ± 5.6 mM of total volatile fatty acids vs 111.6 ± 5.6 mM (TRT) with 8.4 mM higher acetate and 18.1 mM higher propionate, but pH was 0.31 units less (P < 0.050). Shannon diversity increased over time (P < 0.001) and was greater for the TRT at day 200, compared to CON (P = 0.013). Bacterial composition differed at each treatment by time comparison (P ≤ 0.01), with variation increasing over time from 8.6 to 19.2%, suggesting the different diets lead to different microbial successions. The CON steers finished with 12 kg heavier carcasses than the TRT, with 0.8% greater dressing percentage, 1.5 mm more fat at P8, and 1.7 mm more rib fat (P < 0.050). Conclusions Control diets produced better carcass weights, P8 and rib fat, and had more fermentability than the TRT diets, likely reflecting better long-term adaptation. Implications Both diets enabled integration of dairy calves into the red meat supply chain, but with differing lifetime rumen adaptations.

AN25153  Accepted 09 October 2025

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