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

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: (1) short- and long-term rumen adaptation; and (2) lifetime production in Holstein steers. Methods Holstein males, 3–7 days old (n = 72; 36 steers/treatment; six 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 group had a higher risk of subclinical ruminal acidosis at Day 438 than the TRT group (P < 0.001). Liver abnormalities were 17.1% (CON) and 31.3% (TRT). The CON group had greater fermentation, with 138.4 ± 5.6 mM of total volatile fatty acids versus 111.6 ± 5.6 mM (TRT), and 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 group at Day 200, compared with the CON group (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 steers, with 0.8% greater dressing percentage, 1.5 mm more fat at P8 and 1.7 mm more rib fat (P < 0.050). Conclusions CON 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.