Feeding preweaning Holstein calves a synbiotic supplement increases their energy-corrected milk yield as lactating cows

Prebiotics and probiotics are feed additives that can benefit the host by modulating the gut microbiome, which is crucial in digestion, immunity, and overall animal health. This study aims to evaluate the effects of supplementing prebiotics, probiotics, or synbiotics to preweaning Holstein calves on their future milk yield. This study is a retrospective analysis of milk yield records from dairy cows that were randomized at birth to 1 of 4 twice-daily treatments administered during the preweaning period: (1) control, no additive (CON), (2) prebiotic (PRE; 7 mL of Saccharomyces cerevisiae yeast culture), (3) probiotic (PRO; Bacillus subtilis and Lactobacillus plantarum, delivering ∼1 billion and 250 million cfu per head per day, respectively), or (4) synbiotic (SYN; combination of both PRE and PRO at the same dosages as the PRE and PRO treatments). The study was conducted on a dairy farm in Fresno County, California, between 2019 and 2023, involving 1,296 Holstein cows over their first 3 lactations for a total of 2,735 lactations. Monthly test day records for milk yield, fat, and protein were used to calculate ECM, standardized to 4% fat and 3.3% protein, totaling 26,464 monthly test day milk records. A 2-piece splines mixed-effect regression model evaluated the effect of treatments on ECM yield. For the first lactation, ECM yield was estimated at 28.66 kg on the first DIM, peaked at 42.1 kg, and declined to 21.34 kg by 305 DIM. For parity ≥2, ECM yield was 41.06 kg at 1 DIM, peaked at 54.2 kg, and 33.74 kg at 305 DIM. The SYN treatment increased ECM yield by 1.00 kg/d compared with CON. This increase was primarily due to an increase in milk fat yield, with 0.048 kg/d more fat produced compared with the control group. No differences in ECM yield between PRE, PRO, or CON were observed. These findings suggest that supplementing SYN during the preweaning period increased milk, milk fat, and ECM yield across lactations 1, 2, and 3.