<b>Supplementary Materials: The Effects of Offering Adequate-Quality or High-Quality Colostrum on the Passive Immunity, Health, Growth, and Fecal Microbiome Development of Dairy Heifer Calves.</b>

Colostrum quality is influenced by multiple factors, including its microbial load, which is determined by the cleanliness of collection and storage conditions. Additionally, the dam’s diet and immune status play a crucial role in shaping colostrum quality by affecting immunoglobulin (IgG) concentrations. While many factors contribute to colostrum quality, this study will specifically use IgG content as the primary measure of quality. It is well established that feeding low-quality colostrum negatively affects calf health and growth, while feeding good-quality colostrum leads to better outcomes. However, it remains unclear if feeding colostrum above the recommended quality threshold offers additional benefits for calf health and growth. This study compared the effects of adequate versus high-quality colostrum on dairy heifer calf growth, health, and the development of the fecal microbiome during the first 15 weeks of life. We also measured the levels of apparent efficacy of absorption of IgG in both groups. Colostrum quality was initially determined and measured before feeding using a Brix refractometer (only feeding a minimum of 21% Brix); 93 heifer calves were assigned to treatment based on this data. Subsequently, laboratory analyses were conducted using radial immunodiffusion assays to measure the exact IgG levels in the colostrum; 72 calves were selected for continuation in the experiment based on the highest (n=36) and lowest (n=36) IgG levels in the colostrum. For these 72 calves, laboratory analyses were conducted using radial immunodiffusion assays to measure the exact IgG levels in the colostrum at the point of feeding and in the calf serum at 24 hours post-feeding. To ensure a fair comparison, the groups were balanced for calf birthweight, breed, and dam parity. Serum IgG at 24 hours, weight, and health data were analyzed in SAS using linear mixed models and logistic regression. Alpha and beta diversity were analyzed using R with ANOVA, PERMANOVA, and Benjamini-Hochberg p-value adjustments.Calves fed colostrum with a high IgG content (123.0 mg/ml IgG) exhibited higher rates of passive transfer compared to those fed adequate-quality colostrum (85.2 mg/ml IgG). Both groups had passive transfer rates &gt;23 mg/ml IgG. Health outcomes were similar between the two groups, and average daily gain during the pre-weaning period was comparable, with calves gaining an average of 0.62 kg per day. Measures of alpha and beta diversity in the fecal microbiome showed similar development in both groups. Apparent absorption efficacy was lower in calves fed high-quality colostrum (24.9%) compared to those fed adequate-quality colostrum (29.3%).The findings of this study support current recommendations for colostrum quality and suggest that calves may have a limited capacity to absorb higher concentrations of IgG. While feeding higher-quality colostrum did not lead to significant improvements in growth, health, or microbiome diversity, it demonstrated that adequate-quality colostrum can be equally effective when combined with best practice management. Further research is needed to better understand the relationships between immunoglobulin absorption efficiency, calf health, microbiome development, and growth performance.