Predictors of health and growth in neonatal surplus dairy calves: Investigating approaches to identify and manage calves at high health risk early in life

The research described in this thesis is comprised of an exploration of navel healing in male and female neonatal Holstein calves, an evaluation of leukocyte differential cell counts as a biomarker for health and growth in male surplus calves, and an evaluation of a selective antimicrobial therapy strategy based on individual health risk assignment on the time of arrival at a surplus calf rearing facility. This research also involved the validation of a rapid automated leukocyte cell counter in neonatal Holstein calves. A longitudinal study design was used to explore daily navel healing in male and female Holstein calves over a 14 day observation period beginning after birth. Multiple associations were found between navel diameter and length measurements with the day of observation, time of birth, and calf factors (sex, birthweight, and medical treatment). A large proportion of calves had navel diameters above a recently identified 13 mm threshold for navel infection suggesting occult navel infections or that a different threshold may be more appropriate in this population. A cross-sectional diagnostic accuracy study was conducted to validate a rapid automated leukocyte cell counter in neonatal Holstein calves that performed well in estimating neutrophil counts and had fair performance in estimating lymphocyte counts. Leukocyte cell counts were evaluated as a potential biomarker for health and growth in a cohort study that sampled surplus Holstein calves at the time of arrival and 72 hours post-arrival. At the time of arrival, lymphocyte counts were associated with future growth; 72 hours post-arrival lymphocyte counts were associated with morbidity and mortality risk, and neutrophil counts were associated with mortality risk. A randomized control trial was conducted to compare a selective therapy strategy and a conventional group therapy strategy at the time of arrival to a surplus calf facility. Although no difference was found between interventions for morbidity over the study period, the selective therapy strategy had an increased hazard of mortality and lower growth in the winter. These results suggest that a selective therapy strategy may not be appropriate for all facilities in all seasons. Further refinement to risk assignment and therapy protocols are required.