Phenotypic and Genetic Aspects of Health Events and Production Traits in Dairy Cattle

The overall objective of this study was to estimate causal relationships among common health disorder and production traits, and to estimate variance components of common health disorders and production measures. Two separate studies were conducted to identify causal relationships among health disorders and production traits using structural equation models (SEM) implemented in a Bayesian framework. In the first study, the dataset included traits like reproductive health disorders (retained placenta (RP); metritis METR)), metabolic health disorders (ketosis (KETO); displaced abomasum (DA)), mean milk yield (MY) from early part of lactation (1-120 DIM), peak milk yield (PMY), day in milk of peak milk yield (PeakD), and lactation persistency (LP). Three different sets of traits were analyzed which included: recursive effect from each health disorder on culling reason; recursive effects of one health disorder on another health disorder and on MY, and recursive effects of each health disorder on production traits including PeakD, PMY and LP. The estimated structural coefficients of the recursive models revealed how the presence of health disorder increases culling frequency with stronger impact from DA, followed by RP, KETO, and METR. Positive recursive effects of RP to METR and of KETO on DA were estimated, while recursive effects from health disorders to production traits were negligible in all cases. The second study, included mastitis events of first and second lactation, LP, and test day milk yields recorded in three different periods; period 1 (5 to 60 DIM), period 2 (61 to 120 DIM) and period 3 (121 to 180 DIM), and total milk yield recorded for second lactation. The causal relationship between mastitis and milk yield found in this study suggests that mastitis results in a direct decline in test day milk production. There is nonetheless little impact of mastitis on LP. The causal relationship among mastitis events in first and second lactation found in this study indicate that having a mastitis in first lactation is likely to increase the risk of a mastitis in later lactations. Based on the results it was concluded that a health disorder occurring in early lactation has a moderate causal effect on the health disorder occurring in later lactation. Causal links among mastitis events and test-day milk yields were detected that evolved over the course of lactation in the cow's life. Heritability estimates of health disorders ranged from 0.023 to 0.114, in accordance with previous studies. Similarly, genetic correlations obtained between health disorders and production measures were moderate. The SEM should be implemented in analyzing health traits in order to make better farm management decisions, because knowing causal relationships between health disorders will provide information on future health events. This information may then be used to reduce the likelihood of future health challenges via management intervention. In the third study, variance components and heritability of infectious and non-infectious hoof lesions were estimated using pedigree- and genomic-based analyses. In case of genomic-based analysis, a single-step procedure was conducted to estimate genomic variance components and heritability for hoof lesions. The pedigree based analysis produced heritability estimates of 0.11 (+/- 0.05) for infectious hoof lesions and 0.08 (+/- 0.05) for non-infectious hoof lesions. The single-step genomic analysis produced heritability estimates of 0.14 (+/- 0.06) for infectious hoof lesions and 0.12 (+/- 0.08) for non-infectious hoof lesions. Inclusion of genomic data substantially improved young sire reliabilities for hoof lesions. Genomic selection against the occurrence of hoof lesions should be incorporated into breeding programs to improve the health status of animal.