Associations between postpartum phenotypes, cow factors, genetic traits, and reproductive performance in seasonal-calving, pasture-based lactating dairy cows

The objective of this study was to evaluate associations between corpus luteum (CL) status, uterine health, body condition score (BCS), metabolic status, parity, genetic merit for fertility traits, and reproductive performance in pasture-based dairy cows managed for seasonal reproduction. First- and second-lactation (n = 2,600) spring-calving dairy cows from 35 dairy farms located in Ireland were enrolled in the study. Farms were visited every 2 wk, and animals that were at wk 3 (range: 14–27 d in milk) and wk 7 (range: 42–55 d in milk) postpartum were examined. Body condition score was measured using a 1-to-5 scale in 0.25-point increments. Transrectal ultrasound examination was performed at wk 3 and 7 postpartum to determine presence or absence of CL and ultrasound reproductive tract score (scale of G1–G4). Blood samples were collected at each visit, and the concentrations of glucose, β-hydroxybutyrate (BHB), and fatty acids (FA) were analyzed using enzymatic colorimetry. Animals were grouped into 3 BCS categories [low (≤2.5), target (2.75–3.25), and high (≥3.5)], 2 CL categories (present or absent), 2 uterine health status categories (normal or abnormal), and 3 metabolic status categories [good (high glucose, low FA and BHB), poor (low glucose, high FA and BHB), and moderate (all other combinations)]. Fisher's exact test was used to test for associations between variables and was supplemented by logistic regression. More cows with a CL at wk 7 were served during the first 21 d of the breeding period compared with cows without a CL. Cows classified as having a uterine score of G3 or G4 at wk 3 and 7 had lower odds of pregnancy establishment during the breeding period compared with animals with a uterine score of G1 or G2. Animals with low BCS at wk 7 had lower odds of pregnancy establishment than cows with a target BCS. Cows classified as having good metabolic status at both wk 3 and wk 7 had greater odds of pregnancy establishment during the first 21 d of the breeding season than those classified as having poor metabolic status. Overall, primiparous cows had greater reproductive performance than second-parity cows. Animals in the quartiles with the best predicted transmitting ability for survival and calving interval had better reproductive performance compared with animals in the other quartiles. Cows that had better genetic merit for fertility traits and good metabolic status, achieved target BCS, and had a favorable ultrasound reproductive tract score and a CL present at wk 7 postpartum had superior reproductive performance.