title: Event-time analysis of the effect of season of parturition, parity, and concurrent disease on parturition-to-conception interval in dairy cows.
abstract: OBJECTIVE
  To quantify the effect of season of parturition, parity, and various diseases on time to conception.
  
  
  METHODS
  Event-time analysis (survival analysis).
  
  
  METHODS
  44,450 cows delivering calves between September 1985 and September 1986, from 6,227 Ayrshire dairy herds in 80 Finnish communities.
  
  
  METHODS
  Primiparous and multiparous cows were analyzed in separate Cox proportional hazards models for postpartum days 56 to 120. Occurrence of 43 diseases or disorders was recorded; codes were condensed into 25 variables offered to each model. Season of parturition was categorized into spring, summer, and fall-winter; for multiparous cows, 3 categorical variables represented parity. Models controlled for 60-day milk and milk fat production, herd milk production, and community. For all observations, parturition-to-conception interval was plotted against calendar day of parturition.
  
  
  RESULTS
  For multiparous cows, parturition in the spring or summer and being of parity 2 or 3-4 (vs older) increased the chance of conceiving; 10 diseases or disorders decreased this probability. Similarly, in the model for primiparae, parturition in spring or summer increased the probability of conception, and 6 disorders decreased it. Disorders that were detrimental in both models were anestrus, ovulatory dysfunction, other infertility, late metritis, and clinical ketosis.
  
  
  CONCLUSIONS
  The effect of season on the parturition-to-conception interval was marked at this latitude. Parturition during late April, causing confluence of postpartum day 55 with the summer solstice, was associated with the shortest parturition-to-conception intervals. Ketosis was found to be an important factor in lengthening the parturition-to-conception interval, highlighting the effect of negative energy balance on postpartum restoration of reproductive function.
authors.affiliation: Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Harman
authors.forename: Y T
authors.initials: YT
authors.lastname: Gröhn
authors.forename: H N
authors.initials: HN
authors.lastname: Erb
authors.forename: G
authors.initials: G
authors.lastname: Casella
mesh_terms: D000818:Animals; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D002418:Cattle Diseases / Q000453:epidemiology / Q000503:physiopathology*; D005260:Female; D005306:Fertilization / Q000502:physiology*; D015994:Incidence; D007247:Infertility, Female / Q000453:epidemiology / Q000503:physiopathology / Q000662:veterinary; D007662:Ketosis / Q000453:epidemiology / Q000503:physiopathology / Q000662:veterinary; D007743:Labor, Obstetric / Q000502:physiology*; D008414:Mastitis, Bovine / Q000453:epidemiology / Q000503:physiopathology; D008892:Milk / Q000378:metabolism; D010060:Ovulation / Q000502:physiology; D010298:Parity / Q000502:physiology*; D011247:Pregnancy; D016016:Proportional Hazards Models; D012621:Seasons; D016019:Survival Analysis; D013997:Time Factors