Days to calving in artificially inseminated cattle: Alternative models and analyses D.L. Robinson ⁎ ,1 Beef Industry Centre, NSW Department of Primary Industries, University of New England, Armidale, NSW 2351, Australia Received 19 January 2006; received in revised form 19 October 2007; accepted 8 November 2007 Abstract Investigations by Robinson [Robinson, D.L., 2007. Days to calving in artificially inseminated cattle: comparison of potential traits. Livestock Science 110, 174–180] concluded that the most useful trait for assessing fertility of artificially inseminated (AI) beef cows is AI days to calving (AIDC), a trait that mimics days to calving for naturally mated cows. Various fixed and random effects were fitted to AIDC to determine the best way of modelling lactation status of the cow, the effect of service sire, using smaller contemporary groups and lowering the penalty value for non-calvers. Fitting the time interval between calving and the start of mating either as a 10-level factor or a cubic spline function explained considerably more variation than fitting the standard 2- level factor (wet or dry). Estimated permanent environmental effects of the cow were considerably reduced. This suggests that, if a cow calves late in the season (less than 60 days before she is inseminated), her fertility is reduced. Models should therefore account for this effect. If fitted, service sire explained 1.6% of phenotypic variation, compared to a much larger sire × contemporary group interaction (3% of phenotypic variation). It is therefore important to account for sire × contemporary group interactions. When the fertility of service sires is not being evaluated, service sires could be incorporated into the definition of contemporary groups. Ideally, breeders should be encouraged to formally record contemporary (or mating) groups. Reducing the size of contemporary groups (inferred from the data) by limiting the time interval between first and last inseminations from 120 to 60 days had only a marginal effect as did reducing the penalty for non-calvers from 21 to 10 days. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Beef cattle; Female fertility; Artificial insemination; Days to calving; Calving interval; AIDC 1. Introduction Fertility of beef cows is an important aspect of profit- able beef production (Ponzoni, 1992; Phocas et al., 1998). In Australia, days to calving (DC) became the standard trait for genetic evaluation of naturally mated cows, following investigations by Meyer et al. (1990, 1991) and Johnston and Bunter (1996). Many breeders are now using artificial insemination (AI) to access the best available genetics, so a female fertility trait is also required for cows mated by AI. Investigations by Robinson (2007) concluded that the most useful trait was AI days to calving (AIDC). AIDC is calculated in an almost identical way to naturally mated days to calving, by defining the ‘start date’ as the day when the first insemination of the mating Available online at www.sciencedirect.com Livestock Science 117 (2008) 34 – 42 www.elsevier.com/locate/livsci ⁎ Tel.: +61 267 70 1837; fax: +61 267 70 1830. E-mail address: Dorothy.Robinson@dpi.nsw.gov.au. 1 Some of this work was conducted while D.L. Robinson was employed at a unit in the University of New England. 1871-1413/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2007.11.010