THE EFFICACY OF PREDICTING DYSTOCIA IN YEARLING BEEF HEIFERS .2. USING DISCRIMINANT-ANALYSIS

Discriminant analysis was used to develop equations for predicting dys tocia in 2-yr-old, first-calf beef heifers. Data collected on 893 year ling heifers from 34 farms were used to derive four sets of prediction equations (Data Set 1). There was a set of equations for British x Br itish (BRBR) heifers at prebreeding, BRBR heifers at pregnancy check, Continental x British-Continental x Continental (CNBR-CNCN) heifers at prebreeding, and CNBR-CNCN heifers at pregnancy check. A second set o f data collected on 3,728 heifers at prebreeding and 875 heifers at pr egnancy check were used to test these equations (Data Set 2). Measurem ents taken prebreeding and at pregnancy check included heifer age, wei ght, body condition score, pelvic height, pelvic width, pelvic area, h ip height, and shoulder height. A precalving estimate of average calf birth weight (AVGBWT) and actual calf birth weight (BWT) were also rec orded. When actual calf BWT was used, the prediction equations develop ed for BRBR heifers at prebreeding and at pregnancy check correctly cl assified 84.6 and 88.5% of the heifers in Data Set 1, respectively. Th e prediction equations developed for CNBR-CNCN heifers correctly class ified 88.1% of the heifers at prebreeding and 87.9% at pregnancy check . When AVGBWT was used, the accuracy of these four sets of prediction equations ranged from 77.1 to 83.1% for heifers in Data Set 1. In Data Set 2, the prebreeding equations correctly classified 78.5% of BRBR a nd 79.0% of CNBR-CNCN heifers when AVGBWT was used. Culling by these p rebreeding equations would have reduced difficult birth rate in BRBR a nd CNBR- CNCN heifers by 8.5 and 15.7%, respectively. The pregnancy ch eck equations correctly classified 67.7% of BRBR and 90.7% of CNBR-CNC N heifers. However, culling by these pregnancy check equations only re duced rate of difficult births by -1.7% in BRBR heifers and by 3.0% in CNBR-CNCN heifers. Heifers predicted to have a difficult birth were l ighter and smaller and had less pelvic area per unit of heifer BW or h eight than heifers predicted to calve easily. The prebreeding equation s for predicting difficult births in heifers have practical applicatio n, although it is limited by their complexity. If the prebreeding equa tions are to be used they must be part of an overall management plan t o avoid calving difficulty and be tested within each herd.