The objectives of this study were to examine 1) the effect of parity of dam and season of birth on the incidence of twins in Holstein-Friesian cattle in a warm, dry climate and 2) the sex ratio, gestation period and birth weight of calves born. Natural calvings (n=24,843) from nine dairy farms in Saudi Arabia were evaluated over an 8-year period. The calendar year was divided into six bimonthly intervals to identify a possible seasonal effect on the number of natural twins born (twinning rate). The twinning rate increased (P<0.05) from 1.0% (103/10,094) at first calving to 8.0% (1,178/14,749) for all the subsequent calvings. The twinning rate was similar with the second calving (P2) at 7.0% (417/5,929), the third calving (P3) at 7.5% (218/2,896), and the fourth calving (P4) at 7.8% (147/1,864). Correspondingly, cows having >or=five calves had a higher (P<0.01) twinning rate at 9.0% (164/1,805) than cows with too, three or four calvings collectively at 7.3% (782/10,689). The seasonal effect on twinning rate was significant within all parity calvings except in primiparous females. The peak twinning rate occurred in the third season interval (May/June) at 9.3% (66/708) with P2, at 13.0% (34/262) with P3, at 13.8% (27/196) with P4, and at 12.5% (23/184) with P5 females. The male to female ratio was different (P<0.01) for multiparous females (55:46; n=12,305). The ratio of males to females (55:45) was also different (P<0.01) for all twin pairs (n=1,151), evaluated in the study. The gestation period for cows with single calves was 8.5 days longer (P<0.05) than that of cows carrying twin calves. The birth weight of single calves was 13.1 kg greater (P<0.05) than that of twin calves. The reason for the marked increase in the twinning rate during the warmest months may be explained by greater embryo mortality in the hot vs cool months.
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