Relationship among climatological variables and hourly distribution of calvings in Holsteins fed during the late afternoon.

A survey of 924 calvings of Holstein cows and heifers in one large commercial herd during the fall of 1985 revealed a nonuniform pattern of calving over a 24-h period. Pregnant females were attended 24 h/d and fed daily a complete diet between 1500 and 1600 h during the dry period or the last 30 to 60 d of gestation. More calvings occurred from 0601 to 1800 h than from 1801 to 0600 h (56 versus 44%). Cows calving during the last 12 h of the day (1201 to 2400 h) had gestation periods that averaged 1.3 d longer than cows calving during the first 6 h of the day. Length of gestation was increased 1.3 d for every hour decrease in daylength from September to December but was unaffected by any other climatological measure (barometric pressure, precipitation, wind velocity, temperature, percentage sunshine, or relative humidity). The hour of calving was unrelated to any of the climatological measures during the week preceding parturition. However, partial regressions of climatological variables on days to calving revealed significant regressions for barometric pressure [quadratic; beta = (60.1 +/- 2.6) x 10(-13) mm2 Hg/d], precipitation [beta = (2.02 +/- .8) x 10(-1) mm/d], humidity [beta = (1.2 +/- .2) x 10(-2)%/d]. and daylength [beta = (-3.5 +/- .5) x 10(-1) h/d]. It was concluded that pregnant cows and heifers fed during the late afternoon calved in a nonrandom pattern during the 24-h day; more calvings occurred during daylight hours. Changes in various environmental measures may serve as biological cues to initiate parturition in the bovine.