Onset of Puberty and Duration of the Breeding Season in Suffolk, Rambouillet, Finnish Landrace, Dorset and Finn-Dorset Ewe Lambs

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Abstract
Sixty ewe lambs, 12 each of Suffolk, Dorset, Finnish Landrace, Finn-Dorset and Rambouillet born in January and February were used in a study of puberty and duration of the first breeding season. Ovulation was monitored via progesterone assay of weekly blood samples beginning in July, and estrus via daily checking with vasectomized rams. All 60 ewes ovulated but three, one Dorset and two Rambouillet failed to show estrus although each had five to seven ovulatory cycles. Several ewes showed small progesterone rises, 300 to 500 pg/ml, before first ovulation and six (10%) failed to initiate regular cyclicity following the first ovulation as defined by progesterone >500 pg/ml. Mean number of ovulatory events before the first behavioral estrus ranged from 1.25 for Finnish Landrace to 2.20 for Rambouillet ewe lambs (P<.1). Rambouillets were older (277 d; P<.05) at first estrus than Finns (258 d) and Finn-Dorsets (260 d), but differences in age at first ovulation were not significant: Finn-Dorset, 233 d; Dorset, 234 d; Finn, 235 d; Rambouillet, 240 d; Suffolk, 245 d. Most cycles were of normal length, with 91.4% between 14 and 20 d. Repeatability of cycle length within breed group was .38. Suffolks had significantly shorter cycles (16.1 d) than the other four groups (17.0 to 17.4 d). Finnish Landrace and Finn-Dorset ewes continued to cycle much longer than the other groups. The mean number of days from first to last estrus and from first to last ovulation were 127 and 157 for Finn, 131 and 155 for Finn-Dorset, 87 and 127 for Dorset, 77 and 107 for Suffolk and 34 and 110 for Rambouillet. Last ovulation was accompanied by estrus in most Finn, Finn-Dorset and Suffolk ewes but in none of the Rambouillets. Copyright © 1985. American Society of Animal Science . Copyright 1985 by American Society of Animal Science

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