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Variation in the timing of puberty in grazing Holstein-Friesian dairy heifers.

N. Steele




Variation in the timing of puberty in grazing Holstein-Friesian dairy heifers.
N. Steele*1, M. Stephen1, A. Young1, B. Kuhn-Sherlock1, P. Amer2, S. Meier1, C. Phyn1, C. Burke1. 1DairyNZ Limited, Private Bag 3221 Hamilton, New Zealand, 2AbacusBio Limited Dunedin, New Zealand.

Age at puberty varies widely across and within herds, with research indicating that timing of puberty could be an early predictor of cow reproduction. Commercial dairy herds (n = 54) from 3 regions in New Zealand were studied to investigate the phenotypic and genetic (co)variances of age at puberty and measures of fertility in Holstein-Friesian dairy cattle. Herein, we report preliminary phenotypic findings of the timing of puberty and associated BW in 5,010 heifers born in spring 2018. Herds were visited on 3 occasions to collect blood samples when average (�SD) heifer ages for each herd were 299 � 15 d (visit 1; V1), 327 � 15 d (V2) and 355 � 15 d (V3). Body weights were measured at V2. Blood plasma was assayed to determine progesterone (P4) concentration, and the timing of puberty was assigned to each animal as either V1, V2, or V3 when their P4 reached a threshold of ≥1 ng/mL for the first time. A heifer was considered still pre-pubertal after V3, if P4 concentration did not exceed this threshold in any collected samples. We identified that 19.4% (n = 973) of heifers had reached puberty by V1 (average age and BW � SD; 302 � 14 d and 245 � 33 kg), 18.8% (n = 941) by V2 (331 � 16 d and 239 � 32 kg), 16.8% (n = 842) by V3 (355 � 14 d and 229 � 31 kg), and 43.3% (n = 2,170) had not reached puberty by V3 (352 � 14 d and 211 � 30 kg). A further 84 heifers (1.7%) were not assigned an age at puberty due to missing blood samples. A large variation was observed between herds in the percentage of heifers reaching puberty (mean = 61% � 25% of heifers by V3, range 2% to 100%). Research into management factors contributing to between-herd variation is underway. Next, we will examine the phenotypic variation in puberty onset between animals that can be explained by genetics. Relationships between puberty and subsequent fertility measures during lactation will then be investigated to identify earlier, novel traits to estimate genetic merit for fertility.

Keywords: pubertal age, body weight.