Abstract
There were two objectives set forth in this study: 1) To determine if breed differences existed and what effects crossbreeding had on postpartum (PP) cyclicity and fertility in fall calving dairy cows and 2) To evaluate the effects of crossbreeding on the obtainment of puberty in dairy heifers, both in a pasture-based system. Towards the first objective, milk samples were collected from purebred Holstein, purebred Jersey, and Holstein/Jersey crossbred cows twice weekly after calving in both the 2005 and 2006 calving season. Skim milk samples were analyzed for Progesterone (P4) to determine when they returned to cyclicity. Holstein cows weight the most at dry-off, calving, 30d and 60d PP, followed by the crossbred cows; Jersey cows were the lightest at each time point (P <0.01). No differences were seen between breeds in regards to BCS (P = 0.23). At 30d, 60d, and 90d PP over both calving seasons Holstein and crossbred cows had greater milk production than Jersey cows (P < 0.01) and Energy Corrected Milk (ECM) tended to be higher for Holstein and crossbred cows (P = 0.06). A year effect was seen in milk production and ECM such that cows calving in 2006 had higher production and ECM values than those that calved in 2005 (P < 0.05). Fewer Holstein cows were cyclic at 30d, 60d, and 90d PP when compared with purebred Jersey and crossbred cows (P < 0.05). No statistical differences were seen for anestrous interval, but may be present as not all Holsteins had reached cyclicity by 90d PP and were therefore excluded from calculation of breed anestrus interval. Holstein cows also had lower first service conception rates and 90d pregnancy rates than either Jersey or crossbred cows (P < 0.05).
Purebred Holstein, Jersey and various Holstein/Jersey crossbred heifers born in the fall of 2005 were sampled weekly from 4mo until puberty was reached, and serum was analyzed for P4 to determine age at puberty. Serum samples were also analyzed for the hormone Leptin to elucidate its role in obtainment of puberty. Heifers were categorized as 100% Holstein (100% H), >50% Holstein (>50% H), 50% Holstein/ Jersey (50% H/J), >50% Jersey (>50% J), and 100% Jersey (100% J). Age at puberty was positively and linearly correlated to percent Holstein of a heifer, with the greater percentage Holstein of a heifer, the older the heifer will be at puberty (P < 0.01). 100% H and >50% H heifers were heavier at birth than 100% J and >50% J heifers, stayed heavier as time progressed and were heavier at puberty (P < 0.01). There was no effect of breed composition on heifer BCS at anytime from 4 mo to 14 mo of age and little change over time. Differences in wither heights were seen with 100% H heifers being the tallest followed by >50% H, and both were taller than50% H/J, >50% J, and 100% J (P < 0.01). No differences in serum Leptin concentrations were seen regarding percentage Holstein of a heifer from 12wks prior to puberty up to the week of puberty. Changes in Leptin concentrations over time were not significant either. An effect for individual heifers was significant indicating large variability in Leptin for this heifer population (P < 0.05).
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