Milk protein concentration (MP%) has been associated with reproductive performance amongst parous dairy cows. This study aimed to assess whether early lactation MP% of primiparous cows was associated with prior reproductive performance, and to compare the strength of the association between MP% and previous reproductive performance in nulliparous heifers with that in multiparous cows. Primiparous (n = 918 in 35 herds) and multiparous (n = 4,242 in 64 herds) Holstein-Friesian cows with milk production/composition records and sire genetic information in seasonally calving herds were selected from the InCalf database. The date for each herd’s planned start of calving (PSC) was calculated as being 282 d after the date breeding started. The interval between a herd’s PSC date and each cow’s actual calving date (PSC - CI) will therefore reflect time to conception during the previous breeding period. Higher MP% was positively associated with shorter PSC-CI in both primiparous and multiparous cows. Across the range of cows that had a MP% that was ± 0.5% of the herd’s mean MP%, those with the lowest MP% had an average PSC-CI that was either 13 d or from 28 to 32 d later in primiparous or multiparous cows, respectively. These associations were likely due to biological determinants present before and during the cow’s breeding period that were associated with both reproductive performance and subsequent MP%. Since these determinants affected the reproductive performance of non-lactating heifers, some of the associations with MP% were not related to lactation-specific factors even though these same factors may have increased the effect of low MP% on the reproductive performance of the multiparous Holstein-Friesian cows.

J, Fahey, JM Morton, and KL Macmillan

Proceedings of the New Zealand Society of Animal Production, Volume 68, Brisbane, Australia, 69-72, 2008
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