Persistency of lactation is a trait of economic importance in any dairy production system. The objective was to investigate the effect of concurrent pregnancy on the persistency of lactation in a commercial herd of dairy cows milked once a day. The data set included 1962 monthly herd-test records (total 8-9 tests per cow) of milk, fat, protein and lactose yield from 219 pregnant cows and 23 non-pregnant cows of different breeds and lactation number. The records were used to model lactation curves for pregnant and non-pregnant cows using random regression with a linear model (Wilmink function). This was not an experiment initially designed to examine the effects of pregnancy on persistency of lactation in dairy cows. Rather, in the course of normal farm operations, some cows became pregnant during lactation and others did not. Pregnancy did not affect the persistency of lactation of cows milked OAD. After conception, there was no difference in total yields between the two groups, but there was a trend that pregnant cows produced lower yields of fat (P=0.0519) and lactose (P=0.0884). This difference between our results and those of previous studies was probably because our cows were only milked once a day or because the data set was not suitable for testing the hypothesis. Further study is required to investigate persistence of lactation in pregnant and non-pregnant cows milked OAD. Keywords: pregnancy; lactation persistency; cows milked once a day

Y, Fu, SW Peterson, and N Lopez-villalobos

New Zealand Journal of Animal Science and Production, Volume 81, Online, 99-105, 2021
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