Effect of days open on milk yield in Bos taurus and Bos indicus crossbred cows

Productive and reproductive records (n=2483) from crossbred cows Bos taurus (BT; n=870) and Bos indicus (BI; n=1613) in three farms located in Zulia State, Venezuela were evaluated to study the predominant breed (PR) effect on the relationship between days open and milk yield. Four reproductive groups were established according to the length of the open period (PV): G1, pound60; G2, >60pound9O; G3, >90pound2120 y G4, >120 days. Total milk yield (PT), adjusted milk yield to 244 days (P244) and milk index (IRL) were considered as reproductive variables: Data were analyzed according to GLM procedure using the SAS program. PT was different between reproductive groups (P<0.01) showing in both breeds a milk yield increase higher days open (P<0.01). In BI cows the P244 did not differ between different PV and in BT cows the difference was between G2 and G4 (P<0.01). When the IRL is considered in both breeds it decreases when the days open increases (P<0.01). These results indicate that for both breeds of cows it they concive later in the postpartum period they have a higher milk yield; however, when daily milk production between parturition is considered cows with shorter days open are more productive overall.