Relationship between days open and milk yield in dual purpose crossbred cows

Productive and reproductive records from 620 crossbred cows of 2 and more calvings, milked twice a day with suckling of the calf after milking, were evaluated to study the relationship between days open and milk yield. Data was obtained from two farms located at Rosario and Machiques de Perija Counties in Zulia State, Venezuela; in a dry tropical forest and rainy tropical forest, respectively. Days open (PV) was defined as the calving conception interval classified in 4 groups: G1, less than or equal to60; G2, >60 less than or equal to90; G3, > 90less than or equal to 120 and G4, > 120 days. Total milk yield (PT), adjusted milk yield at 244 days (P244) and milk index (milk yield/calving interval) (IRL) were considered as productive variables. Data were analyzed according to GLM procedure using the SAS program. Calving to conception interval according to open days was: G1: 44,8 +/- 9,6 (n=346); G2: 75,4 +/- 8,8 (n=345); G3: 104,4 +/- 8,3 (n=313); G4: 198,5 +/- 70,7 days (n=567). Linear correlation between PT, P244, and IRL with PV were 0,32; 0,11 y - 0,33, respectively (P<0,01). PT was different (P<0.05) between groups 2 and 3 and 2 and 4 with a higher RL in the G2 with the tendency to be lower in the following PV (G1: 1891,2 +/- 63,2; G2: 1927,1 +/- 44,2; G3: 1791,7 +/- 42,5 and G4: 1804,7 +/- 32,4 kg total milk yield per lactation). P244 did not differ among reproductive groups and their means were 1533,8 +/- 45,9; 1623,3 +/- 32,9; 1620,5 +/- 31,7; 1611,4 +/- 20,6 kg of milk for groups 1, 2, 3 y 4, respectively; however, IRL showed a significant decrease (P<0.01) with increasing values of PV: G1: 4,67 +/- 0,12; G2: 4,47 +/- 0,08; G3: 4,14 +/- 0,08; G4: 3,35 +/- 0,05 kg of milk per day of calving interval. These results indicate that cows which get pregnant between 60 and 90 days postpartum had higher milk production average per lactation than other groups. As faras the IRL is concerned, as it was open days increased, milk yield per day of calving interval decreased up to 477 Kg of milk per year (IRL x 365) when comparing cows pregnant before 60 days with those pregnant after 120 days.