RELATIONSHIP BETWEEN DRY PERIOD LENGTH AND PRODUCTION AND REPRODUCTION IN GRAZING JERSEY AND HOLSTEIN COWS IN COSTA RICA
The aim of this study was to evaluate relationships between dry period length (DPL) of Jersey and Holstein grazing cows and succeeding productive and reproductive performance. The study was conducted on 29 dairy herds in the highlands of Cartago, Costa Rica. The predominant pasture grass on the farms was kikuyu (Kikuyuocloa clandestina) and cows were supplemented with 1 kg of a grain mixture/ 2.5 to 3 kg of milk. A total of 4792 completed lactations (Jersey n=3000, Holstein n=1792) were registered from 2009 to 2012. DPL in Jersey (mean= 75.6d, 95% CI: 74.1-77.0d) differed (P < 0.001) from Holstein (mean= 80.7d, 95% CI: 78.2-83.2d). In Jersey cows the DPL was inversely correlated with actual milk yield (-0.23, P < 0.001) and 305d milk yield (-0.27, P < 0.001). Open period length in Jersey cows (mean= 120d, 95% CI: 117-123d) differed (P < 0.001) from Holstein (mean= 150d, 95% CI: 145-154d). Jersey cows with DPL less than 58 d produced more 305d milk (mean= 5583kg, 95% CI: 5480-5687kg, P < 0.001) than cows with a DPL greater than 73 d (mean= 5208kg, 95% CI: 5086-5331kg). Cows in both those dry groups produced less (P < 0.01) milk than cows with a DPL of 58 to 73 d DPL (mean= 5949kg, 95% CI: 5885-6012). Jersey cows with DPL greater than 73 d had shorter (P < 0.001) lactation length (mean= 305d, 95% CI: 299-310d) than those below that threshold (mean= 324d, 95% CI: 321-327d). Calving interval for Jersey cows with DPL less than 58 d (mean= 403d, 95% CI: 398-409d) was greater (P < 0.05) than those with DPL between 58 and 63 d (mean= 392d, 95% CI: 388-397d). 305d milk yield was greater (P < 0.01) in Holstein cows with DPL between 55 and 63 d (mean= 7620kg, 95% CI: 7434-7807kg) than those with DPL less than 55 d (mean= 7162kg, 95% CI: 6964-7361kg) or higher than 86 d (mean= 7066kg, 95% CI: 6830-7301kg). Open period in Holstein cows was greater (P < 0.01) in those with DPL greater than 86 d (mean= 163d, 95% CI: 153-173d) compared to those cows with DPL between 55 to 86 d (mean= 144d, 95% CI: 138-150d). These results suggest that in grazing Jersey and Holstein cow herds milk yield and reproduction performance could be influenced by DPL.
Keywords: Dry period length, grazing cows, dairy cows