Recording system and quality of dairy cattle production data in Tunisia

Investigation of data quality was done through fitting the incomplete gamma function to lactation curves on a sample of 18053 lactations with test-day records of 7032 HolsteinFriesian cows. These cows are from four production sectors, the state herds (OTD), the cooperative herds (UCPA), the groups of private investors’ herds (SMVDA), and the farmers’ herds. After editing 8640 records remained. The effects of production sector, herd, parity, first test-day date, calving year, calving season, and the length of lactation on lactation curve traits were analysed. The factors associated with milk yield at the beginning of lactation and the decreasing phase of the curve, persistency, and peak yield varied significantly (P<0.01) with all variables. The state herds had the lowest peak and total yields. The summer season was unfavourable for milk production. Third lactation cows had the highest peak and total yields. Around 25% of the lactation curves were atypical (a<0 or b<0). Seven factors were significantly associated (p<0.01) with the occurrence of atypical lactations.