Accounting for the effect of pregnancy in the Canadian test day model

Pregnancy has been reported to have a negative effect on milk yield of dairy cows due to hormonal changes, causing regression of the mammary gland (Akers, 2006), and nutrient requirements of the fetus, reducing available nutrients for milk production (Bell et al., 1995). The effect of pregnancy is small at the beginning of gestation and becomes greater at later stages of gestation when growth and nutrient requirements of the fetus are larger. Significant effect of pregnancy on milk yield is usually observed from the 5 month of gestation onwards (Bormann et al., 2002; Haile-Mariam et al., 2003; Olori et al., 1997; Roche, 2003).The current Canadian Test Day Model (CTDM) does not adjust for the effect of pregnancy and therefore breeding values of non-pregnant cows could be inflated.