The effect of pasture nitrate concentration and concentrate intake after turnout on embryo growth and viability in the lactating dairy cow.

This study investigated the effects on embryo growth and survival rate of feeding heavily-fertilised spring grass, containing high levels of quickly-degradable nitrogen, to pregnant cows. Forty-eight lactating Holstein cows between 2 and 8 weeks pregnant were turned-out, after a one-week transition period onto high- or low-nitrate pasture and fed a high- or low-concentrate supplement. Cows grazing the High nitrate pasture had significantly higher milk and plasma urea concentrations than cows grazing the Control pasture, while cows which were fed less concentrate had a notably higher plasma ammonia. However, there was no evidence that an increased quickly-degradable nitrogen (QDN) intake from pasture affected embryo survival or growth from 20 days onwards. This suggests that the impact of turnout on fertility mainly affects ovulation, fertilisation and/or the early embryo.