AbstractChanges in certain blood metabolites, serum progesterone concentration and pregnancy rate, were monitored for single- and twin-suckling cows in two experiments. In experiment 1, cows were managed to achieve condition scores at calving of 2·1 (low) and 3·1 (moderate). Subsequently, these animals were placed on a fixed level of energy intake (80 MJ metabolizable energy (ME) per day) for 16 weeks. Post-partum diets contained differing amounts of digestible undegradable protein (DUP), but similar amounts of effective rumen degradable protein (ERDP). In experiment 2, all cows were fed to achieve a condition score at calving of around 2·75. These animals were subsequently placed on two levels of energy intake (80 MJ and 130 MJ ME per day) for 16 weeks. The diets providing these levels of energy intake also contained differing amounts of DUP. Pregnancies were established by embryo transfer following induced oestrus.Cows in moderate body condition at calving subsequently had higher (P < 0·05) serum non-esterified fatty acid (NEFA) concentrations. Cows on 130 MJ ME per day had lower serum NEFA concentrations than cows on 80 MJ ME per day. Cows given higher DUP diets had higher (P <0 ·05) serum NEFA concentrations when in negative energy balance and lower serum NEFA concentrations when close to energy equilibrium than cows given lower DUP diets. Twin-suckling cows had higher (P < 0·05) serum NEFA concentrations than single-suckling cows, but only during the first 3 weeks post partum. Plasma glucose concentrations were below 3 mmol/lfor twin-suckling cows until week 7 post partum when they became similar to the values for single-suckling cows. Serum D-3-hydroxybutyrate concentrations mirrored those of plasma glucose. Serum urea-nitrogen concentrations were high for twin-suckling cows on 80 MJ ME per day and single-suckling cows on 130 MJ ME per day.Neither body condition at calving nor post-partum energy intake significantly influenced the time taken for cows to overcome anovulation nor did they significantly influence the proportion of cows which eventually resumed normal oestrous cyclicity. DUP concentration in the post-partum diet had no overall effect on the various measures of fertility assessed but thin cows at calving subsequently placed on the high DUP diet took longer to ovulate (55 days; P < 0·05) and had the lowest pregnancy rate (0·30). The post-partum anovulatory period was longer (P < 0·01) by 27 days in experiment 1 and 12 days in experiment 2 for hvin-suckling cows compared with singlesuckling cows. A lower proportion (P < 0·001) of twin-suckling cows regained oestrous cyclicity and became pregnant in experiment 1. Fertility traits were improved when twin-suckling cows were in moderate body condition at calving and/or offered 130 MJ ME per day during the post-partum period.