Plasma urea concentration can reflect the protein intake in animals (Mejia-Guadarrama
et al., 2002) and high urea concentrations (>10 ng/mL) indicate amino acid catabolism
(Kraetzl et al., 1998). In the current study, urea concentrations were numerically higher
in LWL compared with HWL sows. This may be a reflection of higher protein intake
(Mejia-Guadarrama et al., 2002) which seems to correspond with the fact that fewer LWL
sows had feed refusals during lactation (45% vs. 78%). Relations of urea concentration
and reproductive performance have not been clearly established in pigs. In the current
study, urea concentration during lactation was negatively correlated with progesterone
concentrations at day 1-10 and d 11-15 of gestation, however, no clear mechanism can be
found for this relation.