Seasonal fertility differences in synchronised dairy cows: ultrasonic, metabolic and endocrine findings.

The aim of this study was to identify relationships among seasonal differences of fertility, metabolic parameters and appearance of irregular luteal forms in high-yielding dairy cows. Holstein-Friesian cows were put on the Provsynch regimen in winter (n = 10) and in summer (n = 10). Blood sampling (starting 35 days post partum) and rectal ultrasound examinations (starting post insemination) were carried out once a week in each examination period. Metabolic [plasma nonesterified fatty acid (NEFA) and beta-hydroxybutyrate (BHB) levels, ferric reducing ability of plasma (FRAP) and serum beta-carotene] and endocrine parameters [plasma thyroxine (T4), triiodothyronine (T3), insulin-like growth factor (IGF-I) and insulin levels] were measured. In summer, two cows were excluded from the study because of metritis and none of the remaining animals became pregnant, but 6 of the 8 cows had irregular luteal forms (ILF) on their ovaries. In winter, one cow was excluded because of metritis and 6 of the 9 cows became pregnant, while 2 of the 3 open cows had irregular luteal forms. In summer the mean plasma NEFA and BHB concentrations were significantly higher, while serum carotene and plasma IGF-I concentrations were significantly lower than in winter. The high plasma NEFA concentration found in summer seemed to be in association with the lower body condition score (BCS) caused by depressed appetite. In conclusion, statistical analysis supports the hypothesis that increased plasma NEFA and BHB and decreased plasma IGF-I concentrations may result in reduced fertility in summer. These changes may be associated with the more frequent appearance of ILFs and probably have a negative effect on ovarian function and/or oocyte quality.