The influence of season and shade on oestrous behaviour, timing of preovulatory LH surge and the pattern of progesterone secretion in FFPN and Creole heifers in a tropical climate.

The aim of this work was to study the duration of oestrous behaviour, timing of the LH surge and the progesterone pattern with and without shelter during January and again in July in FFPN and Creole heifers in a tropical environment. From days 21 to 28 after a synchronization treatment (progesterone + PMSG), ten Creole heifers and ten FFPN heifers were checked for oestrus and sampled for blood every 3 h to assay plasma LH levels and every day from that oestrus to the following one to assay plasma progesterone content. Five heifers in each group were housed in a covered corral and five were exposed to the sun in a corral without shelter. The mean duration of oestrus for all the heifers was 14.0 h. There was a significant breed/season interaction: 21 h in July and 15 h in January for FFPN heifers. Sun exposure did not significantly affect the mean duration of oestrus. The distribution of onset of heat with respect to time of day was not uniform: 29% of the oestruses began between 5 and 8 a.m. and 19% between 8 and 11 p.m. The mean interval between onset of heat and maximal LH value (3.58 h) only varied significantly with the season (1.6 h in January and 6.0 h in July; P less than 0.05) but was equivalent to those obtained by other authors. The plasma progesterone levels of both breeds were lower than those described for cows in temperate countries and were lower for shaded FFPN heifers in January (2.8 ng/ml) than in July (3.93 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)