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It is well documented that heat stress (HS) causes subfertility in dairy cows. However, during the last ten years we have been observing that, under high temperature-humidity index (THI ≥ 75), despite the overall reduced fertility, some cows conceive at the first artificial insemination (AI). Here, we examined distinctive features of cows with conserved fertility under severe HS. From the databases of three herds, 167 lactating Holstein cows were selected; group TT cows (n = 57) conceived in the previous summer (THI ≥ 75) at the 1st AI, and group TS (n = 110) failed to conceive at the same period after at least 2 consecutive AIs. The animals calved in spring, and in August, blood samples were collected during a hot day (THI ≥ 81) for the determination of cortisol and HSP70 concentrations. In one farm, the validity of fertility data of the previous year was re-examined. In 28 cows from group TT and in 39 cows from group TS, the conception rate was examined during July and August. In 6 cows from each group (TT and TS) the oestrous cycles were synchronized, ovulation was induced with GnRH (THI = 80), and the concentration of the pre-ovulatory LH surge was determined in 9 blood samples. The progesterone concentration in the ensuing cycle was determined in blood samples collected every other day. Overall, cortisol and HSP70 were significantly lower in TT group compared to TS. More (p < .05) animals from group TT conceived at the first AI compared with those from group TS. The induced pre-ovulatory LH surge peaked at higher level (p < .002) in group TT than in group TS, while no difference was recorded among groups either in mean progesterone concentrations or in the duration of the ensuing oestrous cycle. These results are highly suggestive that thermotolerance in some dairy cows is an inherent characteristic, warranting further genetic investigation.

The aims of this study were: to compare total ghrelin concentration throughout pregnancy between lactating cows and heifers, and to study the response to acute feed restriction in pregnant or non-pregnant heifers. Blood samples were collected each month of pregnancy from cows (n = 5) and heifers (n = 5) and analyzed for total ghrelin concentration. Compared to pre-conception values, ghrelin concentrations tended to be greater during 3rd month of pregnancy in heifers, whereas they were higher in the 7th, 8th and 9th months in lactating cows, but no difference was detected between lactating cows and heifers. In experiment two, pregnant (n = 4) and non-pregnant (n = 4) heifers were fasted for 24 h. Blood samples were collected 0, 4, 8, 12, 16 and 24 h of fasting and were assayed for, insulin, glucose, cortisol, BHBA and NEFA concentrations, and at time points 0, 8, 16 and 24 for total ghrelin determination. Compared to satiety, ghrelin concentrations were higher at 8th, 16th and 24th hour of fasting in pregnant and at 8th hour in non-pregnant animals, but no difference was detected between pregnant and non-pregnant heifers. Pregnant heifers had lower glucose concentrations than non-pregnant ones. Insulin concentrations were reduced at 4 and 8 h of fasting in pregnant heifers, and stayed unaffected in non-pregnant ones. Cortisol concentrations increased after 4th hour and remained elevated throughout the sampling period in pregnant heifers, while they increased at 24th h in non-pregnant animals. Here, we provide evidence that total ghrelin concentrations rise in response to feed restriction. Albeit no group effect was evident, our results imply that a) during the last trimester of pregnancy total ghrelin is secreted in different pattern between lactating cows and heifers, b) pregnant animals are more responsive than non-pregnant ones to hunger induced stress.

In two experiments, we studied (a) the changes of LH secretion in heifers under different feeding schedules and (b) total ghrelin concentration at oestrus in cows and heifers. In experiment one, synchronized heifers were allocated in three groups (R, regularly fed controls; F, fasted; and F-F fasted-fed). One day after the completion of the oestrous induction protocol, group F and F-F animals stayed without feed for 24 hr; thereafter, feed was provided to R and F-F cattle; 2 hr later, GnRH was administered to all animals. Blood samples were collected for ghrelin, progesterone, LH and cortisol concentrations. Fasting caused increased ghrelin concentrations in groups F and F-F, while in response to GnRH, LH surge was significantly attenuated in groups F and F-F compared to R. In experiment 2, lactating cows and heifers were used. On day 9 of a synchronized cycle, PGF2α was administered, and blood samples were collected twice daily until the third day after oestrus and analysed for progesterone, estradiol, ghrelin, glucose and BHBA concentrations. No difference was recorded between groups in steroids and BHBA concentrations. In comparison to mid-luteal values, ghrelin concentrations significantly increased at perioestrual period in cows, but not in heifers. This study provides evidence that starving-induced elevated ghrelin concentrations can have suppressing effect on LH secretion, even after ghrelin's restoration to basal values and that during oestrus, ghrelin secretion is differently regulated in cows and heifers, likely being independent from oestradiol concentrations. Further research is required to identify the determining factors that drive the different regulation of ghrelin secretion in cows and heifers.