Livestock Production Science 68 (2001) 231–241www.elsevier.com/ locate / livprodsci

Influence of heat stress or feed restriction on plasmaprogesterone, oestradiol-17b, LH, FSH, prolactin and cortisol in

Holstein heifersa , b c a a*B. Ronchi , G. Stradaioli , A. Verini Supplizi , U. Bernabucci , N. Lacetera ,

d a dP.A. Accorsi , A. Nardone , E. SerenaInstitute of Animal Husbandry, University of Tuscia, via C. de Lellis, 01100 Viterbo, Italy

bDepartment of Animal Production Science, University of Udine, via delle Scienze 208, 33100 Udine, ItalycInstitute of Animal Production, University of Perugia, via San Costanzo 4, 06126 Perugia, Italy

dDepartment of Veterinary Morphophysiology and Animal Production, University of Bologna, via Tolara di Sopra 50,40064 Ozzano Emilia (BO), Italy

Received 7 July 1999; received in revised form 15 February 2000; accepted 25 July 2000

Abstract

The aim of the study was to compare the effects of heat stress and feed restriction on hormonal secretion (progesterone,oestradiol-17b, luteinizing hormone, follicle-stimulating hormone, prolactin and cortisol) in Holstein heifers. Ten pubertalheifers were divided into two groups of five (A and B) and housed in climatic chambers. After a pre-experimental period, theheifers were synchronised for oestrus and monitored for three (group B) or four (group A) consecutive oestrus cycles (OC).In the first OC, both groups were maintained under thermal comfort (TC) and fed on an ad libitum basis. In the second OC,group A was maintained under TC whereas group B was exposed to high air temperatures (HAT); both groups were fed onan ad libitum basis. In the third OC and until day 17 of the fourth OC, group A was kept under TC and fed a restricted diet(the same ration ingested by group B under HAT). At the end of HAT exposure, group B was removed from the study.Exposure to HAT caused development of ovarian cysts in two heifers, an increase in plasma concentrations of prolactin, adecrease in concentrations of cortisol and progesterone, and a 23% reduction in dry matter intake. Feed restriction did notmodify any of the parameters considered. Results of this study indicated that the effects of HAT on the above parameters arenot altered by a reduction in feed intake. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Dairy cattle; Heat stress; Feed restriction; Reproduction; Hormonal secretion

1. Introduction fertility in dairy cows during hot seasons and adecline in conception rate (Ron et al., 1984; Wise et

Several authors have documented depression of al., 1988), an increase in embryo mortality (Ryan etal., 1993), impaired luteal function (Wolfenson et al.,1993; Howell et al., 1994), and disturbances in*Corresponding author. Tel.: 1 39-761-357-444; fax: 1 39-gonadotrophin (Gilad et al., 1993) and oestradiol761-357-434.

E-mail address: ronchi@unitus.it (B. Ronchi). secretion (Wilson et al., 1998) have been observed in

0301-6226/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0301-6226( 00 )00232-3

232 B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241

heat-stressed cattle. Moreover, high air temperatures assigned to two experimental groups (A and B)are known to modify oestrus expression (Gwazdaus- which were arranged by splitting each of the fivekas et al., 1981), increase frequency of silent oestrus pairs of sibs. The two groups were similar in body(Rodtian et al., 1996) and change follicular dynamics weight (426.6668.6 vs. 420.6669.3 kg for groups A(Stradaioli et al., 1994; Wilson et al., 1998). and B, respectively, P . 0.05) and body condition

The effects of heat stress on reproductive function score (2.560.4 vs. 2.660.5 for groups A and B,and hormone changes (Richards et al., 1995) have respectively, P . 0.05). Body condition was scoredbeen thought to be partly related to a reduction in according to Edmondson et al. (1989) and always byfeed intake. The main purpose of this study was to the same person. The heifers were housed in twoinvestigate the common and individual effects of climatic chambers with individual tie stalls. Eachheat stress and feed restriction on ovarian (progester- chamber was 5.6 m wide, 10.0 m long and had a

3one, oestradiol-17b) and pituitary (luteinizing hor- 162.4 m capacity. The two chambers, each capablemone (LH), follicle-stimulating hormone (FSH), of housing a maximum of six mature cows, wereprolactin (PRL)) functions, and on cortisol secretion equipped with individual feeders and water bowls.in Holstein heifers. Ambient temperature and relative humidity were

computer-controlled using heater and refrigeratorunits which were monitored continuously. The

2. Materials and methods photoperiod schedule (14 h light (400 lux) and 10 hdark) and air circulation (0.5 ambient volume/h)

2.1. Animals, experimental design and feeding were maintained during the entire trial.The schematic representation of the experimental

The study was carried out using 10 cyclic 15- to design is shown in Fig. 1. The heifers were main-16-month-old Holstein heifers (two pairs of full-sibs tained for a 14-day pre-experimental period underand three pairs of half-sibs). The animals were thermal comfort (TC) (188C and 70% relative

Fig. 1. Schematic representation of the experimental design.

B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241 233

humidity 5 temperature–humidity index (THI) of weighed once daily at 08:30 h and feed intake was65) to allow them to adapt to the new housing recorded. The diet was adjusted continuously toconditions. During that period no data were col- maintain a constant forage /concentrate ratio. Thelected. heifers had free access to tap water. Individual water

The experimental period lasted for 95 days and intake was recorded once daily at 08:30 h usingstarted immediately after the end of the pre-ex- water meters attached to the water bowls.perimental period. At the end of the pre-experimentalperiod, the heifers of both groups (A and B) were 2.2. Rectal temperatures and blood samplessynchronised with 3 mg of norgestomet (Crestar,Intervet, Italy) which were ear implanted for 9 days. Rectal temperatures (RT) were recorded at 09:00Seven days after implantation, 0.5 mg of clopros- h every 2 to 3 days using a glass precision ther-tenol (Estrumate, Pitman-Moore, Italy) was injected mometer with a prismatic section (Artsana Vet-i.m. Within 72 h of implant removal, all animals erinaria, Como, Italy) and 0.18C accuracy. Blood (20showed signs of oestrus (behavioural (bellowing) ml) was collected from the jugular vein every 15 minand physical (red and swollen vulva, and clear mucus for 12 h (from 08:00 to 20:00 h) on day 0 (oestrus),discharge from vulva)). From the first oestrus (1stO), and on day 12 of the 1st (covariate period), 2nd (TCall heifers were monitored twice daily, in the morn- and HAT treatments) and 4th (TCRF treatment) OCing and in the evening, until the end of the trial to (Fig. 1) to determine plasma LH, FSH and PRL. Thedetect the signs of the subsequent oestrus. The two day before frequent blood sampling, all heifers weregroups were maintained under TC for the first 17 fitted with a jugular catheter (14-gauge, Intraflon 2,days of the first oestrus cycle (OC). That period was Vygon, France).considered as a covariate period. Starting on day 17 In TC and HAT treatments during the 3rdO, bloodof the first OC, after morning blood sampling, heifers samples (10 ml) were collected, at 3-h intervals,in group B were exposed to high air temperatures from the first sign of oestrus until ovulation, which(HAT) (328C and 70% relative humidity 5 THI of was monitored at 6-h intervals using a real time array84) while group A remained under TC. A THI of 84 ultrasound scanner (Toshiba Sonolyer, SAL32A,was chosen in order to induce a condition of Japan) equipped with a 5 MHz intrarectal probe.intermediate heat stress (between mild and severe). Blood samples were utilised to determine plasmaThe passage from TC to HAT took 24 h to complete. LH, oestradiol-17b and cortisol. Blood samples (10The HAT treatment for group B started 4 days before ml) were also taken every 2 to 3 days from day 0the expected second oestrus (2ndO) and ended when until day 17 of the 1st, 2nd and 4th OC at 08:00 hall heifers ovulated in the third oestrus (3rdO) (Fig. (Fig. 1) to determine plasma progesterone,1). After this ovulation, the heifers kept under HAT oestradiol-17b, PRL and cortisol. Blood sampleswere removed from the study. After ovulation were collected from the jugular vein into heparinisedoccurred in the 3rdO, feed was restricted for group A evacuated tubes and immediately centrifuged atheifers (they were given the same ration as that 2000 3 g for 10 min. Plasma was stored at 2 208Cwhich group B heifers voluntarily ingested under until analysis.HAT (see Section 3)) and they remained under TC(TCRF). The TCRF treatment was maintained for 2.3. Hormone assaysgroup A during the third OC and until day 17 of thefourth OC (Fig. 1). During the third OC, no mea- Progesterone and oestradiol-17b were determinedsurements were carried out. by specific radioimmunoassays (RIA). The sensi-

The diet consisted of Italian ryegrass hay and tivity (90% B/B ) of the progesterone antibody waso

commercial concentrate (82/18 ratio on a dry matter 2.5 ng/ml and the cross-reactivities were:(DM) basis). The ration was administered twice daily 9.7% with hydroxyprogesterone-11a, 1.5%(at 09:00 and at 17:00 h) and contained 0.62 fodder with hydroxyprogesterone-17a, 0.3% withunits for milk production /kg and 12% crude protein hydroxyprogesterone-20a, , 0.002% with testoster-on a DM basis. Individual feed refusals were one, 0.05% with cortisol, , 0.0002% with oestrone

234 B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241

and , 0.0001% with oestradiol-17b. The sensitivity 2.4. Statistical analysis(90% B/B ) of the oestrogen antibody was 1.01o

pg/ml and the cross-reactivities were: 100% with Each value of LH, FSH and PRL which was 30%oestradiol-17b, 1.5% with oestradiol-17a, 0.8% with greater than the previous value and was followedoestriol, 0.03% with oestriol sulphate, 1.5% with immediately by at least two lower values wasoestrone and 0.1% with oestrone sulphate. considered as a pulse (Gabai et al., 1994). Data were

Plasma LH concentrations were determined by a analysed using the GLM procedure of SAS (SAS,heterologous double-antibody method. This involves 1996). Dry matter intake, rectal temperature andthe use of rabbit antiserum to bovine LH (NIH-LH- plasma concentrations of hormones were evaluatedB10) at a final dilution of 1:200,000, which dis- utilising two different models. The first model wasplayed negligible cross-reactivity with adrenocor- used to evaluate the differences between groups Aticotrophin hormone (ACTH), FSH, growth hormone and B during the covariate period. It considered the(GH), PRL and thyroid-stimulating hormone (TSH). following effects: group (A and B), heifer withinThe balance point (the concentration of hormone that group, day (day of OC), group 3 day interaction andinduces a displacement of 50% B/B ) was 540 the error term. Differences between groups wereo

pg/ tube and sensitivity (90% B/B ) was 55 pg/ tube. analysed according to the test of hypotheses usingo

NIH-LH-B10 was used as the standard reference and heifer within group as the error term (SAS, 1996).bovine LH LER 1072-2 was used to prepare the The second model considered the following effects:radio-iodinated tracer according to Salacinski et al. treatments (TC, HAT and TCRF), heifers, day (day(1981). The intra-assay and inter-assay coefficients of OC), treatment 3 day interaction and the errorof variation (CV) were 9.7% and 13.8%, respective- term. Least square means were separated with thely. A heterologous double-antibody RIA system, PDIFF procedure of SAS (SAS, 1996). Significanceusing rabbit antiserum to ovine FSH (NIDDK anti- was set at P , 0.05.oFSH-I) at a final dilution of 1:40,000, was utilisedfor the measurement of plasma FSH. The cross-reactivity with ACTH, PRL, GH and LH wasnegligible ( , 0.003%). USDA-bFSH-I-2 was used 3. Resultsas the standard reference and to prepare the radio-iodinated tracer according to Salacinski et al. (1981). 3.1. Rectal temperatures and feed intakeThe balance point was 164 pg/ tube and sensitivity(90% B/B ) was 27 pg/ tube. The intra-assay and Rectal temperatures did not differ between groupso

inter-assay CV were 8.1% and 12.7%, respectively. A and B during the covariate period (38.78C in bothPlasma prolactin (PRL) concentrations were mea- groups). The heifers under HAT showed higher (P ,

sured by a heterologous double-antibody RIA, using 0.01) RT compared with heifers under TC or TCRFa rabbit anti-oPRL serum at a final dilution of (Fig. 2). There were no differences in RT between1:300,000. LER-891-bPRL was used as a standard TC and TCRF treatments (Fig. 2).reference and NIDDK-bPRL-I-1 was used to prepare Dry matter intake (8.861.2 vs. 9.161.6 kg/day/the radio-iodinated tracer according to Salacinski et head, P . 0.05) and water intake (37.5262.1 vs.al. (1981). The balance point was 322 pg/ tube and 38.0262.9 l /day/head, P . 0.05) did not differthe sensitivity (90% B/B ) was 33 pg/ tube. The between groups A and B during the covariate period.o

intra-assay and inter-assay CV were 6.9% and Heifers under HAT ingested | 23% less (P , 0.01)14.7%, respectively. Plasma cortisol concentrations DM than heifers under TC (Fig. 3). Water intake waswere evaluated using a RIA. The sensitivity (90% higher (P , 0.01) under HAT than under TC orB/B ) of the cortisol antibody was 4.93 ng/ml, and TCRF (52.8063.3 vs. 38.7062.3 and 34.4062.1o

the cross-reactivities were: 20.4% with cortisone, l /day /head, respectively). Significant differences74.6% with deoxycortisol-11a, 1.13% with corticos- (P , 0.05) in water intake between TC and TCRFterone, and 0% with progesterone and oestrogens. treatments were also observed.

B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241 235

plitude of PRL pulses, and PRL baseline concen-trations (data not shown).

Before presenting results regarding the hormonalchanges during the experimental period, it must bementioned that two heifers under HAT were diag-nosed as having ovarian cysts. The diagnosis wasmade by ultrasonographic examinations which werecarried out for 15 consecutive days for other pur-poses. The two animals failed to ovulate and pre-sented very low values of plasma LH, FSH andoestradiol-17b. For these reasons, data from the two

Fig. 2. Least square means6S.E. for rectal temperatures (RT) inheifers were not included in the statistical analysis.Holstein heifers kept under thermal comfort (TC d), high ambientHAT heifers had lower (P , 0.01) cortisol concen-temperatures (HAT j) and thermal comfort but restricted fed

(TCRF m). *P , 0.05 and **P , 0.01 between HAT and TC or trations, as compared with TC and TCRF heifersTCRF treatments. No differences were observed between TC and (Fig. 4). Significantly higher concentrations of plas-TCRF treatments. ma cortisol at pro-oestrus and oestrus were evident,

both under HAT and TC, but mean values were3.2. Hormonal and oestrus cycle changes always lower (P , 0.05 and P , 0.01 at pro-oestrus

and oestrus, respectively) under HAT (Table 1).No differences were observed between groups A Mean plasma progesterone concentrations were

and B during the covariate period for mean plasma lower (P , 0.05) under HAT, compared with TC andcortisol concentrations (5.160.5 vs. 4.560.5 ng/ml, TCRF treatments (2.4360.06 vs. 4.1560.09 andP . 0.05, respectively), secretion of progesterone 4.0060.09 ng/ml, respectively), and a treatment byand oestradiol-17b (data not shown), OC length day interaction was detected (P , 0.01). In particu-(21.660.5 vs. 21.160.5 days, P . 0.05, respective- lar, heifers under HAT showed lower plasma proges-ly), frequency and amplitude of LH and FSH pulses, terone (P , 0.05) from days 11 to 17 of OC,LH and FSH baseline concentrations (data not compared with TC and TCRF treatments (Fig. 5).shown), mean plasma PRL (61.766.0 vs. 58.167.7 No differences were observed between TC andng/ml, P . 0.05, respectively), frequency and am- TCRF heifers (Fig. 5).

Fig. 4. Least square means6S.E. for plasma cortisol in Holsteinheifers kept under thermal comfort (TC d), high ambient tem-Fig. 3. Least square means6S.E. for dry matter intake (DMI) inperatures (HAT j) and thermal comfort but restricted fed (TCRFHolstein heifers kept under thermal comfort (TC d), high ambientm). **P , 0.01 between HAT and TC or TCRF treatments. Notemperatures (HAT j) and thermal comfort but restricted feddifferences were observed between TC and TCRF treatments.(TCRF m). *P , 0.05 and **P , 0.01 between HAT and TC

treatments.

236 B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241

Table 1Least square means6S.E. of plasma oestradiol-17b, luteinizing hormone (LH) and cortisol concentrations in blood samples collected at 3-hintervals for 12 h before and after the LH peak at the third oestrus of the experimental period

Hours from Oestradiol-17b (pg/ml) LH (ng/ml) Cortisol (ng /ml)a bLH peak TC HAT TC HAT TC HAT

d c2 12 5.1061.12 2.6761.44 1.8360.35 1.6160.45 6.1060.63 2.3460.55

d c2 9 6.6761.55 4.8861.99 1.6960.44 2.1760.57 6.9560.38 2.5560.16

d c2 6 6.2661.35 5.7161.75 3.5160.90 3.3661.16 6.2861.63 2.5360.26

D C2 3 6.2861.26 5.8461.67 7.6161.75 5.4562.26 8.0961.87 3.2561.32

D C0 7.1361.11 5.9861.43 18.8561.78 17.0762.30 14.0162.77 5.5062.69D C3 5.2261.24 5.0361.60 10.6362.77 7.4663.57 9.0061.21 1.1660.38D C6 3.6460.40 2.3860.52 4.0561.73 3.0362.23 8.3561.52 0.9260.17D C9 2.4060.49 1.4460.63 1.4060.32 1.3060.41 8.0061.91 1.6260.04D C12 1.4260.32 1.1560.41 1.1560.20 0.9060.25 5.2460.88 0.8060.23

a TC, heifers under thermal comfort and fed on an ad libitum basis.b HAT, heifers under high ambient temperatures.c,dMeans within row with different superscripts within hormone differ (P , 0.05).C,DMeans within row with different superscripts within hormone differ (P , 0.01).

Fig. 5. Least square means6S.E. for plasma concentrations of Fig. 6. Least square means6S.E. for plasma concentrations ofprogesterone from days 0 to 17 of the oestrous cycle in Holstein oestradiol-17b from days 0 to 17 of the oestrous cycle in Holsteinheifers kept under thermal comfort (TC d), high ambient tem- heifers kept under thermal comfort (TC d), high ambient tem-peratures (HAT j) and thermal comfort but restricted fed (TCRF peratures (HAT j) and thermal comfort but restricted fed (TCRFm). *P , 0.05 between HAT and TC or TCRF treatments. No

m).differences were observed between TC and TCRF treatments.

(2nd OC) and TCRF (4th OC), respectively). LHNo differences were observed in plasma concen- concentrations in samples collected at 3-h intervals

trations of oestradiol-17b (2.6560.04, 2.5460.06 during the 3rdO did not differ between TC and HATand 2.4760.06 pg/ml, for HAT, TC and TCRF, heifers (Table 1). Frequency and amplitude of LHrespectively) between the various treatments and and FSH pulses, and LH and FSH baseline con-neither was the treatment by day interaction signifi- centrations were not significantly different betweencant (Fig. 6). In blood samples collected at 3-h treatments (Table 2). A marked increase in PRLintervals before and after the LH peak in the 3rdO, concentrations (Fig. 7) was observed in heifers underthe concentrations of plasma oestradiol-17b did not HAT, as compared with their TC contemporariesvary between TC and HAT heifers (Table 1). No (198.85610.1 vs. 53.2768.1 ng/ml, respectively;significant differences were observed in OC length P , 0.01). No differences were detected between TCbetween treatments (19.860.4, 20.760.7 and and TCRF heifers (Fig. 7). The heifers under HAT21.160.4 days (P . 0.05) in HAT (2nd OC), TC had higher (P , 0.01) PRL pulse amplitude and

B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241 237

Table 2Least square means6S.E. of pulse frequency and amplitude, and baseline concentrations of luteinizing hormone (LH), follicle-stimulatinghormone (FSH) and prolactin (PRL) in blood collected every 15 min for 12 h during the second (TC and HAT treatments) and fourth(TCRF treatment) oestrus and dioestrus of the experimental period

aPulse frequency (no. /12 h) Pulse amplitude (ng/ml) Baseline (ng/ml)b c dTC HAT TCRF TC HAT TCRF TC HAT TCRF

LH Oestrus 6.261.9 4.861.1 5.761.2 0.960.12 0.760.13 1.060.11 1.160.03 1.060.03 1.660.04Dioestrus 3.060.7 4.460.9 2.460.5 0.960.31 0.860.24 0.760.27 0.660.02 0.660.02 0.760.02

FSH Oestrus 2.560.6 3.861.0 2.761.3 0.760.08 0.760.07 0.760.09 0.560.01 0.560.01 0.560.01Dioestrus 1.660.5 1.460.2 2.660.7 0.760.06 0.760.06 0.760.07 0.560.01 0.560.01 0.560.01

f e f E F E E F EPRL Oestrus 4.560.9 2.060.7 4.060.4 58.966.0 231.969.4 45.266.0 36.463.2 179.263.1 27.161.3E G F E F EDioestrus 3.860.9 4.060.9 3.060.6 50.4611.3 203.3611.1 28.265.4 20.564.0 133.764.0 21.162.0

a Pulse amplitude 5 pulse height minus average of all samples.b TC, heifers under thermal comfort and fed on an ad libitum basis.c HAT, heifers under high ambient temperatures.d TCRF, heifers restricted fed and kept under thermal comfort.e,f Means within row with different superscripts within hormone differ (P , 0.05).E,F,GMeans within row with different superscripts within hormone differ (P , 0.01).

done et al., 1993) and that of others (Johnson, 1987),the 23% reduction in DM intake observed in HATheifers can be considered within the expected rangeof DM intake reduction for the environmental con-ditions imposed in our trial.

Christison and Johnson (1972) reported a decreasein plasma cortisol concentrations in cattle afterprolonged exposure to HAT. The reduction in cor-tisol is related to an adaptation mechanism. Hydro-cortisone is a thermogenic hormone, and the reduc-tion of adrenocortical activity under heat stress is

Fig. 7. Least square means6S.E. for plasma prolactin (PRL) inconsidered a thermoregulatory protective action pre-Holstein heifers kept under thermal comfort (TC d), high ambientventing increases in metabolic heat production.temperatures (HAT j) and thermal comfort but restricted fed

(TCRF m). *P , 0.05 and **P , 0.01 between HAT and TC or Nevertheless, the significant increase in cortisolTCRF treatments. No differences were observed between TC and during pro-oestrus and oestrus, noted under TC andTCRF treatments. HAT during the 3rdO, suggests that chronic heat

stress depresses cortisol concentrations but does notalter the mechanisms responsible for increased secre-

baseline concentrations during OC, compared with tion during the follicular phase. The depression oftheir TC and TCRF contemporaries, associated with cortisol concentrations on the day of oestrus maya decreased (P , 0.05) pulse frequency (Table 2). have a negative effect on the fertility of cattle (Max,No differences were observed between TC and 1990). The author reported a lower percentage ofTCRF treatments (Table 2). cows with palpable ovarian follicles on the day of

oestrus and a lower conception rate to first insemina-tion in cows with lower blood concentrations of

4. Discussion cortisol on the day of oestrus. It seems that depres-sion of cortisol secretion has a negative effect on

Rectal temperature values showed that heifers ovarian follicle development. As feed restriction hadunder HAT were indeed heat stressed (Turner, no effects on cortisol secretion, our results also1982). As indicated in our previous research (Nar- indicate that the depression in cortisol secretion

238 B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241

observed under HAT is due to heat stress and is not and did not observe any modification in progesteronedue to a decline in feed intake. secretion, even though restricted-fed heifers showed

Our results regarding the development of ovarian smaller CL compared with ad libitum-fed heifers.cysts in HAT heifers concur with those reported by More recently, Rhodes et al. (1996) obtained similarLopez-Diaz and Bosu (1992), who found that stress results in restricted-fed (for 5 months) Bos Indicusfactors, including heat stress, may be associated with heifers. Those authors observed a reduction in CLan impairment of reproductive performance that dimensions during the OC preceding anoestrus,could be related to the development of ovarian cysts. whereas progesterone concentrations did not differ.Moreover, the incidence of ovarian cysts under HAT Similarly, Burns et al. (1997), using non-lactatingmay be associated with the negative effect of heat beef cows, reported that daily progesterone con-stress on follicular dynamics during the OC centrations did not differ between ad libitum-fed(Stradaioli et al., 1994). heifers and restricted-fed cows during the OC pre-

Results of previous studies on blood progesterone ceding anoestrus. Contrasting results were reportedin cattle kept in a hot environment are conflicting. by Richards et al. (1995), who found a reduction inOur results, a reduction of progesterone observed plasma progesterone concentrations in multiparousunder HAT, confirm data reported by Howell et al. cows as a consequence of severe and prolonged feed(1994). Those authors only found lower progesterone restriction. In conclusion, our results indicate that theconcentrations between days 6 and 18 of the OC, decline in plasma progesterone observed under HATduring the summer months. As suggested by Wolfen- during the luteal phase should not be ascribed to theson et al. (1993), the lower progesterone concen- reduction in feed intake which also occurred undertrations under HAT might be a consequence of a those conditions.smaller number of luteal cells. This hypothesis is Our findings on oestradiol-17b concur with thosealso supported by other papers (e.g. Younas et al., reported by Roman-Ponce et al. (1981), but are in1993) which document a reduction in serum proges- contrast with results reported by other authors whoterone in heat-stressed cows during the luteal phase. observed higher (Rosenberg et al., 1982) or lowerA possible explanation for the reduction in progester- (Wilson et al., 1998) oestradiol concentrations inone concentrations in cows reared in hot environ- heat-stressed cattle. Such contrasting results arements might be the lower plasma cholesterol availa- likely to depend on the severity or nature (chronic vs.bility and the impairment of lipid metabolism which acute) of heat stress. In the present study, feedoccurs under such conditions (Ronchi et al., 1999). restriction did not affect the oestradiol-17b secretoryAs is already known, cholesterol represents an pattern. Our findings confirm those of others (e.g.indispensable precursor for the de novo synthesis of Richards et al., 1989), indicating that the plane ofprogesterone (Staples et al., 1998). The alteration of nutrition does not alter serum oestradiol concen-lipid metabolism due to heat stress may, therefore, trations.partly explain the negative effects of HAT on An increase in OC length was observed in cattlefertility. In contrast, in a recent study, Wilson et al. exposed to hot environments (Wilson et al., 1998).(1998) observed higher serum concentrations of The absence of effects of heat stress on the con-progesterone in heat-stressed heifers during the luteal centrations and pattern of secretion of oestradiol-17b

phase compared with contemporaries under TC. In observed under HAT in our study may be related tothe same study, heat stress delayed the regression of the short-term exposure of the heifers to a hotthe corpus luteum (CL), which explained the slower environment, since the failure of adequate oestradioldecline of progesterone observed by the same au- secretion precludes the normal sequence of ovarianthors from days 11 to 21 of the OC. Our results events.regarding the effects of feed restriction on progester- Conflicting results have been reported regardingone concur with those reported by several authors. LH changes in ruminants exposed to hot environ-Apgar et al. (1975) studied the effects of a restricted ments. No changes in the LH secretory pattern indiet (60% of energy requirements) on the sensitivity animals exposed to hot environments have beenof the CL to LH in 10 Holstein heifers over four OC observed in dairy cattle (Gwazdauskas et al., 1981;

B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241 239

Younas et al., 1993). On the contrary, Madan and explanation for the marked and prolonged increase inJohnson (1973) reported a decrease in the LH PRL during heat exposure may derive from the rolepreovulatory peak in heat-stressed cattle and Gilad et played by PRL in controlling renal hemodynamicsal. (1993) found lower LH basal concentrations and (Becker et al., 1985), electrolyte balance (Collier etlower LH amplitude in heat-stressed cows with low al., 1982) and water intake (Schams et al., 1980b).plasma oestradiol. The absence of significant differ- Salah et al. (1995) suggested that PRL might have anences in the LH secretory pattern observed in our important role in thermoregulation during exposurestudy may be related to the relatively short period of to high ambient temperatures. Nevertheless, hightime between the beginning of HAT exposure and plasma concentrations of PRL, even though poten-the LH evaluation. On the basis of these results, it is tially useful for acclimatisation to hot environments,likely that acute heat stress does not affect LH could be responsible for modifying the resumption ofsecretion. As regards the effects of feed restriction ovarian function in heat-stressed cattle (Weiss et al.,on the LH secretory pattern in cattle, in contrast to 1981).our findings, other authors (Richards et al., 1989)observed a reduction in LH concentrations and pulsefrequency. The main discrepancy between our results

5. Conclusionsand those of other studies may be due to the kind offeed restrictions utilised, being longer and more

The data reported in the present study indicate thatsevere than those applied in our study.

heat stress is responsible for significant effects on theGilad et al. (1993) reported lower concentrations

reproductive function of Holstein heifers, as assessedof FSH in acute and chronic heat-stressed cows

by plasma concentrations of progesterone, LH, FSH,which also had lower concentrations of oestradiol.

PRL and cortisol during the oestrus cycle. Further-The same authors found no alterations in concen-

more, our results also indicate that the effects of heattrations of FSH in cows exposed to heat stress but

stress on these aspects of the reproductive functionwhich had normal concentrations of oestradiol. The

are not influenced by a reduction in feed intakeabsence of effects of heat stress on FSH secretion in

which also occurs under high air temperatures. Theour trial might be related to the short-term exposure

role of feed restriction in relation to other aspects ofof the heifers to the hot environment and to the

reproductive function altered by heat stress and theunaltered concentrations of oestradiol observed in

mechanisms by which heat stress affects reproduc-these animals. Feed restriction can affect FSH in

tive efficiency still need to be clarified.several ways. Looper et al. (1996) reported anincrease in FSH concentrations after short-term feedrestriction and a decrease in FSH after long-termfeed deprivation in ovariectomized cows. Gutierrez Acknowledgementset al. (1997) found no effects of feed deprivation onFSH concentrations in Hereford 3 Friesian heifers. The authors thank Prof. M.M. Shafie (DepartmentOur results would indicate that the magnitude of Animal Production, Cairo University, Egypt) for(223%) and the length (20 days) of feed restriction his helpful scientific suggestions. The authors areimposed in our trial were not severe or long enough grateful to Prof. S. Raiti (National Hormone andto alter normal gonadotrophin secretion. Pituitary Program, NIDDK, Baltimore, MD) for

Schams et al. (1980a) found an increase in plasma providing bLH (NIH-LH-B10) and bPRL (NIDDK-PRL in heifers exposed to HAT and hypothesised a B-PRL-I-1), and antiserum to ovine-FSH (NIDDK-direct effect of heat stress on PRL concentrations. anti-oFSH-1), to Dr. D.J. Bolt (USDA AnimalConsidering our findings (no effects of feed restric- Hormone Program, Germplasm Laboratory Beltsvil-tion) and those of Schams et al. (1980a), we can le, MD) for providing bFSH (USDA-bFSH-I-2) andconfirm that the increase in PRL and the changes in to Prof. L. Reichert Jr. for providing bPRL (LER-the PRL secretory pattern observed under HAT are 891-bPRL) and bLH (LER 1072-2). Research sup-directly associated with hyperthermia. A possible ported by RAIZ-Mi.P.A., paper no. RZ-247.

240 B. Ronchi et al. / Livestock Production Science 68 (2001) 231 –241

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