www.elsevier.com/locate/tvjl

The Veterinary Journal 169 (2005) 113–115

TheVeterinary Journal

Short communication

Effect of prolonged use of altrenogest on behaviour in mares

David Hodgson a,*, Stephanie Howe a, Leo Jeffcott b, Stuart Reid c, Dominic Mellor c,Andrew Higgins d

a Faculty of Veterinary Science, University of Sydney, Private Mailbag 4, Narellan Delivery Centre, Narellan, NSW 2567, Australiab Department of Clinical Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK

c Comparative Epidemiology and Informatics, Department of Veterinary Clinical Studies, University of Glasgow Veterinary School,

Bearsden Rd, Bearsden, Glasgow G61 1QH, UKd Federation Equestre Internationale, Avenue Mon Repos 24, P.O. Box 157, 1000 Lausanne, Switzerland

Accepted 15 April 2004

Abstract

Oral administration of altrenogest for oestrus suppression in competition horses is believed to be widespread in some equestrian

disciplines, and can be administered continuously for several months during a competition season. To examine whether altrenogest

has any anabolic or other potential performance enhancing properties that may give a horse an unfair advantage, we examined the

effect of oral altrenogest (0.044 mg/kg), given daily for a period of eight weeks, on social hierarchy, activity budget, body-mass and

body condition score of 12 sedentary mares. We concluded that prolonged oral administration of altrenogest at recommended dose

rates to sedentary mares resulted in no effect on dominance hierarchies, body mass or condition score.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Altrenogest; Regumate; Anabolic; Behaviour; Competition

Altrenogest, a progestagen, is the 17a-allyl derivativeof the potent anabolic agent trenbolone which is struc-

turally similar to the male sex hormone, testosterone.Altrenogest is reputedly used widely, and for long peri-

ods, in female competition horses. This occurs in an at-

tempt to ameliorate reproductive cyclicity, thus making

them more tractable and suitable for training and com-

petition. However, prolonged administration is not in

accordance with manufacturers� recommendations. In

most countries, the drug is registered only for oral ad-

ministration to mares for 10 days for the purpose of in-duction of ovulatory oestrus in the late transitional stage

of the oestrus cycle. Altrenogest suppresses behavioural

oestrus in mares within 2–3 days of commencement of

administration. The effects of long-term administration

1090-0233/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.tvjl.2004.04.020

* Corresponding author. Tel.: +61-2-4655-0743/0777; fax: +61-2-

4655-6942/1212.

E-mail address: davidh@camden.usyd.edu.au (D. Hodgson).

to mares of altrenogest are largely unknown, particu-

larly with reference to those used in competition. Addi-

tionally, it is not clear whether the drug has any anabolicor significant behavioural effects in horses.

A study was therefore designed to provide a prelimi-

nary assessment of prolonged administration of altreno-

gest to sedentary mares in terms of social hierarchy,

body-mass and body condition score (BCS). Twelve

mares of mixed breed (Thoroughbred and Standardbred)

were acquired from a local horse dealer. The animals were

kept at pasture as a single group for a total of 20 weeks.Following acquisition, all mares underwent routine

health care procedures, including de-worming, dental

prophylaxis, hoof care and vaccination. Mares were al-

lowed to acclimate to the new environment for four weeks

duringwhich they stabilised as a social group, andbecame

accustomed to routine management procedures, e.g., be-

ing caught, weighed, placed in a crush, administration of

oral medication by syringe. Throughout the entire period

Table 1

Flow chart of activities

Week 0 Mares acquired

Weeks 1–4 Acclimation (routine health care, acclimation

to scales, weigh and BCS)

Weeks 5–8 Pre-treatment assessment period (weigh, BCS,

determine hierarchy)

Weeks 9–16 Treatment period (weigh, BCS,

determine hierarchy)

Weeks 17–20 Post-treatment assessment period (weigh, BCS,

determine hierarchy)

114 D. Hodgson et al. / The Veterinary Journal 169 (2005) 113–115

of study supplementary lucerne hay was fed. Mares were

weighed and condition score recorded twice weekly.

A summary of the study protocol is given in Table 1.

Following acclimation, at the beginning of week 5,

mares commenced the pre-treatment assessment period,

which ran for four weeks. The hierarchy of the group

was established using a modification of standard tech-

niques (Houpt and Wolski, 1980; Keiper and Sambraus,1986). This assessment was made at regular intervals

throughout each of the periods in the study. Briefly,

dominance was determined using a paired feeding test.

During the 15 min test two horses were in a 3 m · 3 m

yard and had access to one bucket of grain. The domi-

nant horse ate from the bucket for the greater part of

the 15 min. The horses received no food for 16–18 h be-

fore the test. The yard in which the tests were carried outwas familiar to all horses. No horse was tested more

than once a day. Each horse in the herd was paired with

every other horse in the herd with each pair being tested

once. Within the herd the horse dominant to all other

horses was given the rank of 1 (see Table 2). The horse,

which failed to dominate any other horse, was given the

highest numeric rank (11 in a herd of 11). It was possible

for two or more horses to be dominant over the samenumber of animals. In that case, the lower rank (domi-

nance) was ascribed to the horse that was the dominant

member of that particular pair.

Table 2

Hierarchies for mares throughout the period of study

Order in hierarchy Horse no.

week 5 (pre)

Horse no.

week 8 (pre)

Horse no.

week 12 (tre

1 10 10 10

2 3 3 3

3 4 4 4

4 2 2 2

5 11 11 11

6 5 5 7

7 6 6 5

8 7 7 6

9 12 12 1

10 1 1 12

11 8 8 8

Key: Red = Treatment group; Blue = Control group.

Horse 1 in the hierarchy is the dominant mare, whereas 11 is the least domi

Horse 9 died as a result of an intestinal crisis in week 7. Results for this ma

During the test of hierarchy the aggressive actions of

each horse were recorded. The aggressive actions were

scored as follows:

(1) threat to bite (the aggressor laying its ears back and

moving its head towards the opponent);(2) threat to kick (the aggressor turning the hindquar-

ters towards the opponent with ears laid back and

one hind leg lifted);

(3) bite;

(4) kick;

(5) chase (the aggressor moving at least three steps

away from the bucket and towards the opponent).

The aggressive scoring system is an arbitrary onein which a threat is considered the mildest form

of aggression and given a score of one each time

it occurs and a chase, the most severe type of ag-

gression, is given a score of 5 each time it occurs.

Attempts to assess the activity budget (time spent eat-

ing, non-eating activity and resting) for each mare as de-

scribed by Boyd et al. (1988) was attempted in week 5.However, this proved too difficult and was thus not con-

tinued throughout the study.

At the beginning of week 9 (treatment period) mares

were randomly divided into two groups with six mares

treated daily with altrenogest (Regumate, Hoechst

Roussel) at the recommended dose rate of 0.044 mg/kg

body weight per os by syringe. One of the mares in the

treatment group died as a result of an intestinal crisisin week 10 and results for this animal were excluded

from analyses. The remaining six mares acted as con-

trols and were administered a placebo (yogurt) per os

q24h. The treatment protocol was continued for eight

weeks. Again observations to determine the dominance

hierarchy of the group were made weekly. Routine de-

termination of body-mass and BCS were continued as

described. A similar protocol of observation and meas-

at)

Horse no.

week 16 (treat)

Horse no.

week 18 (post)

Horse no.

week 20 (post)

10 10 10

4 4 4

3 3 3

2 2 2

11 11 11

7 7 7

5 5 5

6 6 6

1 1 1

12 12 12

8 8 8

nant.

re were discarded from the analyses.

Table 3

Weights of horses (kg)

Horse no. Week 1: (accl) Week 5: (pre) Week 8: (treat) Week 12: (treat) Week 18: (post) Week 20: (post)

1 568 584 576 556 550 556

2 572 574 548 542 542 530

3 530 536 528 530 534 530

4 466 475 464 463 456 458

5 548 554 536 536 544 556

6 558 560 552 554 556 556

7 544 542 534 539 540 546

8 522 522 516 516 520 526

9 534 534 Deceased Deceased Deceased Deceased

10 546 552 554 550 554 556

11 520 528 529 528 526 528

12 495 499 500 499 498 496

Key as for Table 1.

D. Hodgson et al. / The Veterinary Journal 169 (2005) 113–115 115

urement was followed for the four weeks following ces-

sation of therapy (post-treatment assessment period).

A comparison between each of the recording periods

used Spearman rank correlations. This compared results

for week 5 vs. week 8 (pre-treatment) to assess variation

before treatment. Then, values for week 5 were dropped

and possible variations due to the treatment were as-

sessed, i.e., week 8 vs. week 12 vs. week 16 vs. week 18vs. week 20. A post hoc two-way comparison was then

performed. Significance was ascribed at P<0.05. Statis-

tical analyses revealed no differences in hierarchy in re-

sponse to treatment with altrenogest (Table 2). All

rankings were significantly correlated, even when cor-

recting for multiple comparisons. Thus, given the pri-

mary hypothesis being tested that there is no difference

in the rankings in response to prolonged treatment withaltrenogest, the experimental data do not provide evi-

dence to reject the null hypothesis (P<0.0001).

As shown in Table 3, statistical analyses also revealed

no differences in body weight in response to prolonged

treatment with altrenogest (P<0.001). Similarly there

was no effect of altrenogest on BCS (P<0.001) (data

not shown).

A search of the literature has revealed two papers re-porting the effect of altrenogest on growth (and other

variables) and these are in pigs by the same group (Klu-

ber et al., 1985, 1988). The data concluded that whereas

feeding altrenogest inhibited testicular growth and ster-

oidogenesis in boars it had no effect on body growth,

carcass traits or back fat thickness. This is probably

to be expected as altrenogest�s 17a-C3 side chain

(–CH2CH@CH2), which provides the prostagenic activ-ity, may well restrict binding to anabolic receptors.

When Johnson et al. (1997) looked at the effect of high

dose, prolonged oral administration of altrenogest on

sexual and aggressive behaviour and semen quality in

young stallions, they found no differences in body

weight, BCS or metacarpal measurements between treat-

ed and control groups, although there was reduced libi-

do and decreased flehmen frequency and duration, a fall

in sperm production and a decrease in scrotal circumfer-

ence, all probably as a result of the effect of the drug on

the testes rather than any centrally acting effect. There

is, however, no obvious therapeutic or regulatory need

to consider the use of altrenogest in stallions or geldings.

In the present study, prolonged administration (eightweeks) of altrenogest at recommended dose rates to

sedentary mares caused no effect on dominance hierar-

chies, body mass or body condition score. We conclude

that the evidence to date suggests that altrenogest

when administered in accordance with the manufactur-

er�s data sheet, has no obvious anabolic or behavioural

effects that may affect the performance of a competition

mare.

References

Boyd, L.E., Carbonaro, D.A., Houpt, K.A., 1988. The 24-h time

budget of Przewalski horses. Applied Animal Behaviour Science

21, 5–17.

Houpt, K.A., Wolski, T.R., 1980. Stability of equine hierarchies and

the prevention of dominance related aggression. Equine Veterinary

Journal 12, 15–18.

Johnson, N.N., Brady, H.A., Whisnant, C.S., LaCasha, P.A., 1997.

Effects of oral altrenogest on sexual and aggressive behaviors and

seminal parameters in young stallions. Journal of Equine Veteri-

nary Science 18, 249–253.

Keiper, R.R., Sambraus, H.H., 1986. The stability of equine domi-

nance hierarchies and the effects of kinship, proximity and foaling

status on hierarchy rank. Applied Animal Behaviour Science 16,

121–130.

Kluber, E.F., Pollman, D.S., Davis, D.L., Stevenson, J.S., 1985.

Body growth and testicular characteristics of boars fed a synthetic

progestogen, altrenogest. Journal of Animal Science 61, 1441–

1447.

Kluber, E.F., Minton, J.E., Stevenson, J.S., Hunt, M.C., Davis, D.L.,

Hoagland, T.A., Nelssen, J.L., 1988. Growth, carcass traits, boar

odor and testicular and endocrine functions of male pigs fed a

progestogen, altrenogest. Journal of Animal Science 66, 470–478.