EFFECTS OF BODYWEIGHT AND PMS ADMINISTRATION ON REPRODUCTION RATE IN MERINO EWES
D. A. HEDGES, B.Sc.Agr., and T. F. REARDON*, M.Sc.Agr. C.S.I.R.O. Pastoral Research Laboratory, Armidale, New South Wales, 2350 Introduction
Increased bodyweight (Coop 1962) and the use of pregnant mare serum gonadotrophin (PMS) (Robinson 1951) are both reported to increase the number of lambs conceived by ewes. Coop (1966) suggested that bodyweight exerted 2 independent effects; a ‘static’ effect due to the level of bodyweight at mating and a ‘dynamic’ or ‘flushing’ effect due to increasing bodyweight at mating. Wallace (1955) examined the effect of flushing on the response to PMS treatment in Romney ewes and concluded that the nutritive status of ewes during the breeding season is not an important factor in determining the ovulation rate following PMS treatment. This note reports the effects of bodyweight differences induced some weeks prior to mating and of PMS treatment in a flock of fine-woolled Merino ewes. Materials and Methods The data were obtained during a study reported by Wheeler et a1 (1971), of the contribution of the conceptus to the bodyweight of mature ewes at pasture; the experimental procedures are presented in detail in that report. The ewes were drawn from a flock in which deliberate selection for increased reproduction rate was not practised. In this respect, the flock is typical of many Merino flocks in Australia in which most of the ewes produce only single lambs, and the incidence of twinning is quite low (< 20%) even under favourable conditions. Two initially uniform groups of 6-year-old ewes were grazed from 3 months pre-mating to post-partum a t widely different stocking pressures. Mating began o n 9 M u c h and mean mating weights were 32.3 kg and 40.2 kg. The data for this paper came from randomly selected subgroups which were either mated by fertile rams after detection of oestrus by vasectomised rams, or mated by fertile rams afrer intramuscular injection of 500 iu of PMS on the twelfth, thirteenth or fourteenth day of the preceding oestrous cycle. All but 6 of the ewes which failed to conceive at first mating were remated. A second PMS treatment was not given. The ewes were slaughtered in groups at intervals from 60 days after conception to post-partum and the number of lambs born or found in utcro were recorded. Results
The tendency for those ewes injected with PMS on the twelfth or thirteenth day after oestrus to have a higher twinning rate than ewes injected on * Present address: Ruakura Animal Research Centre, Hamilton, New Zealand.
88
the fourteenth day was not statistically significant (x2 = 2.5). The pooled data are presented in Table 1. The total x2 (24.5; P < 0.001) for the results at the first oestrus with fertile rams (Table 1) was partitioned and revealed significant associations between liveweight and barrenness (x2 = 11.5; P < 0.001) and, within ewes conceiving, between PMS administration and the number of multiple foetuses (x‘ = 12.1; P < 0.001); all other components were negligible. Inclusion of the results from subsequent cycles gave a reduced x2 (7.9; P < 0.01) for the effect of liveweight on barrenness; the number of multiple foetuses among all ewes appeared to be higher in the high bodyweight group (x2 = 3.95; P < 0.05). Again PMS resulted in a higher proportion of multiple foetuses in those ewes which conceived (x2 = 12.4; P < 0.001). Lamb birthweights and survival rates to 2 days post-partum were recorded for the 49 lambs carried to term. Because of the small numbers and their unequal distribution between treatment combinations the results must be interpreted with caution. The mean birthweight of the 14 twin lambs (3.0 kg) was, on average, 1.0 kg less than that of the 35 single lambs though this difference was not reflected in appreciably different survival rates (79% for twins vs 83% for singles). The survival rate of 60% for the 15 lambs born to ewes in the low bodyweight group was considerably lower than the rate of 91% for the 34 lambs of the high bodyweight group; mean birthweights were not markedly different (low 3.6 kg, high 3.8 kg) although there was a higher proportion of twin lambs in the high (35% of lambs born) than in the low (13%) weight group. Discussioa
The lambing percentages indicate that both PMS treatment and increased bodyweight, separately or in combination, improved reproduction rate; the 2 effects appeared to be additive in that the primary effect of bodyweight was on the incidence of barrenness while the use of PMS merely increased the proportion of multiple foetuses in those ewes able to conceive. Examination of the service records suggested that barrenness was very Australian Veterinary Journal, Vol. 51, February, 1975
TABLE 1 The Effect of PMS and Bodyweight on the Number of Ewes with 0, 1 or
No. of Lambs Born or in utero
n 1 2 2 Total* Lambing % f ( i ) (ii)
Low Bodyweight No PMS With PMS
Lambing % f ( i ) (ii)
High Bodyweight No PMS With PMS
13 1 28 53.5 107
22 10 9 41
7 16 3 26
9 16 16 41
68.3 147
88.4 121
119.5 153
8 18 1 27
12 15 11 38
2 19 5 26
5 17 17 39
74.1 11 1
97.4 142
119.2 129
133.3 153
14 - .
$
0 1 3 2 Total*
3 2 Lambs
~
*Differences between totals are due to 6 ewes not being allowed > 1 chance to conceive. ?Lambs. born or in ufero as a % of: ( i ) ewes joined, (ii) ewes conceiving.
$ Results above double line are those obtained from the first
largely due to the failure of fertilisation and not to early embryonic loss. The mean mating weight of our high bodyweight group (40.2 kg) was considerably lower than the average mating weight of 48.2 kg recorded previously during the third year of a 4-year grazing experiment using these same ewes. It was concluded (Hedges et al 1972) from within-year analyses of those results that there was an association between mating weight and the numbers of multiple births. However, because of the low genetic potential for twinning in these ewes, fairly substantial increases in bodyweight may result in only a relatively small number of additional lambs born. It would seem therefore that the greatest potential for response to PMS treatment would exist when bodyweight and nutrition were just adequate to ensure a low level of barrenness but were not sufficiently high to induce twinning; in these circumstances the use of PMS may be less costly and more effective than increasing bodyweight as a means of increasing the twinning rate. It would appear from the results of Wallace (1955) and this experiment that the level of response to PMS will not be affected by increasing bodyweight above that necessary for normal fertilisation. The survival of the lambs is, of course, a primary consideration. The high mortalities among lambs born to ewes of the low weight group in this experiment could well mean that, in terms of the number of lambs weaned, there may be little real benefit from the use of PMS if the ewes re-
main at such low weights until parturition. Provision would also have to be made to ensure adequate growth rates of the lambs. However, these requirements have to be considered with any method of increasing the reproduction rate; the use of PMS may allow resources to be concentrated on the provision of extra feed at and after lambing rather than prior to and during mating or at both times. Robinson (1951) found that 500 iu PMS administered on the twelfth day of the cycle gave the greatest ovulation rate but Wallace (1955) using larger numbers of sheep, observed that the fourteenth day appeared to be the most satisfactory. While the trend in the current study favoured earlier administration the differences were not significant. In any case, practical considerations such as the need for less frequent handling and recording of services are likely to outweigh any small advantage obtained by treating ewes on a particular day of their cycle. Obviously the economics of PMS treatment will depend both on the level of response obtained and on the value to be attributed to that response. However, the current situation of high demand and prices for sheep suggests that the reconsideration of PMS treatment as a means of achieving an immediate increase in the reproduction rate of sheep is warranted.
Austrnlian Veterinary Jouriiul, Val. 51, February, 1975
oestrus only; those below the line include the data from the first and subsequent oestrus cycles.
Summary
Data are presented on the effects of the use of pregnant mare serum gonadotrophin (PMS) and of increased liveweight on the number of lambs 89
born, or in utero, in mature fine-woolled Merino ewes. The primary effect of hcreased bodyweight was to reduce the number of barren ewes whereas the main effect of PMS treatment was to increase level Of twinning among those which conceived. The 2 effects appeared to be additive but it is pointed out that the survival rate of the lambs will be an important consideration in assessing the relative merits of the 2 treatments.
90
References Coop, I. E. (1962)-N.Z. J1 agric. Res. 5: 249. Coop, I. E. (1966)--WId Rev. Anim. Prod. 2: 69. Hedges, D. A., Williams, W. T. and Wheeler, J. L.
(1972)-Aust. 1. agric. Res. 23: 839. Robinson, T. J. (1951)--J. ugric. Sci., Camb. 41: 6. Wallace, L. R. ( i 9 5 5 ) - ~ . ugric. Sci., Camb. 45: 60. Wheeler, J. L., Reardon, T. F., Hedges, D. A. and Rocks, R. L. (1971)-J. ugric. Sci., Carnb. 76: 347. (Received for publication 19 September 1973)
Australian Veterinary Journul, Vol. 51, February, 1975
D. A. HEDGES, B.Sc.Agr., and T. F. REARDON*, M.Sc.Agr. C.S.I.R.O. Pastoral Research Laboratory, Armidale, New South Wales, 2350 Introduction
Increased bodyweight (Coop 1962) and the use of pregnant mare serum gonadotrophin (PMS) (Robinson 1951) are both reported to increase the number of lambs conceived by ewes. Coop (1966) suggested that bodyweight exerted 2 independent effects; a ‘static’ effect due to the level of bodyweight at mating and a ‘dynamic’ or ‘flushing’ effect due to increasing bodyweight at mating. Wallace (1955) examined the effect of flushing on the response to PMS treatment in Romney ewes and concluded that the nutritive status of ewes during the breeding season is not an important factor in determining the ovulation rate following PMS treatment. This note reports the effects of bodyweight differences induced some weeks prior to mating and of PMS treatment in a flock of fine-woolled Merino ewes. Materials and Methods The data were obtained during a study reported by Wheeler et a1 (1971), of the contribution of the conceptus to the bodyweight of mature ewes at pasture; the experimental procedures are presented in detail in that report. The ewes were drawn from a flock in which deliberate selection for increased reproduction rate was not practised. In this respect, the flock is typical of many Merino flocks in Australia in which most of the ewes produce only single lambs, and the incidence of twinning is quite low (< 20%) even under favourable conditions. Two initially uniform groups of 6-year-old ewes were grazed from 3 months pre-mating to post-partum a t widely different stocking pressures. Mating began o n 9 M u c h and mean mating weights were 32.3 kg and 40.2 kg. The data for this paper came from randomly selected subgroups which were either mated by fertile rams after detection of oestrus by vasectomised rams, or mated by fertile rams afrer intramuscular injection of 500 iu of PMS on the twelfth, thirteenth or fourteenth day of the preceding oestrous cycle. All but 6 of the ewes which failed to conceive at first mating were remated. A second PMS treatment was not given. The ewes were slaughtered in groups at intervals from 60 days after conception to post-partum and the number of lambs born or found in utcro were recorded. Results
The tendency for those ewes injected with PMS on the twelfth or thirteenth day after oestrus to have a higher twinning rate than ewes injected on * Present address: Ruakura Animal Research Centre, Hamilton, New Zealand.
88
the fourteenth day was not statistically significant (x2 = 2.5). The pooled data are presented in Table 1. The total x2 (24.5; P < 0.001) for the results at the first oestrus with fertile rams (Table 1) was partitioned and revealed significant associations between liveweight and barrenness (x2 = 11.5; P < 0.001) and, within ewes conceiving, between PMS administration and the number of multiple foetuses (x‘ = 12.1; P < 0.001); all other components were negligible. Inclusion of the results from subsequent cycles gave a reduced x2 (7.9; P < 0.01) for the effect of liveweight on barrenness; the number of multiple foetuses among all ewes appeared to be higher in the high bodyweight group (x2 = 3.95; P < 0.05). Again PMS resulted in a higher proportion of multiple foetuses in those ewes which conceived (x2 = 12.4; P < 0.001). Lamb birthweights and survival rates to 2 days post-partum were recorded for the 49 lambs carried to term. Because of the small numbers and their unequal distribution between treatment combinations the results must be interpreted with caution. The mean birthweight of the 14 twin lambs (3.0 kg) was, on average, 1.0 kg less than that of the 35 single lambs though this difference was not reflected in appreciably different survival rates (79% for twins vs 83% for singles). The survival rate of 60% for the 15 lambs born to ewes in the low bodyweight group was considerably lower than the rate of 91% for the 34 lambs of the high bodyweight group; mean birthweights were not markedly different (low 3.6 kg, high 3.8 kg) although there was a higher proportion of twin lambs in the high (35% of lambs born) than in the low (13%) weight group. Discussioa
The lambing percentages indicate that both PMS treatment and increased bodyweight, separately or in combination, improved reproduction rate; the 2 effects appeared to be additive in that the primary effect of bodyweight was on the incidence of barrenness while the use of PMS merely increased the proportion of multiple foetuses in those ewes able to conceive. Examination of the service records suggested that barrenness was very Australian Veterinary Journal, Vol. 51, February, 1975
TABLE 1 The Effect of PMS and Bodyweight on the Number of Ewes with 0, 1 or
No. of Lambs Born or in utero
n 1 2 2 Total* Lambing % f ( i ) (ii)
Low Bodyweight No PMS With PMS
Lambing % f ( i ) (ii)
High Bodyweight No PMS With PMS
13 1 28 53.5 107
22 10 9 41
7 16 3 26
9 16 16 41
68.3 147
88.4 121
119.5 153
8 18 1 27
12 15 11 38
2 19 5 26
5 17 17 39
74.1 11 1
97.4 142
119.2 129
133.3 153
14 - .
$
0 1 3 2 Total*
3 2 Lambs
~
*Differences between totals are due to 6 ewes not being allowed > 1 chance to conceive. ?Lambs. born or in ufero as a % of: ( i ) ewes joined, (ii) ewes conceiving.
$ Results above double line are those obtained from the first
largely due to the failure of fertilisation and not to early embryonic loss. The mean mating weight of our high bodyweight group (40.2 kg) was considerably lower than the average mating weight of 48.2 kg recorded previously during the third year of a 4-year grazing experiment using these same ewes. It was concluded (Hedges et al 1972) from within-year analyses of those results that there was an association between mating weight and the numbers of multiple births. However, because of the low genetic potential for twinning in these ewes, fairly substantial increases in bodyweight may result in only a relatively small number of additional lambs born. It would seem therefore that the greatest potential for response to PMS treatment would exist when bodyweight and nutrition were just adequate to ensure a low level of barrenness but were not sufficiently high to induce twinning; in these circumstances the use of PMS may be less costly and more effective than increasing bodyweight as a means of increasing the twinning rate. It would appear from the results of Wallace (1955) and this experiment that the level of response to PMS will not be affected by increasing bodyweight above that necessary for normal fertilisation. The survival of the lambs is, of course, a primary consideration. The high mortalities among lambs born to ewes of the low weight group in this experiment could well mean that, in terms of the number of lambs weaned, there may be little real benefit from the use of PMS if the ewes re-
main at such low weights until parturition. Provision would also have to be made to ensure adequate growth rates of the lambs. However, these requirements have to be considered with any method of increasing the reproduction rate; the use of PMS may allow resources to be concentrated on the provision of extra feed at and after lambing rather than prior to and during mating or at both times. Robinson (1951) found that 500 iu PMS administered on the twelfth day of the cycle gave the greatest ovulation rate but Wallace (1955) using larger numbers of sheep, observed that the fourteenth day appeared to be the most satisfactory. While the trend in the current study favoured earlier administration the differences were not significant. In any case, practical considerations such as the need for less frequent handling and recording of services are likely to outweigh any small advantage obtained by treating ewes on a particular day of their cycle. Obviously the economics of PMS treatment will depend both on the level of response obtained and on the value to be attributed to that response. However, the current situation of high demand and prices for sheep suggests that the reconsideration of PMS treatment as a means of achieving an immediate increase in the reproduction rate of sheep is warranted.
Austrnlian Veterinary Jouriiul, Val. 51, February, 1975
oestrus only; those below the line include the data from the first and subsequent oestrus cycles.
Summary
Data are presented on the effects of the use of pregnant mare serum gonadotrophin (PMS) and of increased liveweight on the number of lambs 89
born, or in utero, in mature fine-woolled Merino ewes. The primary effect of hcreased bodyweight was to reduce the number of barren ewes whereas the main effect of PMS treatment was to increase level Of twinning among those which conceived. The 2 effects appeared to be additive but it is pointed out that the survival rate of the lambs will be an important consideration in assessing the relative merits of the 2 treatments.
90
References Coop, I. E. (1962)-N.Z. J1 agric. Res. 5: 249. Coop, I. E. (1966)--WId Rev. Anim. Prod. 2: 69. Hedges, D. A., Williams, W. T. and Wheeler, J. L.
(1972)-Aust. 1. agric. Res. 23: 839. Robinson, T. J. (1951)--J. ugric. Sci., Camb. 41: 6. Wallace, L. R. ( i 9 5 5 ) - ~ . ugric. Sci., Camb. 45: 60. Wheeler, J. L., Reardon, T. F., Hedges, D. A. and Rocks, R. L. (1971)-J. ugric. Sci., Carnb. 76: 347. (Received for publication 19 September 1973)
Australian Veterinary Journul, Vol. 51, February, 1975
