Applied Animal Ethology, 10 (1983) 301-317 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

301

SEPARATION OF EWES FROM TWIN LAMBS: INCIDENCE IN SEVERAL SHEEP BREEDS

G. ALEXANDER’, D. STEVENS’, R. KILGOUR’, H. de LANGEN2, B.E. MOTTERSHEAD’ and J.J. LYNCH’

‘CSIRO Division of Animal Production, P.O. Box 239, Blacktown, Sydney, N.S.W. 2148 (Australia)

zMinistry of Agriculture and Fisheries, Ruakura Animal Research Station, Hamilton (New Zealand)

TSIRO Division of Animal Production, Armidale, N.S. W. (Australia)

(Accepted for publication 23 June 1982)

ABSTRACT

Alexander, G., Stevens, D., Kilgour, R., de Langen, H., Mottershead, B.E. and Lynch, J.J., 1983. Separation of ewes from twin lambs: incidence in several sheep breeds. Appl. Anim. Ethol., 10: 301-317.

The ability of ewes and their lambs to keep together during the first day or two of life, when the motherqffspring bond is consolidating, was examined in fine-wool Merino, Dorset Horn and Crossbred (Border Leicester x Merino ewes mated to a Suffolk ram) sheep, lambing on a sparse pasture in the Armidale district of N.S.W., and in a Romney flock, lambing on good pasture in the Mangakino district of New Zealand. Ewes in the N.Z. flock had been selected for their ability to rear more than one lamb.

46% of the twin-bearing Merinos became permanently separated from a lamb, mostly on the day after giving birth, and in at least 54% of these cases these was no obvious pre- cipitating factor such as birth difficulties or interference by other sheep. In addition, a further 34% of twin-bearing Merinos experienced temporary separation from one lamb. By contrast, in the other flocks, permanent separations were 17, 0 and 8%, respectively, and some precipitating factor could always be identified. Few separations were associated with human interference.

The sheep at Armidale moved from the birth-site much more rapidly than the Rom- neys (means of 2 h versus 6.5 h), possibly due to sparse pasture conditions. In the Meri- nos, the proportion of separations decreased as the time that ewes remained near the birth-site increased. The observations indicate that fine-wool Merino ewes are slow to recognize the size of their litters and inherently poor at maintaining contact with more than one lamb in the neonatal period.

INTRODUCTION

Lambs born as multiples suffer a much higher rate of mortality than singles during the first days of life (Vetter et al., 1960; Watson and Elder, 1961; Purser and Young, 1964; Holmes, 1976; Piper and Bindon, 1977;

0304-3762/83/$03.00 0 1983 Elsevier Science Publishers B.V.

302

Dalton et al., 1980). Abnormal behaviour of ewes and their newborn lambs has been identified as one of a number of significant causes of lamb death (for example Alexander, 1960; Winfield, 1970; Arnold and Morgan, 1975). Most studies on behavioural attributes which lead to mortality in lambs born as multiples have been done under close confinement in pens or yards, where the complex behavioural interactions that lead to successful suckling can be readily observed (Holmes, 1976; Owens et al., 1980). However, a recent study (Stevens et al., 1982) indicated that one cause of this higher mortality in multiples was failure of the ewes and their twins to remain together during the first day or two of life, while roaming free in 12-ha paddocks. By the second day after birth, about half of the ewes were caring for only one lamb. Direct observation indicated that the separation was largely due to the ewe moving and leaving one lamb behind; the ewes appeared satisfied if accom- panied by one lamb only. Since most of the abandoned lambs were judged to be strong and healthy, the prime cause of separation appeared to be inade- quate maternal care.

To provide information on whether these separations are common in sheep generally or are characteristic of fine-woolled Merinos, the study was repeated a year later with fine-wool Merino ewes, Dorset Horn and Border Leicester X Merino crossbreds. In a separate study, a flock of New Zealand Romney sheep selected for the ability to rear more than one lamb, was examined.

METHODS

Study 1 (Armidale, N.S. W.)

The same flock and the same four 12-ha paddocks were used as in the pre- vious study, and winter-weather conditions during the July/August period of study were similar to those reported previously (Stevens et al., 1981, 1982). About 500 fine-wool Merino ewes that had been mated to Merino rams and were due to lamb over 17 days were divided into 3 groups of approximately 170 each and placed in 3 of the paddocks, 2 of which contained 0.5 ha of contrived shelter in the form of grass hedges l-l.5 m high and 15 m apart. Fifty-eight Dorset Horn ewes mated to Dorset rams were placed in the fourth paddock, together with 72 crossbred ewes (Border Leicester X Meri- no) mated to Suffolk rams. The ewes were shorn a week before lambing was due to start, and were placed in the paddocks on the day after shearing. All ewes carried numbers paint-branded on their sides.

Due to drought conditions, the pasture was extremely short, so sheep were offered wheat grain every second day at the rate of 700 g per head per day, increasing to 800 g nine days after lambing began. Lucerne hay was also offered at the rate of 140 g per head per day. The wheat and hay were trailed along the ground at night away from the main concentration of sheep in each paddock.

The flocks were watched through binoculars by an observer in a 13-m high tower and by 2-4 observers who walked through the paddocks continuously between 0830 h and last light at 1800 h. The sheep were also watched by 2 observers for new births from first light at about 0530 h to 0830 h.

Birth-sites were identified on paddock plans and the time at which ewes moved more than an estimated 20 m from this site was recorded to the nearest 15 min. Lambs born overnight without recorded birth-sites were “tagged” early in the day, i.e. they were ear-tagged, weighed, sexed, assessed for vigour and numbered with branding fluid. Lambs born during the day were tagged after the ewe had moved at least 20 m from the birth-site, but where this had not occurred by late afternoon, the lambs were tagged at the end of the day’s observations. All 70 ewes with twins and 41% (170/411) of ewes with singles were observed closely.

The four paddocks were each divided into 7 zones (Stevens et al., 1981) and the zone in which the ewe was positioned was recorded at intervals of 0.5-l h during daylight on the day that she gave birth.

Details of other procedures and observations have been described pre- viously (Stevens et al., 1981,1982).

Study 2

This study was conducted in spring (17-27 September) on the Waihora block of the Department of Lands and Survey, in the Mangakino district of New Zealand. The ewes constituted part of the “elite” flock of 800 ewes in the Department’s Romney sheep-breeding scheme (Hight et al., 1975). All ewes in this flock, selected from a base of 250 000 ewes, had reared either twins when lambing for the first time at 1 year of age or a single at this lambing and twins at 2 years of age; their retention in the flock depended on them rearing multiples at most lambings.

A randomly selected sub-flock of approximately 400 ewes was observed under the husbandry conditions practised with these ewes. Ages of ewes ranged from 2 to 11 years, and the number of previous lambings from 1 to 10. The ewes carried g-months wool at lambing and each was identified by a numbered neck-tag. To facilitate identification of lambs and of ewes in need of obstetrical intervention, the unlambed ewes were moved (“drifted” or “shedded”) once, or occasionally twice, daily into another paddock, leaving the recently lambed ewes behind, where they remained for several days be- fore being joined with larger flocks of ewes and lambs. These practices are tradition4 for this flock. The drift circuit contained 7 paddocks, ranging in size from 5 to 15 ha and varying widely in shape and topography; most were undulating and contained small hills, some up to 20 m high, with steep sides, and some paddocks contained swamps, watercourses and gullies up to 5 m deep. There were few trees within the paddocks, but some had windbreaks approximately 5 m high along one boundary. Pastures were mainly white clover (Trifolium repens), cocksfoot (Dactylis glomeruta) and perennial rye

304

grass (Lolium perenne) showing moderate spring growth. Detailed weather records were not kept, but during the lo-day study the weather was largely overcast, windy and showery, with temperatures usually between 5 and 12°C.

The lambing flock was kept under surveillance from dawn to dusk (0600- 1800 h) by 2 observers, who had also participated in the Armidale study, The observations made were similar to those in Study 1, but birth-sites were marked by yellow pegs.

Lambs were tagged l-12 h after birth; lambs born overnight were tagged when first seen and lambs born during the day were tagged in the late after- noon. The flock was drifted between 0900 and 1000 h, but with the smallest lambing paddocks, it was drifted again between 1700 and 1800 h. Tagging was done by 2 observers together, and drifting was done by 2-3 people, assisted on some days by a quiet well-trained dog or horse.

The remainder of the flock was also under study by other observers, who provided some additional comparable data (Tables I and IV).

Statistical comparisons

Times for ewes to move at least 20 m from the birth-site were compared by conventional analysis of variance, using the log,, transformation to stabi- lize variances. Differences between populations in the proportions of animals falling into various categories were tested by contingency x2 analysis, or where significance appeared marginal, by Fisher’s exact test (Freeman and Halton, 1951).

RESULTS

Study 1

Response of sheep to observers and to the feeding routine The sheep habituated to the presence of observers within 3 days, so that it

was possible to approach individuals to within about 10 m without distur- bance.

The response of ewes to handling of their lambs at tagging varied widely (Fig. 1). About half of the ewes of each breed moved 20-50 m away, but re- turned immediately the operator left. However, a higher proportion of Dorsets (9/36, 25%) remained with the lambs during tagging than Merinos (37/453, 8.2%) and Crossbreds (2/61,3.3%) (P < 0.01).

In general, the ewes showed little response to the night-feeding routine, but during the day, ewes and lambs frequently gathered at the wheat and hay and occasionally some ewes were observed to cease grazing and move rapidly to the wheat from a distance of more than 100 m.

305

8Or Fine-wool merino I- Cross-breed

60

s Dorset horn 60

t

40

20

n ”

\ ,

returned:- at once,later, never

I I 1 0 <5 >5

L ,

returned:- at once,later, never

I I I

0 <5 ==5 Distance retreated by ewe at tagging (ml

Fig. 1. Distribution of bebaviour categories in ewes of the 4 breeds when their lambs were handled at tagging. The 5 categories were: ewe remained with lamb; ewe did not move further than 5 m; ewe moved further than 5 m and returned at once; ewe moved further than 5 m but returned later; ewe never returned.

Movement of ewes Ewes remained within 20 m of the birth-site for widely varying periods,

mostly less than 5 h (Fig. 2). Mean periods for the three breeds were all about 2 h and there were no clear differences between ewes with singles and ewes with twins (Fig. 2). However, in the pooled data for Merino ewes, those giving birth within the sheltered areas (described by Stevens et al., 1981) tended to remain on the birth-site for longer than ewes giving birth in the open (Fig. 2). Respective means of log,, transformed data were 0.447 + 0.047 versus 0.241 f 0.045 (P< 0.01).

Movement away from the birth-site was usually gradual (Stevens et al., 1982), with care-giving behaviour being supplanted by grazing. The 6 Merino ewes that. moved from the birth-site within 15 min displayed little interest in their lambs and resumed grazing almost immediately.

There was no obvious tendency for the Merinos to return to the birth-site, but this aspect was not specifically examined in Study 1.

306

Merinos: birth in shelter

Mean 2.8h

- 7 9

0 Merinos: birth in open II

Mean 1.8h

Clorset horns: birth in oaen

Mean 2.4h ’

Cross- breeds: bit-t h in open W

Mean 2.2h

n lm L...~~~~I~~~I~.~I~~1I~..l~~~I...l

1 2 3 4 5 6 1 8 Time taken for ewes to move more than 20m

from the birth site (h)

Fig. 2. Distribution of time taken for ewes in Study 1 to move more than 20 m from the birth-site (15-min class intervals). Ewes with singles are indicated by open columns, ewes with twins by filled columns. Mean times were calculated using a log,, transformation to stabilize variances.

A crude index of the amount of travelling by ewes during the first day after giving birth was provided by dividing the number of zones that the ewe was known to enter by the number of observations of position. Observations for less than 5 h were excluded to reduce bias due to initial post-pa&urn immobility. The Merinos showed no significant differences between pad- docks or between single and twin-bearing ewes, but twin-bearing Merinos moved through significantly more zones than twin-bearing Dorset ewes [respective means (zones/observation) +_ SE, 0.61 + 0.11 and 0.17 f. 0.06, P < 0.051. Insufficient single-bearing Dorsets were observed to provide a comparison with Merinos.

TAB

LE I

No.

of

ewes

sep

arat

ed f

rom

lam

bs i

n St

udie

s 1

and

2 (p

erce

ntag

es

in p

aren

thes

es)

Mer

ino

Dor

set

Hor

n C

ross

bree

d N

ew Z

eala

nd R

omne

y

Sing

les

Twin

s Si

ngle

s Tw

ins

Sing

les

Twin

s Si

ngle

s Tw

ins

Trip

lets

Perm

anen

t’ 7

(4.1

) 32

(45

.7)

0 (0

) 3

(16.

7)

0 (0

) 0

(0)

3 (6

.1)

15

(8.8

) 5

(35.

7)

[l (2

.0)

17

(7.1

) 5

(20.

8)]3

Te

mpo

rary

* 9

(5.3

) 24

(34.

3)

1 (1

2.5)

7

(38.

9)

0 (0

) 2

(40)

1

(2.0

) 13

(7

.6)

1 (7

.1)

[l (2

.0)

6 (2

.5)

2 (8

.3)]3

Nev

er s

epar

ated

15

4 (9

0.6)

14

(20

.0)

7 (8

7.5)

8

(44.

5)

22 (

100)

3

(60)

45

(91

.8)

143

(83.

6)

8 (5

7.1)

[4

9 (9

6.1)

21

6 (9

0.4)

17

(70

.8)]

3

Tota

l 17

0 70

8

18

22

5 $]

17

1 [2

391

[El

‘Per

man

ent

sepa

rati

ons

wer

e th

ose

in w

hich

the

ew

e an

d la

mb

wer

e ne

ver

seen

tog

ethe

r ag

ain

unle

ss d

elib

erat

ely

reun

ited

by

the

shep

- he

rds

in S

tudy

2

afte

r it

beca

me

appa

rent

th

at t

here

was

litt

le c

hanc

e of

spo

ntan

eous

re

unio

n;

the

shep

herd

s in

terv

ened

in

12 o

f th

e 23

per

man

ent

sepa

rati

ons

in t

his

stud

y.

‘Sep

arat

ions

wer

e re

gard

ed a

s te

mpo

rary

if

the

ewe

and

lam

b re

unit

ed s

pont

aneo

usly

. ‘D

ata

from

2nd

sub

-floc

k.

308

TABLE II

Major “causes” of permanent separation of ewes from lambs

Breed Stage at which separation occurred

No. of ewes with singles

Total Weak Birth Inter- P00r Misc. or lamb Problem’ ference* mother unknown

Merino At birth 2 - 1 1 - - (Armidale) After birth but

before tagging 3 - 2 1 - - <lhaftertagging 1 - - 1 - - > 1 h after tagging 1 - - - - 1

Total 1 - 3 3 - 1

Romney At birth 1 - 1 - (Waihora) After birth but

before tagging - - - - - < 1 h after tagging - - - - - - > 1 h after tagging 2 - 2 - - -

Total 3 - 3 - - -

‘Birth of fit lamb obviously protracted (> 2 h). long interval between twins, lamb malpresented. or lamb’s head swollen indicating protracted birth.

‘Excludes those that also experienced a birth problem: includes ewes that showed interest in lambs of other ewes, and ewes whose lambs attracted the attention of other ewes.

Incidence of separation of ewes from lambs Merinos. Forty-six percent (32/70) of the Merino ewes that produced live twins became permanently separated from at least one lamb (Table I); 30 became separated on the first day after birth and the remaining 2 on the second or third day. Temporary separation (ewes being > 30 m away from a lamb and showing no response to its presence) of up to 6 h was observed in another 34% ewes, and only 20% were never observed to be separated from either twin. By contrast, few ewes with singles were observed to be separated at any time (Table I), and 90% were never observed separated. All but one of the 16 observed temporary and permanent separations from singles were on the first day after birth.

Weakness of lambs at birth, birth difficulties or interference with, or by, other sheep appeared to contribute to permanent separation in only 12 of the 26 twin-bearing ewes for which the circumstances of separation were known (Table II). Maternal abandonment of a twin in the absence of obvious contributing factors occurred in the remaining 14. The circumstances sur- rounding these 14 separations are given in Table III. In cases of permanent separation from single lambs, one or more of the contributing factors were involved in at least 6 of the 7 cases. The incidence of birth difficulties in ewes that became permanently separated from their single lambs (3/7, 43%) was higher than with ewes that remained with their singles (7/154, 4.5%) (PC 0.001) (Table II). BY contrast, in twin-bearing ewes there were few birth difficulties. The incidence of interference was also higher in ewes that

309

No. of ewes with twins No. of ewcc with triplets

Total Weak Birth Inter- Poor Misc. 01 Total Weak Birth Inter- Poor MiSC. Or

Iamb problem’ ference’mother unknown lamb problem’ fercnce’mother unknown

4_- - 3 1

32 4 1 7 14 6 I 1 6 - - - 1 - - - -

4 - 2 2 - - - - - - I 1 - 1 - - - - - - 1 - - 312 --- - - - 2 - -

16 2 11 2 - - 5 - 1 3 - 1

TABLE III

Circumstances of separation of Merino ewes from twin lambs, where poor maternal be- baviour was the only factor implicated

Three ewes separated from lamb at birth Two ewes showed no interest in either lamb One lamb wandered away during 2nd birth - returned later but was rejected

Seven ewes left strong lamb on birth-site after birth Ewe grazing or went to water - one lamb left in shelters Ewe went to water -one lamb left in open Ewe paid more attention to one lamb - moved slowly away from the other, grazing Ewe grazing - left one lamb asleep on birthsite. Lamb finally followed another ewe Ewe moved to wheat trail between births - returned to first-born only - later butted

second-born Ewe wandered from first-born to give birth to second - first lamb found ewe 2 h later,

but ewe showed no interest Ewe showed little interest in first-born - lamb finally followed other ewes

One ewe left strong lamb after hauing moved fkom birth-site Ewe grazing - some interference by a stray lamb - separation gradual

One ewe left strong lamb soon after tagging Ewe grazing - lambs 20 m apart before tagging

Two ewes left strong lamb behind long after tagging Ewe walking quickly with flock - one lamb dropped behind Ewe went to water -one lamb left behind

o-1 > l-2 >2-3 ‘3-4 Time the ewe remained on the birth

AL ‘4

site (h)

Fig. 3. The proportion of twin-bearing Merino ewes that became permanently separated from a twin, according to the time that ewes remained within 20 m of the birth-site. Dotted columns represent ewes for which the time at the birth-site was known to the nearest hour. Shaded columns represent ewes for which a minimum time only was known. The total number of ewes in each class is shown above each column.

became permanently separated from their singles (3/7, 43%) than in ewes that remained with their lamb (14/154, 9% ) (P < O.Ol), and the same trends were seen with twin-bearing ewes (7/32, 22% versus O/14, 0%) (P < 0.10) (Tables I and II).

Fifty-nine percent (26/44) of twin-births during daylight were followed by permanent separation compared with only 23% (6/26) of night births (P < O.Ol), and regardless of contributing factors, the incidence of separa- tion decreased as the time that the ewes remained on the birth-site increased (Fig. 3) (P < 0.01). All lambs that were left behind were alive at the time the ewe moved from the birth-site.

Dorset Horns and Crossbreds. The incidence of permanent separations in the Dorsets and Crossbreds was significantly less than in the Merinos (P = 0.014). Three of the 18 twin-bearing Dorset ewes became separated from a twin, all

311

during the 24 h following birth (Table I). With 2, there was interference by other ewes during the births, and with the third, a strong lamb vanished from the paddock overnight, presumably taken by a fox after becoming separated while the ewe grazed near dusk. The only Dorset ewe with live-born triplets became separated from the smallest lamb during inclement weather, and her remaining 2 were stolen by another ewe.

None of the 5 twin-producing crossbreds became permanently separated from a lamb, but, as with the Merinos, about 40% of the Dorset and Cross- bred ewes experienced a temporary separation during the first day after birth.

Fate of separated lambs Ten of the 39 separated Merino lambs were spontaneously adopted by

other ewes and only 2 of these lambs died, apparently due to inadequate lac- tation. The remaining 29 permanently separated Merino lambs died. Only one of the 3 permanently separated Dorset twins was fostered and survived.

Study 2

Response of sheep to observers and handling procedures The Romney ewes were accustomed to frequent handling and were little

disturbed by the presence of observers unless approached closer than about 10 m; ewes with newly-born lambs also showed little or no response to the observer pegging the birth site. The Romneys showed less disturbance at the tagging of their lambs than the Merinos, Dorsets or Crossbreds of Study 1 (P < 0.001); for example, 28% of Romneys remained with the lamb, and another 62% remained within 5 m (Fig. 1).

The Romneys were accustomed to the drifting procedure, and unlambed ewes walked quietly before the operators even when a dog, a horse or both were employed.

Movement of ewes from the birth-site The Romneys remained within 20 m of the birth-site for periods that

ranged from less than 1 h to 54 h (Fig. 4), and 13% were still within 20 m at the first observation on the day following their recorded birth, i.e. 12-36 h after birth. There were no significant differences between the means for ewes with singles, twins or triplets, and the overall means was 6.6 h (mean of trans- formed data 0.822 & 0.035), which is much higher than the means of ap- proximately 2 h in the other breeds (P < 0.001). Overall, 41% of ewes (26% in second sub-flock) were recorded as returning to the birth-site or had never left by 36-48 h after birth (Table IV).

At the positional observations, approximately 1 h after tagging or drifting which were usually done at different times, 59 and 46%, respectively, of ewes that were still on the birth-site an hour earlier had now moved, but when the 2 operations were done together, only 11% were still on the birth- site at the next observation (Fig. 5).

312

60

$60 aJ r

After drifting

Y- 0

d 40 2

20

0

After tagging & drifting together

0 0 >O- >5- >lO- '25->50- >100->200-

5 10 25 50 100 200 400 Distance moved (m1

Fig. 4. Approximate distance moved by twin-bearing Romney ewes during the hour following tagging, drifting, or tagging and drifting together. Similar patterns were shown by the smaller number of single- and triplet-bearing ewes.

There were no clear effects of litter size. The movements following tagging were mostly 10-25 m, but a few individuals had moved 100 m or more. The movements following drifting were also widely variable, but did not peak at lo-25 m (Fig. 4).

There was a tendency for movement associated with drifting to be more when done by men with a horse than by men alone, men with a dog, or by men with dog and horse; the proportions of ewes moving more than 10 m during the hour following drifting were 52, 22, 21 and 31%, respectively (P = 0.071).

313

TABLE IV

Numbers of Romney ewes returning to, or remaining near, birth-site during first 2 days after birth

No. of ewes with:

Singles

Remained on birth-site for 36-48 h 1[11’ Returned to birth-site within 24 h

of birth 9[51 Returned to birth-site 24-48 h

after birth 5[01

Total observed 25[31]

‘Data from second sub-flock shown in square brackets.

Twins Triplets

lO[ 141 2[31

42[ 241 2[21

6[91 2r11

143[174] 14[20]

Times known to nearest hour

Minimum times only known

1~1111111~~~~~~~~~~‘~‘~““““~“’

1 5 10 15 20 25 30 34% Time taken for ewes to move more than 20m

from the birth site (h1

Fig. 5. Distribution of time taken for twin-bearing N.Z. Romney ewes to move more than 20 m from the birth-site. The cross-hatched, dotted and filled portions of the columns, respectively, indicate ewes that moved from the birth-site within an hour of tagging, drifting or both operations together. Similar patterns were shown by the smaller number of single- and triplet-bearing ewes.

Incidence of separation of ewes from lambs Few Romney ewes were temporarily separated from their lambs, regard-

less of litter size (Table I). The low incidence in ewes with twins is in marked contrast with that in the twin-bearing Merinos (13/171 versus 24170,

314

P< O.OO,l) or Dorsets (13/171) versus 7/18, P < 0.001). Similarly, the inci- dence of permanent separations in twin-bearing Romneys was much less than in Merinos (15/171 versus 32/70, P < O.OOl), but was of the same order as in Dorsets (15/171 versus 3/18, N.S.). There were no marked differences be- tween the 4 breeds in the incidence of separations by ewes with single lambs (Table I).

All permanent separations among Romney ewes were associated with ob- vious predisposing factors such as birth difficulties, lamb weakness, the lamb rolling down-hill, or with interference by ewes or by straying lambs (Table II). Birth difficulties were the major factor associated with permanent sepa- ration in ewes with singles (3/3, 100%) and twins (11/15, 73%), but appar- ently not in ewes with triplets (l/5, 20%), (P = 0.078) (Table II). With singles, all 3 permanent separations were associated with birth difficulties, which occurred by contrast in only l/45 ewes that were never separated from their single lambs (P < 0.001). There was a much higher incidence of birth difficulties in permanent separation of twins (11/15, 73%) than in ewes that were never separated (14/143, 10%) (P < 0.001). The incidence of interference in twin-bearing ewes that were never separated (29/143,20%) was not very different from that in cases of permanent separation (2/15, 13%) (N.S.), when there were no difficulties at birth. With single-bearing ewes, the only cases of interference not associated with difficulty of birth were in ewes that were never separated from their lambs (6/45,13%) (Tables I and II).

Survival of separated lambs to end of observation Since observers were obliged to minimize mortality, they intervened in 12

of the 23 apparently permanent separations. Intervention took the form of reuniting ewes and lambs, assisting lambs to suck after being separated, and in 2 cases penning ewes and lambs. Six of the 12 lambs survived and a further 4 survived after being accepted by other ewes that were in labour or had recently lambed.

DISCUSSION

These observations revealed major differences between the flocks in the ability of ewes and their twin lambs to keep together during the first day or so after birth when the mother-offspring bond is consolidating (Alexander, 1980). Such differences were not seen with ewes and their single lambs. The fine-wool Merinos appeared to separate from their lambs much more readily than Dorsets and Crossbreds, all recently shorn and held under the same pas- ture conditions, and than the woolly Waihora Romneys held under good pasture conditions. This separation of ewes from twins in fine-wool Merinos conforms with previous observations on the same flock (Stevens et al., 1982), and could be a major cause of the high twin mortality in this and

315

other fine-wool Merino flocks in the Armidale district (J. George and T.S. Ch’ang, personal communication, 1979).

A more recent study (G. Alexander, D. Stevens and B. Mottershead, un- published data, 1981) has also shown that twin separation occurs in other strains of Merinos and under good pasture conditions, albeit at a lower level than reported here.

Separation of ewes from lambs in the present study was apparently caused by a variety of factors (Table II), the importance of which varied from flock to flock. Difficulties of birth were a major factor predisposing to separation, and the low incidence of birth problems in ewes that remained with their lambs reinforces this conclusion. Birth difficulty was an important factor in cases of separation of ewes from single Merino and Romney lambs, and par- ticularly from twin Romneys, but not from twin Merinos, and this may reflect the higher birth weight in Romney twins (4.4 kg, unpublished data) than in Merino twins (2.9 kg; Stevens et al., 1982).

Interference, in the form of continued interest by other ewes and lambs, in newborn lambs and lambing ewes, without any associated birth difficulty, was also a significant cause of separation, particularly in Merinos. This was at least partly due to the large number of abandoned lambs in the flock. How- ever, the major factor in twin-bearing Merinos was the failure by apparently normal ewes to attend to both lambs and to ensure that they both followed when the mother moved. Since most of the Merino lambs appeared to be strong and healthy, the likely cause of the failure was maternal, and the data relating incidence of separation to time spent at the birth-site (Fig. 2) in- dicate that the Merino ewes may move from the birth-site before their lambs are sufficiently mobile, or before the mother-offspring bond is fully es- tablished. This is supported by a lower incidence of separation after births at night, when the Merinos moved little (Lynch et al., 1980), than after day- births, when the ewes were actively grazing. Although the Dorsets and Cross- breds, like the Merinos, remained with their twins, there was some evidence that the Merinos were more mobile, crossing more zones than the Dorsets; this could contribute to the difference in the incidence of separation from twins. The New Zealand Romneys tended to remain on the birth-site for much longer periods than the Armidale sheep, and this difference could ac- count for much of the corresponding differences in the incidence of separa- tion between the Merinos and Romneys. The difference in the tendency to move could be due to environmental factors, such as the degree of handling, excessive pressure on the Armidale sheep to graze due to recent shearing, which can increase appetite (Wodzicka-Tomaszewska, 1964; Arnold, 1976), and sparse pasture conditions, or, in the case of Merinos, it could be inherent. The temperamental differences in behaviour of the ewes at tagging (Fig. 1) could also reflect handling or inherent differences, and are consistent with other data (Morgan et al., 1974). However, some less obvious maternal qua- lity, such as awareness of the number of offspring, and/or previous expe- rience of multiple births, may be involved in minimizing separation, in addi-

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tion to a tendency to remain near the birth-site and travel little. Alternative- ly, the lamb genotype may be important; some breeds may become coordi- nated and able to follow their darns sooner after birth than others.

In addition to differences in shearing and grazing conditions in Studies 1 and 2, the Merinos were held on flat, almost featureless, paddocks and con- gregated in the contrived shelter (Lynch et al., 1980) at night, while the Romneys were held on broken terrain and showed little or no tendency to congregate. It is not clear how these differences affected the relative per- formance of the Merinos and Romneys, although congregation at night could have aggravated interference in the Merinos.

The tendency of the Romneys to move less than the Armidale sheep, and to separate from fewer twins, was observed under drift management, which stimulated some movement (Fig. 5), especially when done in conjunction with tagging. However, few separations could be attributed to this procedure (Table II), thus providing further evidence of a superior ability of the Romneys to care for twins, and indicating that the use of a well-trained dog in a drift system need not cause undue disturbance and loss of lambs.

Failure of ewes and lambs to stay together is clearly prejudicial to lamb survival, even though some abandoned or lost lambs are fostered by other ewes. Birth difficulties were an important cause of separation in Romneys and in Merinos with singles, and the observations indicate that moderately prolonged birth extends the period of maternal receptiveness that precedes delivery, so increasing the chances of a ewe fostering a stray lamb or stealing a lamb from its mother, while markedly prolonged birth suppresses maternal behaviour and can lead to recumbency (Alexander, 1960), so that newborn lambs are not cared for. Hence, difficult parturition can have marked adverse effects on peri-partum behaviour of ewes, in addition to increasing cold- sensitivity of lambs, as reported by Alexander et al. (1980), inducing central nervous damage in the lambs and adversely affecting their sucking behaviour (Haughey, 1980).

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the cheerful help of Geoff Green, Bob Elwin, Laurie Bradley, Chris Donovan, Ann Hall, Chris Arave and Pam Gould who assisted with the observations. They are especially grateful to Mr. A.E. Gibson, Superintendent of Land Development, Department of Lands and Survey, at Rotorua, New Zealand, for permission to work with the Wai- hora Romneys, and to Mr. Peter Roberts, who manages this flock, for his invaluable cooperation and tolerance. CSIRO Division of Mathematics and Statistics provided advice on some of the statistical analyses.

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