studies of face cover in the new zealand romney sheep
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Studies of face cover in theNew Zealand Romney sheepF. Cockrem aa Massey University , Palmerston North , NewZealandPublished online: 09 Jan 2012.
To cite this article: F. Cockrem (1968) Studies of face cover in the New ZealandRomney sheep, New Zealand Journal of Agricultural Research, 11:3, 560-574, DOI:10.1080/00288233.1968.10422436
To link to this article: http://dx.doi.org/10.1080/00288233.1968.10422436
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560
STUDIES OF FACE COVER IN THE NEW ZEALAND ROMNEY SHEEP
III. The body weight growth of Iambs of different face cover grade
By F. COCKREM*
(Received 19 March 1968)
ABSTRACT
Two aspects of the components of the live weight differences between open- and woolly-faced Romney ewes were studied: first, the stage of lamb growth at which the differences arose; and, secondly, the relative importance of maternal and genetic effects to such differences.
When classified by their face grade at 18 months of age, openfaced lambs showed the greater weight gain between three months of age (weaning) and 14 months of age. By this age they were on average 6 to 16 lb heavier than woolly-faced lambs depending on the year. In two of the three years studied the differences arose over the winter and in the remaining year directly after weaning and shearihg at three months of age. It is suggested that the live weight differences result from cold and/or nutritional factors affecting the growth of woolly-faced lambs to a greater degree than open-faced lambs.
Evidence from the mating group comparisons indicated that the phenotypic relationship between face cover and weight gains had a large genetic component but that maternal effects were probably not of importance.
Data on changes in face grade from weaning to 18 months of age showed that, on average, open-faced lambs became open-faced over this period but that woolly-faced lambs showed little change.
INTRODUCTION
The experiments described in this paper were part of an investigation into the causes of the phenotypic relationships between face cover, live weight, and fertility. The first stages of the investigation were the examination, in field experiments, of the components of these traits in order to determine those aspects worth more detailed examination. Also, as face cover is strongly inherited (Morley 1955), evidence on the relative importance of genetic and environmental factors was of interest.
The present paper examines the components of the live weight differences found between open- and woolly-faced ewes at 18 months of age (Cockrem and Rae 1966). Two main aspects were studied:
.. Massey University, Palmerston North.
N.Z. II agric. Res. (1968,1, 11: 56(}-74
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F. COCKREM 561
the first was to test whether the differences arose at any specific stage of growth; secondly, the possible importance of maternal and genetic effects on lamb growth was examined. This was necessary, as Terrill (I949) and Coop (1956) have reported differences in weaning weight of lambs when classified by their dam's face cover. Because of the high heritability of face cover, this could arise from the partial confounding of maternal effects with those of the lamb's own face cover.
The mating plan was intended to overcome this confounding as far as possible and at the same time to give information on genetic relationShIpS from differences between sire groups.
EXPERIMENTAL Plan of experiment
The data analysed in this paper were obtained from lambs born in 195!S, 1960, and 1961. The 1959 lambs were used in a separate experiment. The lambs were the offspring of the open- and woolly-faced ewe flock described by Cockrem and Rae (1966). "Eye-wigged" ewes were included as woolly-faced. The ewes were three years old in 1958. Only data from lambs alive at 18 months of age were analysed. Ram lambs were castrated at about three weeks of age.
One open-faced ram and one woolly-faced ram were used for the matings each year, different rams being used in each of the years concerned. The open- and woolly-faced groups of ewes were each initially randomised into two equal groups for mating, and the resulting four mating groups are shown in Table 1, the same ewes being in a particular mating group for successive years.
The data collected
The lambs were weighed, identified, and the ewe parent recorded at birth. Subsequently lambs were weighed every two or three months, and their face cover graded according to the system of Cockrem and Rae (1966). In addition, grades I + and 2+ were used as approximate intermediates. In adult sheep such grades would be I and 3 respectively. The lambs were shorn after weaning in December. Poll staple length (Cockrem 1966) was measured in June (10 months of age), and the animals were again shorn in October (14 months), all fleeces being weighed. Clean scoured yield of wool was estimated from a mid-side sample. Wool-blind animals had the wool clipped from
Ewes
Open-faced
Woolly-faced
TABLE 1. The mating groups
Open-faced ram
I
III
Woolly-faced ram
II
IV
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562 Face cover in the Romney. Ill.
around their eyes after the June gradings and measurements were taken. October body weights and grades are those taken a few days before shearing.
Unless specified. the face grade used throughout this paper is the final grade taken at 18 months (February). This should correspond to the subsequent adult grade (Cockrem 1966). Any grade 4 animals were pooled with grade 3. The final weight and grade for the lambs born in 1960 were taken at 16 months of age. when the experiment was terminated early because of drought.
Methods of analysis
The general system of analysing growth data which was described by Cockrem (1965) was used. This involves the analysis of variance of weights taken at a particular time. followed by covariance analyses to adjust for age or for previous weights. As one sample of lambs (those alive at 18 months) is analysed throughout. with the same sub-groupings. then one matrix of coefficients from a linear model will cover all of one set of analyses.
The analyses were carried out following the methods of Harvey (1960).
Estimates on which the analyses of variance were based came from the following two linear models:-
Model 1: For differences between lambs of different face cover:Y(k)iJmnp = p. + Ci + dJ + fm + go + (cg)in + elJmop
Model 2: Sire and ewe (mating group) effects added to model 1:Y(k)ijJmnp = p. + Cj -I- hij + dJ + fm + gn + (cgLn + eijJmnp
where Ci are years. i = 1-3 (1958, 1960, 1961) h ij mating groups within years. j = I-IV (Table 1) d J sex, 1 = wether, 2 = ewe fm birth rank. 1 = single. or twin reared as single.
2 = twin lambs' face cover grades at 18 months, n = 1-3 year-face cover interaction
eijkJmnp error term. assumed to be normally distributed with zero mean and constant variance.
y (k) ij Jmnp is the p th observation for the particular sub-group shown by the sub-scripts and for the variable denoted by k.
Thus model 1 estimated and tested effects of the lambs' own face cover at various stages of growth when seasonal, sex, and birth rank differences were allowed for. The differences found would reflect phenotypic associations between face cover and live weight growth.
In model 2 the mating group differences reflect a genetic component (sires). a maternal and genetic component (dams), with a residual phenotypic component (genetic and environmental) between the lambs' own face cover grades.
For the tests of significance, the h ij were regarded as random effects but those of face cover grade (gn) as fixed. Test for sex and
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F. COCKREM 563
birth rank were valid for either situation, with the assumption that there were no interactions between either of these terms and any other. Initial examination of the sub-class means indicated that this was a reasonable a priori assumption.
The equations were reduced before solution by applying the restriction that each of the sub-set estimates should sum to zero (e.g. ~gn = 0).
The choice of independent varieties was based on separate analyses of the significance of the difference of the slope from zero and of the homogeneity of the slopes within the groupings of years and face cover grades. Birth date (age) caused no significant reduction in the variance of weights after weaning if weaning weight was also adjusted for. For Ya, Y4, and Y5 the regression on weaning weight was found to be homogeneous, and therefore the following analyses were made:
x Grade 04 (04)
w3 ~' o SHORN ~ ~ w'''iv: ' ________ x------' Grad .. 2+3 (S8)
~ __ ~--x ~ 2 )(
x ______ 'X Grade 2
------(39)
_"..----x ___ x ____ x-
Oct. Dec. Feb. 1959
6
April June
10
Aug. Oct.
12 I~
Dec.
16
-x Grade. 1.1+ (18)
Feb. 1960
18 Age(month.)
Fig. l.--Changes in the average face cover grade up to 18 months of age plotted according to the 18 months' grade. The number of lambs in each group jn
parentheses.
Analysis of variance of: birth date (z). birth weight (y d, four month (weaning) weight (Y2), 10 month (June) weight (Y3)' 14 month (pre-shearing) weight (Y4), 18 month (final) weight (Y5), clean fleece weight (Y6), poll staple length (Y7). Analyses of covariance:-
Y2 adjusting for z and Yl
Y3' Y4' Y5 each adjusted for Y2 Y7 adjusting for Y6'
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564 Face cover in the Romney. Ill.
Some confounding of face cover grade effects with mating group effects could occur in model 2; poll staple length, as an independent estimate of face cover, was used to obtain an estimate of the extent of the confounding. The final conclusions were based on the relative results of all these analyses.
RESULTS The face cover grades
As face cover does not change markedly after 18 months of age (Cockrem 1966) the grades used for the classification in the main analyses were made at this time. Fig. 1 shows the changes in the average grade of groups classified according to 18 months' grade, from weaning until the 18 months' grade was attained. It is apparent that there is less difference between the lambs at weaning than subsequently and that part of the difference observed at 18 months had developed during the period from weaning onwards. The changes appeared to result from differences in the rate of re-growth of wool after shearings and after some fibre shedding observed to take place in July and August.
The analyses of variance
The number of lambs in each sub-group is shown in Table 2. Although the sub-classes are unequal, any particular estimate was based
TABLE 2. The numbers of lambs in the major sub-groups in each year
Year
Singles/twins
Wether/ewes
Face cover grade 1/2/3
Mating g,oups I/II jIll/IV 19
1~58
119
69 50
45 74
18 39 62
26 41 33 16
1960 1961
98 82
56 42 45 37
42 56 49 33
25 43 30 30 23 29
19 30 33 20 16 25 21
Mating groups classified by lamb's 18 month face cover grade
Mating group I I II III IV
------ - -- - ----
.... 1 2 6 26 39 11> ""11> 0-0
2 10 19 33 43 "os 11> .... "co os ~ 3 43 36 37 5
I
I Total
299
299
299
299
299
Total -~
73
105
121
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F. COCKREM 565
on 16 or more lambs. However, when the mating groups are classified by face cover grade it can be seen that there were very few woollyfaced (grade 1) lambs from open-faced ewes (groups I and II). Also mating group IV has few open-faced lambs. Thus there was still likely to be some confounding in the analyses by model 2 between the mating group and face cover -grade effects. However, the classification in this way also demonstrates the strong inheritance of face cover.
Results from the analyses of variance from model 1 are given in Table 3, together with the estimated least square means of face cover grade over the three years. The interaction at 14 and 18 months between grade and year resulted from grades 2 and 3 having the same body weights in .1961 but grade 3 being heavier than grade 2 lambs in 1958 and 1961. Also grade 3 were 18 lb heavier than grade 1 in 1958 but only 6 lb heavier in 1960 and 1961. Results for the other effects were similar to those for model 2 given below.
The weight differences arose between four and 14 months of age and did not further increase by 18 months of age. They were such that open-faced lambs had a significantly greater body weight by 10 months of age. The results in Table 3 also show that the woolly-faced lambs
Source
TABLE 3. The analyses of variance (mean squares) and estimates (IL + g.) of face cover grade effects from the solution of model I
I z Body weights Y6
d.f. Y7
Poll staple length
\
i ~~:~ Yl I Y2 -i --Y~-;-Y4 I Y5 ~!:a~ Birth ~ean-II0 months 114 months 18 months weight
_____ ~.-- , ___ -'-____ mg , --,-', ___ __ _ _____ -----, ____ _
: ,Ii Face cover
grade
Grade-year interaction
Residual
Estimated means
Grade 1
2
3
2 1 6.04 1.55
I
1 I I 52.81 I 855.27 .... 11823.07 .... 11582.93 ...... 1
, I 1 '
I I
4
288
67.40 0.31 75.15 I 144.08 428.46" I 380.22" I
I 91.361 3.03 75.491 90.81
18.5
1 18.5
I I i 18.1 i
10.3 61.4
10.4 61.8
10.5 62.8
82.8
86.1
89.1
129.43 1 134.00
92.7
I
I :~:: 116.4
99.1
102.0
.. P <0.05 ... P <0.01 .u P <0.001
1.64
0.14
0.81
5.4
5.7
5.6
Note: Sex and birth rank mean squares were similar to those in model 2 and have therefore been omitted.
1.03
0.84
6.25
6.20
5.76
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4
VI
01
0
1
TA
BL
E 4
. T
he
anal
yses
of
vari
ance
of
bo
dy
wei
ghts
fro
m m
odel
2 -
pres
ente
d as
the
mea
n sq
uare
s (l
b)
z
dJ.
1
Bir
th d
ate
Yl
Y2
Y3
Y-l
Y5
I Y6
Y7
•
lAug
ust
Bir
th
4 m
onth
s 10
m
onth
s 14
mon
ths
18
nth
Cle
an
Pol
l st
aple
! 1 =
1) m
o S I f
leec
e w
t le
ngth
(em
)
----
-------------------
-_
__
__
_ L _
__
__
__
__
__
_
'":j I:l
Sou
rce
'"I ~
Yea
rs
2 1
66
1.4
4"
1.56
40
.29
8.82
16
86.7
0 I 8
970.
41·u
13
.82
·"
34
.19
···
8 -.:: ~
Mat
ing
grou
ps
.., w
ithi
n ye
ars
9 16
5.48
2.
46
85.8
0 26
1.35
"-5
30
.99
···
54
8.8
2"·
0.
93
3.1
1"·
S·
... :::t-
Sex
118.
21
.~7.07···1
84
5.0
4"
418.
50·
1043
.11·
· 2
16
0.0
3··
· 1.
15
0.79
~
1210
.03 ..
·1 ~
Cl
Bir
th r
ank
236.
00
17
5.3
6"·
1
31
81
.78
·"1
2
14
1.2
7··
·1
17
08
.27
···
11
.46
·· •
0.27
~
I ;:s
I
I ~
I I
'<
Fac
e co
ver
2 0.
85
2.65
2.
89
137.
27
401.
95·
435.
87·
1.35
1.
75
.....
.....
:---Y
ear-
fal'.
e co
ver
inte
ract
ion
4 62
.67
0.31
70
.40
63.9
1 87
.07
189.
25
0.21
0.
24
Res
idua
l 27
9 88
.97
3.05
75
.15
85.3
1 11
6.47
12
1.65
0.
81
0.76
No
te:
"Yea
rs"
was
tes
ted
agai
nst "
Mat
ing
grou
ps w
ithi
n ye
ars"
and
oth
er s
ourc
es a
gain
st t
he r
esid
ual
term
.
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F. COCKREM 567
(grade 1) had the longer poll staple although there were no differences in fleece weight. Further analyses omitting the interaction term from the model for those cases where it was not significant did not markedly change the estimates or the conclusions from the analyses of variance.
The analyses of variance arising from model 2 are shown in Table 4. Where sex and birth-rank effects were significantly different. wethers were heavier than ewes and singles heavier than twins. The significant difference between years for the 18-month body weight was the result of the early finishing of the experiment in 1960.
There were significant differences in body weight at 10 months for the mating groups and at 14 and 18 months of age for both face cover grade and the' mating groups. However, poll staple lengths were significantly different only for the mating groups.
In Table 5 are shown the estimated effects for the lO-month body weights and poll staple lengths for the mating groups in each of the three years. together with those for the face cover grade. The standard errors were estimated from actual sub-class numbers and the residual variances. It can be seen that the poll staple length differences are more apparent between mating groups than between grades and that in general they are associated with the body weight differences.
The lack of a poll staple length effect in 1960 could be associated with the greater length in that year leading to a poorer discrimination of face cover (Cockrem 1966). The different order of the 1961 body weight effect from other years could be sire or seasonal interactions and, in part. arise from the sampling chances indicated by the standard errors.
The differences between the ram groups and between the ewe groups, as unweighted averages of the effects over the three years. are also presented in Table 5. The differences are subdivided according to the face cover of the group mated with. Thus the differences between rams mated with open ewes are averaged over the effects from mating groups I and II. These results show that open rams have offspring with greater body weights and a lesser poll staple length than those by woolly-faced rams. The effect holds for matings with both open- or woolly-faced ewes.
When the ewe parents are compared. the differences are smaller for poll staple length only when a woolly-faced ram was used. This could arise from the fact that poll staple length does not distinguish well between woolly-faced animals (Cockrem 1966).
When the analyses of variance from models 1 and 2 are compared it can be seen that the residual variances have not changed markedly. This suggests that the addition of the mating group effects (model 2) has sub-divided the variance from face grades in model 1 and not accounted for additional effects. This is reflected in the reduction in the face cover mean squares in model 2, in particular those for poll staple length. which is an independent estimate of face cover.
Thus the phenotypic differences shown in model 1 show in model 2 as genetic differences between sires and genetic and maternal effect
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Mat
ing
-I II
Gro
up
-
III
IV
Yea
r ef
fect
TA
BL
E 5
. E
stim
ates
fro
m m
od
el 2
of
mat
ing
grou
p an
d fa
ce c
over
gra
de e
Uee
Ts
for
lO-m
on
th b
od
y w
eigh
t (l
b)
and
poll
sta
ple
leng
th (
cm)
1958
I
1960
I
------
------
----
-1--
-----
--B
ody
wei
ght
!I P
oll
stap
le
Bod~ w
eigh
t I
Pol
l st
aple
B
ody
wei
ght
3.7
±
2.1
-0.4
8
±
0.20
4.
6 ±
2.
3 I
-0.0
7
±
0.22
4.
1 ±
2.
1
-1.0
±
1.
8 0.
31
±
0.17
-1
.4
±
2.1
I
-0.0
2
±
0.20
1.
2 ±
2.
3
1.3
±
1.4
-0.2
4
±
0.13
1
2.2
±
1.7
0.07
±
0.
16
-3.8
±
1.
9
--4
.0
±
1.6
0.41
±
0.
15
-5.4
±
1.
6
I 0.
02
±
0.15
-1
.5
±
2.0
__ J __
~.4 ___
J _~7
4 __
__
__
_
-0.3
__ J
0.59
I
-0.1
I
__
__
__
.
__
_ 1..
. __
__
__
__
1961
Po
ll s
tapl
e
-0.7
6
±
0.20
0.45
±
0.
22
-0.2
1
±
0.17
0.52
±
0.
19
0.15
-
------
Gra
de
1 G
rad
e 2
Gra
de
3 G
rad
e ef
fect
-1
.8
I 0.
02
0.5
I 0.
10
1.3
I -0
.12
Not
e: A
lon
ger
poll
sta
ple
indi
cate
s a
mo
re w
ooll
y-fa
ced
(low
er g
rade
) sh
eep.
Ave
rage
dif
fere
nces
ove
r th
e th
ree
year
s as
op
en
> W
oolly
_L
Mat
ed w
ith
Bod
y w
eigh
t (l
b)
Pol
l st
aple
(em
)
--------
Bet
wee
n si
res
I O
pen
C(
4.5
-0.6
8
Woo
lly
C(
3.5
-0.4
8
Bet
wee
n ew
e gr
oups
O
pen
~
4.6
-0.3
1
Woo
lly
~ 3.
9 -0
.07
Vt
C\
00
~ ~ 8 ~ ~ .., ~
. -::l- ~ S' 3 ~ .... ... :---
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F. COCKREM 569
differences between ewe parents. The grade effect in model 2 will reflect the phenotypic effects within the mating groups.
The ewe groups differed as much in average face cover as the sires but showed smaller weight and poll staple differences in their offspring; as the maternal effects. would be additional to the genetic ones, it therefore seems unlikely that the former were important.
A nalyses of covariance
Although they were not significant, the face cover grade differences at weaning (Y2) were in the same sequence as those at later ages. These weights may therefore have some influence on subsequent differences. Birth weight and birth date may also have had effects on weaning weight differences. Relevant sections of the analyses of covariance made to examine these possibilities are shown in Table 6 for models 1 and 2. Such an analysis can be considered as equivalent to one of weight gains.
Adjusting for birth date and birth weight did not alter the conclusion that face cover grade and mating group effects were not of importance at weaning. Furthermore adjustment of weights at later ages to the mean weaning weight had little effect on the value or the significance of the differences associated with face cover grade found before adjustment. This applied to the models with and without the mating group effects included. These additional estimates adjusted for the regressions are not therefore presented. For birth rank, adjustment for weaning weight resulted in no difference between singles and twins; that is, the weight differences after weaning merely reflected those already present at weaning.
Where a year-face grade interaction was significant (Y4, Y5, Y7)' it resulted from a change in the magnitude of the differences between grades for different years. There was no change in ranking, and openfaced lambs always had the greatest adjusted weight and the shortest poll staple length. Where the interaction was not significant, re-analysis, omitting the interaction term from the model, had little effect on the estimates or conclusions. .
The year difference for Y5 was the result of the early finish of the experiment in 1960. There was also a marked year difference in poll staple length after adjustment for fleece weight. This effect could be either a seasonal or a genetic effect associated with the different sires used each year. It could also be an effect from the age of the ewe, but a sire effect seems the most likely.
Details of the growth curves
The above analysis will smooth out short-term fluotuations in growth both by the time intervals used and by the averaging of many individual effects (Zuckerman 1950). Individual and sub-group mean growth curves were therefore plotted to assist in the interpretation of the main analysis. Two typical sets of curves are shown in Figs. 2 and 3 for wether single lambs in 1958 and ewe single lambs in 1960. The curves for 1961 were similar to those for 1960, while neither
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Mod
el
2
Sou
rce
Yea
rs
Mat
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U.
-.J o ~ "" 8 -<: "" ., ;;- ... ;::-. ~
~
C ;: ::x ~ - ::::
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F. COCKREM 571
sex nor birth rank altered the general pattern. The standard errors of the means were derived from the residual terms of the main analysis of variance and the actual sub-class number; they are, therefore, approximations.
~ 0 ..s-
>-I \'! w So >-0 0 ..,
140
130
120
110
100
90
80
70
60
50
40
30
20
10
'l( I Open-faced / , 1ln<ermedlate
~./ ",,;x Woolly-faced
x ...... /'
, " - - 1. / ~
x __ ~.",/'""" . _----,-I--------- _1/"
/~_:- --- _--.x-I---------- 1
- L_-)(" ---~xr-./,
xl<
,~x ...
Birth Oct. Nov. Dec. Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Jan.
Fig. 2.-The mean weights of single wether lambs born in 1958. The vertical lines show twice the estimated S.E.M. to indicate variation. For significance
of differences see analyses of variance.
120
110
100
90
"'0 80 0 ~ .;
70 >-I
60 \'!
~ 50 >-0
40 0 .., 30
20
10
x Intermediate
x ~ Open-faced
x--+-~~.:r: Woolly-hced
/
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y~
~:r'"W--
/.~
/
:r,. t: . , ,
x
Birth Oct. Nov. Dec. Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.
Fig. 3.-The mean weights of single ewe lambs born in 1960. The vertical lines show twice the estimated S.E.M.
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572 Face cover in lhe Romney. Ill.
The mean growth curves suggest that the weight differences between face cover grade groups can arise over relatively short periods; that is, between four and six months of age (after shearing and weaning) in 1958, and between nine and 10 months, that is. at the beginning of winter in 1960. Lambs of grade 2 born in 1958 also showed a relative slowing of weight gain during the winter.
DISCUSSION
The overall results establish that the differences in body weight associated with face cover grade in the adult Romney ewe (Cockrem and Rae 1966) are the result of growth rate differences between weaning and 14 months of age.
The face cover grade used was that at 18 months of age, and because of changes in face grade from weaning to this time the fa-.:e grade might not be suitable for predicting lamb growth. This change in average face cover grade between four and 18 months could be the result of a number of factors. At weaning a lamb's head and face are still comparatively small, and considerable elongation occurs after weaning (about 4 cm increase to 14 months for the 1958 lambs; see also Palsson and Verges (1952)). This could change the patterns of visible wool growth on which the grading system is based. Shearing at four and 14 months will affect the face grading for a short period until wool is again visible above the kemp in the appropriate areas. This would not be true if the time of shearing coincided with a period when facial wool growth was declining. A third factor is the shedding of face wool which occurred in these lambs during the period July to September (12- I 4 months of age). This would cause an incre:J.se in face cover grade (more open) followed by a decrease depending on the rate of re-growth of the wool.
On the basis of these observations a probable interpretation of the observed changes in average face cover grade is that there is a general increase in the average grade after weaning associated with the elongation of the head and that this effect is emphasised by shedding of wool up to 14 months. The final differences at 18 months, which remain for the next few years (Cockrem 1966), seem likely to be associated with differences in the rate of regrowth of face wool after shedding and shearing.
The lower variability of face grade at weaning means that if selection of animals before 18 months for face cover is desirable then a more accurate grading system or an alternative method of measuring face cover is required. With practice, it is possible to pick potentially open-faced lambs at six months of age but it is difficult to describe the method objectively.
Factors to be considered in the interpretation of analyses of body weight changes by the methods used in this paper have been discussed by Cocbem (1965). Some of the present data were used to illustrate various points which require consideration in the interpretation. Allowing for these it would appear that there was some environmental influence which slowed the growth of woolly-faced lambs more than that
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F. COCKREM 573
of open-faced ones. The effect could be continuously related to face cover in that the intermediate (grade 2) lambs had growth curves between the two extreme groups.
The analysis by model I showed these body growth effects as phenotypic ones occurring after weaning. This suggested that any maternal effects were indirect and not associated with milk production or mothering ability. The model 2 analysis indicated that the relationship between face cover and lamb growth has a high genetic content and confirmed that maternal effects were unlikely to be of importance. Ch'ang (1967) reported a genetic correlation between 14 month live weight and fertility in Romney sheep. The present data show that 14 month live weight and face cover have a strong genetic relationship, while Roberts (1959) has shown a genetic relationship (for the Merino) between face cover and fertility. It is therefore possible that face cover could be the common factor in the live weight-fertility relationship. If this is so, then the relationship reported by Ch'ang (1967) might not hold in a flock with a low variability of face cover. In any case identification of the factors underlying these correlations is clearly of importance to their interpretation.
One possible environmental influence coinciding with the weight gain differences is cold stress, as shearing of the 1958 lambs coincided with a southerly storm. A second possibility is a nutritional effect, as feed availability can be restricted over the winter. The effective nutrition is, however, difficult to assess in the paddock. Preliminary evidence of an association of body temperature after shearing with face cover and weight gain (Cockrem 1962) would suggest that cold stress may be important, and further evidence on temperature effects has been discussed by Cockrem (1967). Such effects could account for the year-face grade interactions found in the analyses of variance.
The possibility that lamb growth differences arise from maternal effects such that woolly-faced ewes had the lighter lambs was not confirmed by these data and analyses. It would appear to be a factor associated with the lamb's own face cover that is of importance. The possibility that body growth affects the face cover seems unlikely in that face cover differences increased from 14 to 17 months when growth rates were similar. Nor have face cover differences been detected between singles and twins with their different growth rates up to weaning.
I
As the lambs had their eyes clipped free of wool in June and as growth rate differences were detected after shearing for the 1958 lambs, any wool blindness effect would appear to be negligible. It would appear, therefore, that there is some underlying factor which leads both to a lower rate of body growth of the lamb under certain conditions and also to more wool growth on the face. It may be this underlying factor which also affects fertility in the ewe, in which case the lower live weight of adult wooliy-faced ewes is a secondary effect and possibly not of intrinsic importance.
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574 Face cover in the Romney. Ill.
ACKNOWLEDGMENTS
I should like to acknowledge the assistance of: Messrs L. Hawthorne and P. Whitehead in the management and shepherding of the flocks; Messrs B. Thatcher and E. Ormsby in the collection of data; and Professor A. L. Rae for discussion of the analysis system.
The work was financed by the New Zealand Wool Board until March 1961, and then by the New Zealand Wool Research Organisation.
REFERENCES
CH'ANG, T. S. 1967: Ph.D. Thesis Massey Univ. (Lodged in Massey Univ. Library.).
CocKREM, F. 1962: Proc. N.Z. Soc. Anim. Prod. 22: 45.
---1965: Ibid. 25: 164.
----1966: Aust. 1. agric. Res. 17: 975.
~---1967: Proc. N.Z. Soc. Anim. Prod. 27: 193.
CocKREM, F.; RAE, A. L. 1966: Aust. 1. agric. Res. 17: 967.
CooP. I. E. 1956: Proc. N.Z. Soc. Anim. Prod .. 16: 55.
HARVEY. W. R. 1960: Least squares analysis of data with unequal sub-class numbers. U.S. Dep. Agric. Publ. Ars.-20....8:85
MORLEY. F. H. W. 1955: Aust. 1. agric. Res. 6: 873.
PALS SON. H.; VERGES. J. 1952: 1. agric. Sci., Camb. 42: 1.
ROBERTS. E. 1959: Ph.D. Thesis N.S.W. Univ. of Technology. (Lodged in N.S.W. Univ. of Technology Library.).
TERRILL, C. E. 1949: 1. Anim. Sci. 8: 353.
ZUCKERMAN, S. 1950: Proc. R. Soc. B. 137: 433.
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