ovulatory activity in mature romney ewes in new zealand
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This article was downloaded by: [Case Western Reserve University]On: 02 December 2014, At: 23:17Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK
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Ovulatory activity inmature Romney ewes inNew ZealandR. L. W. AVERILL aa Endocrinology Research Department,Medical Research Council of New Zealand ,Otago University Medical School , DunedinPublished online: 06 Jan 2012.
To cite this article: R. L. W. AVERILL (1964) Ovulatory activity in matureRomney ewes in New Zealand, New Zealand Journal of Agricultural Research,7:4, 514-524, DOI: 10.1080/00288233.1964.10416379
To link to this article: http://dx.doi.org/10.1080/00288233.1964.10416379
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514
OVULATORY ACTIVITY IN MATUREROMNEY EWES IN NEW ZEALAND
By R. L. W . AVERILL
Endocrinology Research Department,Medical Research Council of New Zealand,
Otago University Medical School,Dunedin
(Received 16 April 1964)
ABSTRACT
Causes of variation in time of onset and levels of ovulatory activitywere studied in 7,782 mature Romney ewes from several regions withinNew Zealand in 1959, 1960, and 1962.
The median date of onset of seasonal ovulatory activity in OtagoRornneys was the same in both 1959 and 1960-15 March. Significantvariations in the proportion of ewes with multiple versus single ovulationswere found in comparisons within- and between-flocks and within- andbetween-regions studied.
Multiple ovulation was significantly less frequent in ewes fromNorthland and Poverty Bay than in similar ewes from Southland in eachof the months March, April, and May 1962. Multiple ovulation wassignificantly less frequent in March than in April in all regions studiedexcept Northland.
These results are discussed in relation to ewe fertility.
INTRODUCTION
The Romney Marsh breed of sheep is numerically predominantin both islands of New Zealand. Lambing percentages in the mostsoutherly provinces consistently exceed those for ewes of similar breedand age in most of the North Island (Hart and Stevens 1952).Wallace (1954) has reported that the low incidence of multiple births inNorth Island Romney ewes contributes greatly to the low lambingpercentages and has found ovulation rates of 1.17 to 1.33 in such sheep.These figures contrast with a much higher incidence of multiple ovulationsin Romney ewes examined in Otago (Averill 1959).
These studies were undertaken to gain further information on timeof onset of seasonal ovulatory activity and the degree and causes ofvariation in levels of ovulatory activity in Romney ewes throughout NewZealand.
METHODS
Collections of reproductive organs (uteri and ovaries) from nonpregnant mature Romney ewes were made between January and July
N.Z.]. agric. Res. 7: 514-24
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at freezing works sited at different latitudes throughout New Zealand, asshown in Table 1. (Note. Sheep killed at Mataura were not necessarilyliving farther south than those killed at Burnside) .
TABLE 1. Distribution of collections of sheep ovaries in New Zealand
District I Latitude I Years E I Farms___________I~)__ (No.) wes_~._)_
323 I 43685 8194 2
_~J__~__
North Island Northland
Poverty Bay
Taranaki
Hawke's Bay
35.4
38.7
39.0
39.5
S196011962
1962
)195911960
1960
475382
521
South Island Canterbury
Otago
Southland
43.5
45.9
46.2
1960
\195911960
\196111962
1,350
2,319697
167773
13
7024
344
The size and composition of collected samples varied. In 1959 and1960, samples of 100 + organs were obtained (generally from one or twofarms) at fortnightly intervals from regions other than Otago. FromOtago, and the regions sampled in 1962, composite collections of 100+ organs were made weekly or twice weekly and were drawn from ewesoriginating on several different farms.
All sets of organs were sent frozen to Dunedin, where the ovarieswere examined individually for the presence and number of corporalutea as described previously (Averill 1955, 1959).
In deriving monthly mean values for levels of ovulatory activity,collections made during the first eight days in any month were attributedto the previous month to correct for the fact that the corpora luteaobserved would have been formed at ovulations occurring throughoutthe preceding 16 days.
RESULTS
General
Forms of luteal tissue not included as ovulations below were foundas corpora lutea atretica «3 mm diameter) and luteinised follicles(> 15 mm diameter) which were lined with a thin layer of lutein tissue.Both forms were found in the ovaries together with normal corporalutea. Luteinised follicles were found also in ewes without corpora luteain the month preceding seasonal activity.
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516 Ovu!atoiY acttuity in Romney ewes
In the ovaries of 11 ewes with large active mid-cycle corpora therewere, as well, recent ovulations of two-three days in age. Histologicalexamination of these ovaries revealed that ovulation had occurred in thepresence of existing corpora lutea and that lutein cells in both typesof corpora were active. In four of these 11 ewes tubal and uterine ovawere recovered which corresponded to the appa1rent ages of the corporalutea. Polycystic enlargement of the ovaries was seen in 20 ewes, sevenof which had mid-cycle corpora lutea in their ovaries. None of theabove abnormalities was present in significant numbers from anyoneof the regions under study. They probably represent "normal" accidentsin the regular reproductive pattern. Data from these ewes, togetherwith those from 12 with infantile genitalia, 18 with partial agenesis ofsegments of uterine horns or tubes, and 14 with ovaries deeply embeddedin adhesions of horn and broad ligament or with unilateral ovarianagenesis, were not included in the ovulation studies.
Onset of seasonal activity
To detemine an approximate date by which 50 per cent of ewes inthat locality might be expected to have begun seasonal activity, probitvalues for the percentage of ewes already ovulating in each collectedsample were plotted against the date of collection in both the season1959 and 1960 for Otago ewes. This estimated date for 50 per cent ofewes to have begun ovulatory activity (E.T.50) was plotted also for theCanterbury collections (1960) and the Southland collections (1962).
The E.T. 50 for Otago was:-15 March -+- 2.1 days in 1959, and16 March -+- 3.1 days in 1960. There was no difference between thesedates. The E.T. 50 for Canterbury ewes in 1960 was 5 March -+- 4.3days, and for Southland ewes in 1962 was 12 March -+- 4.5 days. TheCanterbury date was significantly earlier than those for Otago (P <0.05by t. test), but the Southland date was not significantly different fromthe Otago dates.
Earlier collections from other regions were too sporadic to enablestatistical comparison or calculation of E.T. 50.
Thus, from Northland in 1960, on 11 February 1960, 12.9 per centof 101 ewes were ovulating, on 1 March 1960, 20.9 per cent of 92 eweswere ovulating, and on 18 March 1960, 93.3 per cent of 90 ewes wereovulating.
From Hawke's Bay on 22 February 1960, 7.8 per cent of 77 eweswere ovulating, but no collections were made during March.
From Taranaki, no ewes were ovulating amongst 150 examinedon 5 January 1959, and only one of 137 on 23 January 1959. By 18February 1959, when two farms were sampled, 39.5 per cent and 87.5 percent of 49 and 47 ewes respectively were ovulating. (Only 8.4 per cent of107 Taranaki ewes examined on 12 February 1960 were ovulating). On4 March 1959, 31.4 per cent of 121 ewes examined were ovulating, andon 25 March 1959, 97.0 per cent of 99 ewes examined were ovulating.
The 1962 collections from Northland and Poverty Bay began on23 March 1962 and 15 March 1962 respectively. Between these dates
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and the end of March, 94.3 per cent and 86.8 per cent of ewes examinedalready had corpora lutea in their ovaries.
It was concluded that ovulatory activity began earlier in the yearin the northern regions than it did in the southern regions of NewZealand and that there was considerable between-farm variation in timesof onset.
Variations in the number of ovulations per ewe
The number of corpora lutea in the ovaries of each specimen wasrecorded, and for each sample a mean value for the number of ovulationsper ewe ovulating was calculated. While these mean values express ina single index the relative level of ovulatory activity of the ewessampled, for comparative purposes Chi-square tests were made betweenthe proportion
Number of ewes with multiple ovulationsNumber of ewes with single ovulations '
to test the validity of differences in activity between the various groupsof ewes.
TABLE 2. Monthly uartations in mean numbers of ovulations per ewe ovulatingin Otago 1959 and 1960 and Canterbury 1960
-----,--- _._-
IOtago collections
----
I
II
Month I 1957 I 1958 1959 I 1960 CanterburyI 1960
I(from Averill
Present studies i1959) II .. --,,-'---
February Ewes examined (No.) 276I
367 364 114 216Ewes ovulating (%) 4.2 ! 8.9 8.5 11.4 17.8
C.L. per ewe 1.25 ! 1.30 1.26 1.31 1.25
March Ewes examined (No.) 63 I 424 604 379 191Ewes ovulating (%) 61.9 65.1 59.9 56.5 95.8
C.L. per ewe 1.54 1.57 1.42(a) 1.35(d) 1.30
April] Ewes examined 417 357 541 76 188C.L. per ewe 1.86 1.82 1.60(b) 1.82 1.64
May] Ewes examined 132 : 354 I 337 118 205C.L. per ewe 1.67 1.76 1.65 1.51 1.52
I
Junet Ewes examined (No.) 137 314 ! 166 - 213C.L. per ewe 1.60 1.52 11.25(c) - 1.33
t In April, May, and June 98-100 per cent of ewes examined had corporalutea in their ovaries.
Between-year comparisons:
1959 \ signif. <1958 (P <0.01, Chi-square = 10.8)a- l. not signif. <1957 (Chi-square = 2.5)b--1959 sign if. <1958 (P <0.001, Chi-square = 26.21) :. <1960 and 1957c-1959 signif. <1958 (P <0.001, Chi-square = 36.5) :. <1957d-1960 signif. <1957 (P <0.05, Chi-square = 6.40) :. <1958
No differences between February or May means were significant.
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518 Ovulatory activity in Romney ewes
(a) Within-season trends. In Tables 2 and 3 the values for meannumbers of ovulations per ewe ovulating in consecutive months of thebreeding season are shown where such monthly means were derivedfrom a reasonably large number of different farms. A test was made in1959 to evaluate day-to-day variation in sample means; and from 155
TABLE 3. Monthly uariation. in mean numbers of ovulations per ewe ovulatingin Northland, Poverty Bay, and Southland, 1962
Month I Northland .Poverty Bay 1 Southland
February Ewes examined (No.) - - 17Farms (No.) - 2C.L. per ewe 1.23
March Ewes examined (No.) 64 79 217Farms (No.) 5 19 16C.L. per ewe 1.20 (a) 1.20 (d) 1.46 (g)
April Ewes examined (No.) 123 118 197Farms (No.) 8 11 10C.L. per ewe 1.35 (b) 1.41 (e) 1.78 (h)
May Ewe examined (No.) 84 71 278Farms (No.) 6 7 15C.L. per ewe 1.21 (c) 1.39 (f) 1.62 (i)
June Ewes examined (No.) 16 16 14Farms (No.) 1 2 1C.L. per ewe 1.19 1.31 1.07
July Ewes examined (No.) 29Farms (No.) 4C.L. per ewe 1.06
Between-region comparisons:
g signif, > a (P <0.001, Chi-square = 13.0) :. g > d.h signif. > b (P <0.001, Chi-square = 48.4)h signif. > e (P <0.001, Chi-square = 27.2)i signif. > c (P <0.001, Chi-square = 34.1)i signif. > f (P <0.01, Chi-square = 8.9)
No differences between Northland and Poverty Bay monthly means weresignificant.
Otago Romneys from four farms, killed on 20 April 1959, the meannumber of ovulations per ewe was 1.54 -+- 0.04, while that from 127 ewesfrom a further four farms, killed on 21 April 1959, was 1.52 -+- 0.05ovulations/ewe. The day-to-day difference was not significant.
Between-month comparisons of levels of ovulatory activity weremade for the data in Tables 2 and 3, i.e., for ewes killed in Otago in1959 and 1960, Canterbury 1960, and in Northland, Poverty Bay, andSouthland in 1962.
The Chi-square values from these comparisons are shown in Table 4.
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TABLE 4. Chi-square values from between-month comparisons of levels ofovulatory activity in New Zealand Romney ewes
Comparisons
RegionI April> May> IApril> I April> I May>
March March May I June ; June
I
f 1959 23.36 66.61 I 2.77 54.63 63.80Otago 1 1960 45.83 1.89 16.62
Canterbury 1960 32.26 15.74 4.99 33.31 12.41
Northland 1962 3.09I
3.04
Poverty Bay 1962 10.10
I
6.83
Southland 1962 32.54 9.35 9.30
I Chi-square values >3.8 = P <0.05N.B. For one degree of freedom ~ Chi-square values >6.6 = P <0.01
J Chi-square values> 10.8 = < 0.001
It was concluded that in the collections from Otago, Canterbury,Poverty Bay, and Southland, there were significant within-season variations in the proportions of ewes with multiple versus single ovulations.A similar trend in Northland ewes was not significant. The highestmeans occurred in April and May.
(b) Between-seasons. In view of the significant between-monthvariations within the breeding season (see above) between-season comparisons of levels of ovulatory activity were made by comparing eachof the months between years for Otago data. (Comparable data tromcollections in 1957 and 1958 (Averill 1959) are included in Table 2) .
Between-year comparisons for 1957-60 revealed no significantdifferences between values for Otago ewes in either February or May.For March, the 1960 value was significantly lower than those for 1957and 1958 but not significantly different from the 1959 value. For Aprilthe 1959 value fell significantly below those for the other three years,and for June the 1959 value fell below those for 1957 and 1958. TheChi-square values for these comparisons are shown as footnotes toTable 2.
Thus of 27 possible comparisons, only eight comparisons dependenton three of the 19 tabulated means gave rise to significant betweenseason differences. These differences were probably due to interruption insampling, which gave rise to non-representative samples. Thus the March1960 mean was derived from only three collections; that for April 1959from four collections, of which two were on consecutive days; and thatfor June 1959 from only two collections.
It was concluded that while between-season variations in the meannumber of ovulations per ewe ovulating in Otago did occur, there wasa remarkably close agreement over four consecutive seasons for themeans calculated for each of the months studied.
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520 Ovulatory activity in Romney ewes
(c) Between-region comparisons. In Table 3 the March, April, andMay mean numbers of ovulations per ewe derived from the differentdistributions of single- and multiple-ovulating ewes within samples fromNorthland, Poverty Bay, and Southland in 1962 are shown.
Comparisons for each month between regions showed that meansfor Southland were significantly greater than those for Northland ewesand for Poverty Bay ewes. Northland monthly values did not differsignificantly from those from Poverty Bay ewes. Comparisons betweenmonthly means from Otago and Canterbury ewes in 1960 showed nosignificance for differences in means for March or for May (Chi-square= < 1.0), but the Otago April mean was significantly higher than thatfor Canterbury (P <0.05, Chi-square = 5.35).
Data from single flocks in Northland and Hawke's Bay in1960 are shown in Table 5. Specimens were collected as nearly aspossible on the same day to obviate within-season variation. Differencesbetween values for the two Hawke's Bay farms sampled in April andthe two Northland farms sampled on the same days were significant(P <0.05; Chi-square = 5.40 for 1 April 1960, and P <0.001;Chi-square = 32.2 for 20 March 1960).
Multiple ovulation occurred significantly less frequently in Northlandand Poverty Bay than in Southland in 1962 and was less frequent ontwo Northland farms than on two Hawke's Bay farms in 1960.
TABLE 5. Variation in mean ovulations per ewe in single flock samples fromNorthland and Hawke's Bay, 1960
C.L. per ewe(mean ± S.E.)
Ewes (No.)
C.L. per ewe(mean ± S.E.)
Ewes (No.)
C.L. per ewe(mean ± S.E.)
Ewes (No.)
C.L. per ewe(mean ± S.E.)
Ewes (No.)
Date killed
13.3.60
5.4.60
20.4.60
12.5.60
Northland IIj
1.30 ± 0.06 (a) I
34 .
1.43 ± 0.06 (b)93
1.31 ± 0.04 (c)97
Hawke's Bay
1.62 ± 0.02 (d)35
1.35 ± 0.07 (e)92
1.33 ± 0.05 (f)99
Between-farm comparisons:No differences were significant for comparisons a v. b, b v. c, d v. e.
d signif, > f. (P <0.01, Chi-square = 7.5)e signif, > f. (P <0.001, Chi-square = 19.9)
Between-region comparisons:
5.4.60-d. sign if. > b (P <0.05, Chi-square = 5.4)20.4.60-e. signif. > c (P <0.001, Chi-square = 32.2)
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( d) Between-farms variations. Variations in the incidence of multipleversus single-ovulating ewes are shown for three Northland farms andthree Poverty Bay farms in Table 5. Chi-square tests of differencesbetween Northland farms failed to reach significance, while those forthe Poverty Bay farms were significant in two of the three comparisons.In the 1962 samples considerable between-farm differences were apparent,
TABLE 6. Variations in the number of corpora lutea in the ovaries of ewes fromNorthland farms in April 1962
Ewes with
1 C.L. I 2 C.L. I 3 C.L.
i'Mean C.L.
per ewe
Total
Ewes
Farm A
Farm B
Farm C
Farm D
Farm E
Farm F
Farm G
Farm H
11
6
8
7
8
17
14
12
16
2
8
7
2
1.08
1.83
1.20
1.33
1.50
1.12
1.33
1.17
12
24
10
9
16
18
21
13
All farms 83 37
I
3-1--~-123
but the small numbers of ewes per farm (10-20) precluded directstatistical comparison. The extent of this variation may be seen in theNorthland subsampJes for April 1962 shown in Table 6.
(e) Within-farm variations. Samples were obtained on two differentdates from ewes on a Canterbury farm in 1960 and from a Southlandand a Poverty Bay farm in 1962.
TABLE 7. Variations in the mean number of ovulations per ewe in ewes fromthree farms sampled on two occasions
Farm Collection date C.L. per ewe
INo. of ewes
origin (mean ± S.E.)-------- ----- -- - --- "rCanterbury 3.5.60 1.73 ± 0.06 88
8.6.60 1.42 ± 0.05I
105
Poverty Bay 30.4.62 1.70I
104.7.62 1.50 10
Southland 7.5.62 1.62 ± 0.04I
211.6.62 1.50 ± 0.01 20
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522 Ovulatory activity in Romney ewes
The mean number of ovulations per ewe in the separate samplesare shown in Table 7.
The difference between the two means from the Canterbury farmwas significant (P <0.001; Chi-square = 17.8), but those between thetwo means from the other two farms were not significant. The differencespossibly reflect within-season variations in ovulatory activity.
(f) Effect of weight of ewe. On 20 April 1959 samples were collectedfrom lines of fat Otago ewes (carcass weight > 80 lb) from which themean number of ovulations per ewe was 1.74 + 0.06 from 68 ewes.A second sample was drawn from noticeably thin ewes (carcass weight< 64 lb), for which the mean number of ovulations was 1.39 + 0.05from 87 ewes. The difference between the proportions of multiple- tosingle-ovulating ewes in the two samples was significant (P <0.001;Chi-square = 14.98).
DISCUSSION
Onset of seasonal activity
The estimated dates by which 50 per cent of mature Otago Romneyewes had begun seasonal activity in 1959 and 1960 did not differsignificantly from the dates previously estimated for similar Otago ewesfor 1957 and 1958 (Averill 1959). In confirmation of these dates Lewisand Averill (unpublished) found that of 216 six-tooth Romney ewesjoined with raddled rams on 2 March 1960 only 20 per cent were servedby 22 March 1960 and 45 per cent by 28 March 1960, i.e. 45 per centwere probably experiencing pre-seasonal "silent heats" (Grant 1933)by 11 March 1960, a few days earlier than the E.T. 50 of 16 Marchfound for that year. The median date of onset for Canterbury ewes m1960 was significantly earlier than this, i.e., 5 March 1960. This datewas similar to that of 7 March for 4,000 Romney and Romney cross ewesin South Canterbury in 1963 (Woolff, J. E., pers. comm.).
Yeates (1949) found that onset of seasonal breeding activity wasinduced by the photoperiodic stimulus of declining daylight hours, and theannual consistency shown in the Otago figures above showed that thestimulus must be relatively constant from year to year. Data from Goot(1949) confirm that North Island Romneys are similarly consistent withrespect to the proportions of ewes mated during consecutive weeks inconsecutive breeding seasons. Data on the onset of ovulatory activityin North Island Romneys were incomplete in this study, but seasonalactivity clearly begins earlier there than in the South Island. Thus in theweek ending 15 March 1962, 80 per cent of ewes from seven PovertyBay farms were already ovulating, compared with only 57 per cent otewes from four Southland farms, while Wallace (1951) found that atleast 90 per cent of aged ewes in the Waikato had had oestrous periodsbefore the end of March.
Factors other than the light environment are known to exertmodifying influences on the time of onset of seasonal activity. Thesefactors include lowered environmental temperature (Dutt and Bush
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1955) and the unaccustomed presence of the ram (Schinckel 1954;Edgar and Bilkey 1963). Stress associated with transportation of ewesabout the time of onset of the breeding season is claimed to inducepremature ovulation (Braden, A. W'. H., pers. comm.). Transport otRomney ewes to saleyards and freezing works in New Zealand did notappear to induce ovulation during January in these studies. Thus of394 North Island ewes killed in January in 1959 and 1960 only 10 hadovulated, and of 307 South Island ewes also killed in January in thoseyears only six had ovulated. (See also Averill 1959). The proportionsof ewes ovulating in the February killings appeared to he consistent witha normal pattern of seasonal onset.
Of greater practical interest were the considerable differences between farms within regions in times of onset of activity. Furtherresearch may reveal whether such differences are dependent on nutrition,climate, aspect, or altitude of the farms concerned or are due to otherfactors.
The number of ovulations per ewe
As each ovulation implies a potential lamb (Hammond 1921),considerable importance attaches to the possible causes of variation inthe mean numbers of ovulations per ewe or the relative incidence ofmultiple ovulations in sheep. In these studies, significant variations inmeans were found within a population restricted as far as possible tomature ewes of the Romney breed. Levels of ovulatory activity variedsignificantly both between flocks and within flocks, the latter possiblyreflecting within-season trends. In one comparison it was found that inheavier ewes the incidence of multiple ovulations was significantlygreater than that in noticeably thin ewes. (See also Wallace (1958), andCoop (1962)).
Confirmation of within-season rise and fall in the mean numbers ofovulations per ewe (McKenzie and Terrill 1937; Hammond 1944;Watson 1952; Averill 1955, 1959; McDonald 1958; and others) wasfound in data from mature Romney ewes from Southland, Otago,Canterbury, and Poverty Bay (Tables 2 and 3), and was suggested inthe data from Hawke's Bay (Table 5), although the samples fromthat region were from individual flocks. Of the six regions studied,between-month comparisons for Northland ewes only failed to showsignificant differences. As the 1962 collections in that region did notbegin until 22 March 1962, the mean for March was probably anover-estimate of the true level of activity in that month. The importanceof the within-season variation and its relationship to potential lambingpercentages has been stressed (Averill 1959). I t is clearly importantalso to allow for such variation in comparisons of potential fertility madebetween regions.
The present studies have shown significant between-region differencesin levels of ovulatory activity in mature Romney ewes. Thus significantlyfewer ewes from 57 farms in Northland and Poverty Bay had multipleovulations in each of the months of March, April, and May 1962 whencompared with similar ewes from 41 farms in Southland killed duringthe same period. The incidence of multiple versus single ovulation in
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524 Ovulatory activity in Romney ewes
Northland ewes was so low that the within-season variation did notreach significant levels. On the other hand, the monthly mean valuesfor the Southland ewes indicate levels of ovulatory activity closely similarto those in Romney ewes in Otago and in Canterbury. The generalisationthat Romney ewes tend to shed greater numbers of eggs per ewe inthe south than they do in the north of New Zealand confirms thehitherto discrepant data of both Wallace (1954) and Averill (1959) andoffers at least a partial explanation for the lower lambing percentages inNorth Island Romneys.
Assuming a normal over-all post-ovulation wastage of 40 per centof all ova shed (Brambell 1948; Averill 1955), the potential lambingpercentages of mature Romneys lambing in Northland and PovertyBay would be 90-112 per cent on the March / April ovulation figures,whereas in Otago and Southland the lambing percentages would be112-133 per cent from similar ewes. Judged against those from the south,northern Romneys might be considered to be contending against anadverse environment and therefore unsuited to it as a breed. The sameapplies in comparisons of Northland lambing percentages with those ofewes of the parent stock from Romney Marsh (Gordon 1958). Whetherthe source of this "adversity" is climatological, nutritional, dependent onlatitude (Hart and Stevens 1952), or not, or whether it is a result of along period of unconscious selection against fertility, is unknown. Furtherresearch may reveal the reasons for the lower levels of ovulatory activityin North Island Romneys.
ACKNOWLEDGMENTS
Thanks are due to the Supervising Meat Inspectors at freezing worksat Moerewa, Kaiti, Waitara, Whakatu, Islington, Burnside, and Mataura,who made the collections of material. Finance for transport of materialto Dunedin was generously provided by the New Zealand Wool Board.
REFERENCES
AVERILL, R. L. W. 1955: Stud. Fertility 7: 139.------1959: N.Z. J. agric. Res. 2: 575.DRAMBELL, F. W. R. 1948: Bioi. Rev. 23: 370.Coor-, I. E. 1962: N.Z. J. agric. Res. 5: 249.DUTT, R. H.; BUSH, L. F. 1955: J. Anim. Sci. 14: 885.EDGAR, D. G.; BILKEY, D. A. 1963: Proc . N.Z. Soc. Anim. Prod. 23: 79.GOOT, H. 1949: N.Z. J. Sci. Tech. A30: 330.GORDON, I. 1958: J. agric. Sci. 50: 123.GR.ANT, R. 1933: Nature, Land. 131: 802.HAMMOND, J. 1921: J. agric. Sci. 11: 337.HAMMOND, J. (Jnr.) 1944: Ibid. 34: 97.H\P"T, D. S.; STEVENS, P. G. 1952: Proc. N.Z. Soc. Anim. Prod. 12: 67.M.~DoNALD, M. F. 1958: Sheepfmg Annu.: 193.Mr.KE!'iZIE, F. F.; TERRILL, C. E. 1937: Mo. agric. expo Sta. res. Bull.
264: 328.SCHINCKEL, P. G. 1954: Aust. J. agric. Res. 5: 465.WALLACE, L. R. 1951: Proc. Ruakura {mrs' Conf. p. 1.------1954:J. agric. Sci. 44: 60.-------1958: Proc. Ruakura {mrs' Conf. p. 62.WATS0N, R. H. 1952: Aust. vet. J. 28: 1.YEATES, N. T. M. 1949: J. agric. Sci. 39: 1.
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