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Effects of pasture allowance in winteron liveweight, wool growth, and woolcharacteristics of Romney ewesH. Hawker a & K. F. Thompson b ca Invermay Agricultural Centre, MAF , Private Bag, Mosgiel ,New Zealandb Woodlands Research Station, MAF , Private Bag, Invercargill ,New Zealandc Department of Animal Science , Lincoln College , Canterbury ,New ZealandPublished online: 16 Jan 2012.

To cite this article: H. Hawker & K. F. Thompson (1987) Effects of pasture allowance in winteron liveweight, wool growth, and wool characteristics of Romney ewes, New Zealand Journal ofExperimental Agriculture, 15:3, 295-302, DOI: 10.1080/03015521.1987.10425574

To link to this article: http://dx.doi.org/10.1080/03015521.1987.10425574

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New Zealand Journal of Experimental Agriculture, 1987, Vol. 15 : 295 - 302 0301 - 5521187/1503 - 0295$2.5010 © Crown copyright 1987

295

Effects of pasture allowance in winter on liveweight, wool growth, and wool characteristics of Romney ewes

H. HAWKER Invermay Agricultural Centre, MAF Private Bag, Mosgiel, New Zealand

K. F. THOMPSON! Woodlands Research Station, MAF Private Bag, Invercargill, New Zealand

Abstract In each of two years, pasture allowances of 0.7, 1.2, 1.7, or 2.2 kg dry matter (DM)/ewe per day were offered to groups of Romney ewes for 6 weeks in June - July. The ewes were then run together until weaning in December. In Year I there were 70 2-year-old ewes per group; mean initialliveweight was 47 (SE 0.25) kg. In Year 2 each group comprised 25 2-year-old ewes with a mean initialliveweight of 48 (SE 0.49) kg and 25 6-year-old ewes with a mean initial liveweight of 57 kg. For liveweight change (g/day), the inverse linear relationship with pasture allowance (kg DM/ewe per day) for ewes with one lamb in utero was: [liveweight change = 136 (SE 4.4) - (132 (SE 5.0) allowance)]. On average, the pasture allowance that resulted in zero liveweight change was predicted to be 1.0 kg DM/ewe per day. With a pre­grazing pasture mass of c. 2000 kg DM/ha, zero liveweight change was associated with a residual pasture mass of 760 kg DM/ha, a utilisation of 660/0, and an apparent intake of 0.59 kg DM/ewe per day. For ewes with zero, one and two lambs in utero, zero liveweight change was predicted to occur on pasture allowances of 1.3, 1.0, and 0.8 kg DM/ewe per day, respectively. However, a similar allowance (c. 1.3 kg DM/ewe per day) would maintain body weight (conceptus free) for ewes in all three categories. Pasture allowance in June - July affected liveweight in August by up to 3.4 kg but this effect had largely disappeared by

1 Present address: Department of Animal Science, Lincoln College, Canterbury, New Zealand. Received 29 October 1986; revision 2 June 1987

December. Pasture allowance did not affect lamb birth weight. Wool growth, fibre diameter, and staple length growth rate during the period of differential feeding in June - July, and subsequently in July - August, increased significantly with increasing pasture allowance in June - July. The maximum effect of pasture allowance on clean wool production between June and December was 0.12 kg. The relative effect of pasture allowance on staple strength was similar to that on wool growth in July - August. Fibre growth was continuous through the winter, reaching a minimum in July - August, about 6 weeks before the start of lambing. In June - July (mid pregnancy) pregnant ewes grew 0.8 g (180/0) less wool per day than non­pregnant ewes; in July - August (late pregnancy) ewes with one lamb in utero grew 0.7 g (21 %) less wool per day than non-pregnant ewes and 0.4 g (17%) more wool per day than ewes with twins. There was no significant interaction between number of lambs in utero and pasture allowance.

Keywords pasture allowance; sheep; ewes; pregnancy; winter; liveweight change; birth weight; wool growth; fibre diameter; staple length growth; staple strength

INTRODUCTION

Although the nutritional requirements of breeding ewes are lower in winter than in the autumn or spring, production may be affected by the usual practice of rationing feed during winter when the rate of pasture growth is low. Coop & Clark (1969) and Monteath (1971) studied the effects on production of reducing the quantity of winter supplements. They fed ewes at approximately half maintenance for 5 - 10 weeks after joining and found that, despite liveweight reductions of 4 - 10 kg (7 -16% of initial liveweight), there were no significant reductions in the birth weight of lambs or in lamb growth rate during lactation. However, depending on the severity and duration of the low nutrition, the low feeding levels led to reductions in both greasy fleece weight (by 0.2 - 0.7 kg) and staple strength.

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296 New Zealand Journal of Experimental Agriculture, 1987, Vol. 15

Intensive grass wintering (ewes stocked at about 2000/ha and shifted at intervals of 1 - 3 days onto new areas) has been adopted in many New Zealand environments, with strict rationing of pasture to reduce or eliminate the need for supplementary winter feed (Halford 1972).

Although the relationship between intake and production in the winter (and in other seasons) is reasonably well known (Geenty & Rattray 1987), the amount of pasture that has to be offered to give the desired intake, and therefore the desired level of production, is poorly defined. The problem of measuring the intake of grazing sheep has been avoided by looking directly at the relationship between pasture allowance (the amount of pasture offered) and production (Rattray et al. 1987). In some experiments the residual pasture mass remaining after the sheep have been removed has been estimated. The proportion of the offered pasture apparently eaten and the intake have then been calculated.

In experiments at Ruakura Research Station the production responses of mixed-age ewes to pasture allowance in winter (mid pregnancy) have been examined (Jagusch et al. 1981). Extremes of differential feeding for 6 - 8 weeks led to differences in liveweight of 6 - 13 kg and in greasy fleece weight of 0.2 - 0.5 kg, but there were no significant differences in either lamb birth weight or growth during lactation. The results suggested that, if pasture mass was 2000 - 2500 kg dry matter (DM)/ha, zero liveweight change would occur when residual pasture mass was 350 -700 kg DM/ha, utilisation 68 - 870/0 and apparent intake 0.7 - 0.8 kg DM/ewe per day.

To test whether the Ruakura results could be extrapolated to other New Zealand environments, Romney ewes at Woodlands Research Station, Southland, were offered a range of pasture allowances in mid pregnancy (June - July) in each of two years. There were two major objectives:

(1) to establish the liveweight change/pasture allowance relationship in mid pregnancy for a typical Southland situation;

(2) to estimate the effect of level of winter feeding on wool production and wool characteristics.

MATERIALS AND METHODS

Design and conduct Year 1 Two-year-old Romney ewes were joined with entire rams from 4 April until 29 April 1978. Mobs of 70 ewes (initial mean liveweight 47 (SE 0.25) kg) were offered pasture allowances (measured to ground level) of 0.7, 1.2, 1.7, or 2.2 kg DM/ewe per day from 13 June until 25 July.

Year 2 Romney ewes were joined with entire rams from 1 April until 20 April 1979. The same allowances as Year 1 were offered from 23 May until 5 July to mobs of 50 ewes in which 25 ewes were 2 years old with an initial mean liveweight of 48 (SE 0.37) kg and 25 were 6 years old, averaging 57 (SE 0.49) kg in liveweight.

After the period of differential feeding in each experiment the ewes were run as one mob until weaning in early December, except for dry ewes which were run separately after lambing. The number of lambs born to each ewe was recorded. The mean (and median) lambing date was 13 September in Year 1 and 9 September in Year 2. The ewes were shorn one week after the lambs were weaned.

Pasture measurements Pasture mass to ground level before grazing was estimated by a double sampling method using a weighted disc meter and quadrat cutting as described by Kelly et al. (1983). The mean pasture mass before grazing was 1920 and 2225 kg total DM/ha in Years 1 and 2 respectively. The average composition of the pasture before grazing in Year 2 (the only year for which results were available) was 73% grass, 1 % legume, and 26% dead matter. In vitro digestibility (OMD) was 730/0 in both years. Pasture allowance treatments were effected by fencing off areas of the appropriate size, depending on pasture mass before grazing and grazing duration. Each mob was moved to a new area (a pasture shift) every 1 or 2 days.

In Year 2 residual pasture mass was estimated twice a week for the 0.7, 1.2, and 2.2 kg DM/ ewe per day pasture allowance treatments by cutting to ground level four randomly selected 0.25 m2

quadrats per pasture allowance treatment. Utilisation (%) and apparent intakes were calculated from the estimates of pasture mass and residual pasture mass.

Animal measurements

Liveweights

Liveweight was recorded after a 24 h fast at the beginning and end of the differential feeding period, and at weaning in December. In Year 2, the ewes were also weighed straight off pasture 2 weeks before the start of lambing, and lamb birth weights were recorded.

Wool Growth and Characteristics A midside patch was clipped at the start and end of the differential feeding period, 2 weeks before the start of lambing, and at weaning. At shearing

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Hawker & Thompson - Effect of winter pasture allowances on wool 297

in December, greasy fleece weight was recorded and a midside sample collected.

Each fleece sample was scoured in a 4-bowl experimental scouring unit, exposed on racks at 20±2°C and 65 ±2OJo relative humidity for 48 h, oven-dried yield determined and oven-dried clean fleece weight estimated. Clean patch weights were determined by the same method. Clean dry wool growth (g/ day) in each period of measurement was estimated by partitioning clean fleece weight according to the weight of clean wool harvested from the patch on each clipping date, and dividing by the number of days in the period.

For the fleece samples, measurements made were: staple length; mean fibre diameter by sonic fineness tester (Andrews et al. 1987); and staple strength by an Instron tensile strength tester (2 staples/sheep). For the patch samples, 'staple' length was estimated and mean fibre diameter measured by liquid scintillation spectrometry (Andrews et al. 1987). Fibre volume was calculated from the length and diameter results.

The soundness of whole staples randomly selected from the mid side region of each fleece was subjectively assessed manually using a three-grade system:

sound - staple does not break; slight tender - staple breaks under considerable

tension; tender - staple breaks easily. Fifteen randomly selected ewes per pasture

allowance per year were dyebanded (Wheeler et al. 1977) at the beginning and end of the period of differential feeding and (Year 2 only) in mid August. At shearing, the dyebanded staples were broken by hand and the position of the break matched with the time of application of the dyebands. In Year 2, staples were plucked from the dyebanded ewes at 3-week intervals through the winter and the root ends of the fibres examined by projection microscope for the presence or absence of brush ends and fibre breakage (Doney & Smith 1969).

Statistical analyses

Pasture data Relationships between pasture allowance and residual pasture mass, utilisation, and apparent intake were examined by analysis of variance and regression, using pasture shifts as replicates.

Animal data In Year 1, complete Iiveweight records were available from 247 ewes, of which 55, 150, and 42 had zero, one, and two Iambs in utero, respectively,

as determined from subsequent lambing records. Detailed wool measurements were made on the samples from a maximum of 10 ewes randomly chosen from each pregnancy category within each pasture allowance treatment; i.e., totals of 40,39, and 37 ewes with zero, one, and two Iambs in utero, respectively (n = 116).

In Year 2 liveweights and wool measurements were recorded on 100 2-year-old ewes, of which 2, 72, and 26 had zero, one, and two Iambs in utero, respectively (estimated from lambing records), and on 96 6-year-old ewes, of which 4, 39, and 53 had zero, one and two Iambs in utero, respectively (n = 196).

Because the data were non-orthogonal, least squares regression analyses using GENST A T were performed, using individual animals as experimental units. For each production characteristic, factors initially included in the model were the reciprocal of pasture allowance, year, pregnancy category and age of ewe, and first order interactions. Backwards elimination procedures were followed to give a minimal model. All results quoted come from the final model fitted, with each main effect adjusted for all other significant main effects, including the effects of year.

Relationships between production characteristics and pasture allowance are curvilinear, so these were fitted empirically by the linear regression of the production characteristic on the reciprocal of allowance,

production = IX - J3/allowance, where IX represents potential production on an unrestricted pasture allowance (kg DM/ewe per day), and J3 (the slope parameter) the extent of depression in production on a restricted allowance. Note that this equation applies only to the observed range of allowances and that the relationships may be presented graphically as hyperbolic functions (Fig. 1).

RESULTS

(1) Pasture measurements In Year 2, when residual pasture mass was measured, it and apparent intake increased and utilisation decreased with increasing pasture allowance (Table 1). For each variable the relationship with pasture allowance was linear; curvilinear relationships did not give significantly improved fits.

(2) Animal measurements

Table 2 shows coefficients IX and J3 for each significant relationship between production

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298 New Zealand Journal of Experimental Agriculture, 1987, Vol. 15

Table 1 Effect of pasture allowance in mid pregnancy on residual pasture, 0,70 utilisation, and apparent intake (Year 2).

Residual pasture (kg DM/ha) Utilisation (0,70) Apparent intake (kg DM/ewe per day)

characteristics and pasture allowance, and summarises effects of pregnancy category and age on production. Neither pregnancy category nor age of ewe significantly affected the slope parameter for any production characteristic/ allowance relationship, and interactions are therefore not mentioned in the subsequent results.

Liveweight and Iiveweight gain

Effects of pasture allowance and pregnancy category From the liveweight gain/pasture allowance equation in Table 2 it is predicted that zero liveweight change for ewes with one lamb in utero would be achieved on a pasture allowance of 1.0 (SE 0.04) kg DM/ewe per day. It is further predicted from Table 1 that zero liveweight change in Year 2 would have been associated with a residual pasture mass of 760 (SE 39) kg DM/ha, a utilisation of 66 (SE 1.5) 070, and an apparent intake of 0.59 (SE 0.03) kg DM/ewe per day.

There was a positive relationship between the number of lambs in utero and potentialliveweight gain, with ewes subsequently producing twins having a mean liveweight gain of 53 (SE 6) g/day more than that of dry ewes. It can be predicted that zero liveweight change would have occurred on a pasture allowance of 1.3, 1.0, and 0.8 kg DM/ewe per day for ewes carrying zero, one, and two lambs, respectively.

After adjustment for pregnancy category and age there was considerable variation in liveweight change between ewes within allowance treatments (pooled standard deviation 36 g/day). This variation was homogeneous with respect to pasture allowance, ewe age, and number of lambs in utero, indicating that no category was differentially affected by feeding level.

Pasture allowance in June - July had a carry over effect on ewe liveweight in August (Table 2). Mean birth weights of single and twin lambs were, respectively, 5.3 (SE 0.11) and 4.0 (SE 0.12) kg for 2-year-old ewes and 5.7 (SE 0.15) and 4.7 (SE 0.07) kg for 6-year-old ewes. There was, however, no

Pasture allowance (kg DM/ewe per day) 0.7 1.2 2.2 SED

636 72 0.50

~ "0

C <1l Q)

U

~ "0 :§ .r: ~ o 0,

~

5.0

4.0

3.0

2.0

1.0

944 58 0.70

1356 40

0.87

75 3

0.06

• June 0 July

fi / __ J"'Y - A"9""

June· July Wool growth = 50(020) - 1.38(O.lS)Jaliowance

July - August: Wool growth = 3.6(0.13) - 1 24(O.13)/allowance

r OO_

O -1

0.7 1.2 1.7 2.2

Pasture allowance (kg DM/ewe/day)

Fig. 1 The effect of pasture allowance in June - July on the wool growth in June- July and July- August of ewes with one lamb in utero. Hyperbolic relationships, pasture allowance treatment means, and equations (standard errors bracketed) are shown.

evidence of any effect of pasture allowance on lamb birth weight. At weaning on 5 December, the effect of midwinter pasture allowance on ewe liveweight was no longer significant, the difference between the lowest and highest allowance being 1.3 (SE 0.8) kg. Lamb weaning weights were not recorded.

Effects of ewe age The 6-year-old ewes present in Year 2 were 9 kg heavier than the 2-year-old ewes at the start of differential feeding and this difference remained until August, but had decreased to 6 kg at weaning (P<O.OOI).

Wool growth and wool characteristics (Table 2)

Effects of pasture allowance (Fig. 1)

As pasture allowance increased from 0.7 to 2.2 kg DM/ewe per day, wool growth in June - July increased by 1.3 g/day (35%). There were

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Hawker & Thompson - Effect of winter pasture allowances on wool

Table 2 Regression coefficients and significance levels for significant linear relationships between production characteristics and the reciprocal of pasture allowance l for 2-year-old ewes bearing and rearing one lamb, and deviations in potential production for ewes with zero or two lambs, and for 6-year-old ewes (with SEDs in parentheses).

Deviations

Coefficients Pregnancy category Ewe age Production characteristic ex (3 0 2 6 year-old

Liveweight gain (g/day) 136 132*** -32*** +21*** + 17* (4.4) (5.0) (5) (4) (7)

Liveweight in August (kg) 55.7 3.4** -2.9 +6.4** + 8.9*** (1.3) (1.5) (2.3) (1.9) (0.8)

Wool growth (g/day) June-July 5.0 1.38*** +0.8** 0.0 -0.1

(0.20) (0.15) (0.20) (0.20) (0.13) July - August 3.6 1.24*** +0.7** -0.4* -0.3

(0.13) (0.13) (0.13) (0.16) (0.16) Wool production Jun - Dec 1.13 0.12** NA -0.09** -0.20*** (kg) (0.03) (0.03) (0.03) (0.03) Fibre diameter (JLm)

June-July 32.3 I. 7*** +0.4 -0.2 + 1.6** (0.5) (0.4) (0.6) (0.6) (0.5)

July - August 31.5 3.1*** +0.7 0.0 + 1.6* (0.5) (0.6) (0.7) (0.7) (0.7)

Staple length growth (mm/day) June-July 0.38- 0.07*** +0.01 0.00 -0.01

(0.01) (0.01) (0.01) (0.01) (0.01) July- August 0.35 0.04*** +0.04* -0.01 -0.01

(0.01) (0.01) (0.02) (0.01) (0.01) Staple strength (N/ktex) 26.9 10.0*** +0.08 -0.26 +0.26

(1.0) (1.2) (0.16) (0.17) (0.16)

1 Model used: measurement = ex - (3 allowance, where ex potential production, (3 slope parameter, allowance = kg DM/ewe per day. NA = not applicable.

299

concomitant increases of 0.07 mm/day (220/0) in staple length growth rate and 1.6 /tm (5%) in fibre diameter. The increases in length growth rate and diameter were estimated to increase fibre volume by 32%, an increase consistent with that in wool growth.

pasture allowance on the fibre diameter or staple length of the annual fleece, indicating that variability in fibre growth in summer - autumn and spring masked the nutritional effects on fibre growth in the winter.

Differential feeding in June - July had a significant carryover effect on wool growth in July- August, with a difference of 1.2 g/day (49%) between the allowances of 0.7 and 2.2 kg DM/ ewe per day. The equivalent differences in staple length growth rate and fibre diameter were 0.04 mm/day (12%) and 3.0/tm (100/0), respectively, giving a difference of 33% in estimated fibre volume.

Winter pasture allowance did not significantly affect clean wool growth, staple length growth, or fibre diameter during August - December (parturition -lactation). The cumulative effect of pasture allowance in June - July was a difference between the extreme allowances of 0.12 (SE 0.04) kg in clean fleece production between June and December. There were no significant effects of

Staple strength increased significantly with winter pasture allowance (Table 2). The proportions of ewes with tender fleeces were 0.80,0.65,0.46, and 0.28 on pasture allowances of 0.7, 1.2, 1.7, and 2.2 kg DM/ewe per day, respectively.

When the dyebanded staples were broken, the position of break in Year 1 was, with few exceptions, below the dyeband applied in July (end of differential feeding). In Year 2 it was between the dyebands applied in July and August (pre­lambing). This indicates that July- August included the period of minimum fibre growth for most ewes, irrespective of pasture allowance. Projection microscope observations made on the plucked fibre roots revealed very few brush ends or broken fibres, indicating that fibre growth was continuous through the winter.

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Effects of pregnancy category (Table 2) Non pregnant ewes grew, on average, 18070 more wool than pregnant ewes during the period of differential feeding (June - July). In July - August, wool growth rates were 26% higher for the non­pregnant ewes than for the ewes with one lamb in utero, with the ewes carrying twins a further 15% lower. Ewes bearing and rearing single lambs grew 0.6 (SE 0.33) g more wool per day (9%) in August - December than ewes bearing and rearing twins.

During July - August (late pregnancy) staple length growth rate was 0.04 (SE 0.015) mm/day (13%) higher for the non-pregnant than pregnant ewes. There were no other significant effects of pregnancy category on wool characteristics.

Effects of ewe age The 2-year-old ewes had a 20% higher wool growth rate than the 6-year-old ewes in spring (7.3 v. 5.7 g/day; SED 0.2), but differences during the winter were small and non-significant. The 2-year-old ewes had fibre diameters c. 1.5 /Lm less than the 6-year­old ewes.

DISCUSSION

Liveweight gain The relationship between liveweight change and pasture allowance is similar to results reported by Jagusch et al. (1981) for ewes in mid pregnancy grazing swards of pasture mass of ;;<: 2000 kg DM/ha. It is also broadly similar to relationships established for breeding ewes in summer and autumn (Hawker et al. 1984), seasons when there is no major confounding with physiological state. The allowance of 1.0 kg DM/ewe per day predicted to give zero liveweight change in the present experiment is within the range of estimates (0.8 - 1.2 kg DM/ewe per day) calculated from the results of Jagusch et al. (1981). However, relative to their results, zero liveweight change was associated with a higher residual pasture mass (760 v. 350 -700 kg DM/ha), and hence a lower utilisation and apparent intake. When experimental results are used as the basis for planning winter grazing management the variation observed between experiments should be recognised or liveweight changes could differ markedly from those expected.

When planning winter feeding the objective on many New Zealand sheep farms is to hold average ewe liveweight constant. The appropriate feeding level is frequently referred to as 'maintenance' but this overlooks the growth of the conceptus. The liveweight gain of ewes with twins in utero of 53

g/day more than that of dry ewes is consistent with reported increases in conceptus weight of some 2.5 kg between days 50 and 90 of pregnancy (Robinson et al. 1977). As a result of conceptus growth, the predicted allowance associated with zero liveweight change decreased markedly from 1.3 kg DM/ewe per day for non pregnant ewes to 0.8 kg DM/ewe per day for ewes carrying two lambs. However, if offered an allowance of 0.8 kg DM/ewe per day ewes carrying two lambs would lose considerable maternal body weight. Liveweight change is thus an inappropriate criterion for rationing pasture for ewes in mid pregnancy, particularly if the potential lambing percentage is unknown. The present results suggest that a pasture allowance of 1.3 kg DM/ewe per day would be close to that required for the maintenance of body weight (conceptus-free) by ewes in mid pregnancy carrying one or two lambs.

However, the present results indicate that maintenance of maternal body weight in mid pregnancy is not essential. Although differential feeding affected ewe liveweight in mid pregnancy by up to 5.4 kg, and in late pregnancy by up to 3.4 kg, there was no effect on the birth weight of single or twin lambs. This is not surprising, because more extreme differential feeding regimes in other studies (Monteath 1971; Jagusch et al. 1981) have not affected birth weight, indicating the considerable buffering ability of the ewe in mid pregnancy. In the study of Smeaton et al. (1985) undernutrition during mid pregnancy had to reduce conceptus-free body weights below 42 kg before effects on production, other than wool growth, were observed. It is suggested that ewes in good body condition can tolerate liveweight loss for up to 6 weeks in mid pregnancy (days 40-100).

Wool growth and characteristics The effects of differential feeding in winter on liveweight and fleece weight are closely associated. In the present experiment the difference between the lowest and highest winter pasture allowance in clean wool production between June and December was small (0.12 kg) and there was no difference in fleece fibre diameter or staple length. Larger effects on greasy fleece weight (0.3-0.7 kg) have been recorded when the differential feeding period has been longer (8 -10 weeks) and the effect on ewe liveweight greater, i.e., 6-12 kg (Monteath 1971; Jagusch et al. 1981).

Pasture allowance in June - July significantly affected wool growth and its components, fibre diameter and length growth, in both June - July and July - August but not in August - December. In each winter period the wool growth response to pasture allowance and potential wool growth were both much less than in experiments at Woodlands

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in other seasons (Hawker et al. 1984). This is consistent with the interaction of season with the effect of nutrition on the wool growth of long­woo lied breeds demonstrated in several studies (Sumner 1979; Hawker et al. 1984; Hawker & Crosbie 1985). The relative response of wool growth to feeding level is similar in all seasons but the absolute response is directly related to the wool growth rate of ewes held at zero liveweight change, i.e., the winter response is only 30 - 50070 of that in summer.

The relative effect of pasture allowance on wool growth (and on fibre diameter) was considerably greater in July - August than during the differential feeding period itself (June - July). This large carryover effect has two components, viz., a lag period of several weeks before wool growth rate adjusts fully to a change in intake (Black 1984), and a measurement lag (the 'emergence time') before changes in the rate of mitosis and/or protein synthesis in the follicle bulb can be detected above the skin (Downes & Sharry 1971). To quantify the relative contribution of these two components, reliable estimates of the emergence time for long­woolled breeds in winter would be required.

The relative effects of pasture allowance on wool growth in July - August and on staple strength were very similar. This between-treatment relationship is not surprising because, when ewes are shorn annually in summer, staple strength closely reflects the minimum dimensions attained by the fibre (Fitzgerald et al. 1984; Hawker 1986). The difference of 50% in strength between the lowest and highest pasture allowance would be expected to result in a significant difference in fibre length post-carding and therefore in processing performance (Ross 1982). The level of nutrition in winter would not affect staple strength if ewes were shorn before lambing.

The observation on dyebanded staples and fibre roots showed that fibre growth was continuous through the winter, reaching a minimum in July - August, which is about 4 - 6 weeks before lambing in Otago and Southland. As is also clear from the results of Horton (1978), the so-called 'lambing break' is a misnomer - it is the expression of an earlier event. Feeding level did not affect the position of break, no doubt because it did not alter the timing of minimum fibre diameter and wool growth.

Although of little practical importance, the effects of pregnancy category on wool production are of interest. Relative to ewes producing one lamb, non-pregnant ewes grew 22% and 26% more wool in June - July and July - August, respectively, whereas the ewes producing twins grew 0 and 15% less in the same periods. These effects are similar

to average figures from several experiments reported by Hawker (l984) and to average figures in the literature reviewed by Corbett (l979). Estimation of the effects of pregnancy category from the wool production and wool characteristics of ewes naturally having 0, 1 or 2 lambs in utero can be criticised because it ignores possible inherent differences between the ewes in the three categories. However, Reid (1978) reported similar patterns of wool growth for ewes that were naturally non­pregnant or artificially aborted on days 23 - 24 of pregnancy. No equivalent data have been reported for ewes carrying single or twin lambs.

The differences in wool growth between the pregnancy categories were consistent across allowances, as found by Oddy (1985). Oddy calculated the reduction in the wool growth of single and twin-bearing ewes from the wool growth/feed intake regression for dry ewes and found that this did not change significantly over a wide range of feed intake and wool growth values. The influence of pregnancy on wool growth cannot, therefore, be attributed solely to competition for available nutrients between body functions. Hormonal changes during pregnancy which occur independently of feeding level are presumably implicated in the reduced wool growth.

CONCLUSION

This experiment has demonstrated the curvilinear production response of pregnant Romney ewes to differential feeding in a rotational grazing situation. The levels of nutrition were quantified in terms of pasture allowance, a concept that sheep farmers can easily utilise for practical feed budgeting. The pasture allowance/liveweight gain relationship was similar to relationships observed at Ruakura in winter and Woodlands in summer and autumn, suggesting wide applicability in New Zealand.

On average, the mob attained zero liveweight change on a pasture allowance of 1.0 kg DM/ewe per day. However, to maintain maternal body weight, the mob would require an allowance of approximately 1.3 kg DM/ewe per day. The liveweight changes induced by 6 weeks of differential feeding did not affect lamb birth weights and had little effect on ewe liveweight at weaning.

Although pasture allowance in June - July significantly affected fibre diameter and staple length growth through the winter, and hence wool growth, the difference in clean fleece weight between allowances of 0.7 and 2.2 kg DM/ ewe per day was only 0.12 kg. The small absolute response of wool growth to feeding level in winter suggests that feeding Romney ewes to gain liveweight in winter would not be financially viable.

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302 New Zealand Journal of Experimental Agriculture, 1987, Vol. 15

ACKNOWLEDGMENTS

We wish to thank W. M. Jarman, S. F. Rutherford, and H. W. Davey for field work; R. N. Andrews, D. L. Grant, J. A. Edge, and D. C. Taylor for wool metrology; S. F. Crosbie and R. P. Littlejohn for advice and assistance with statistical analyses; and J. C. McEwan for help with the preparation of the manuscript.

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