Published December 4, 2014
Adaptability of pregnant Merino ewes to the cold desert
climate in Nevada1
W. M. Rauw,*2 D. S. Thain,* M. B. Teglas,* T. Wuliji,* M. A. Sandstrom,†
and L. Gomez-Raya*
*Department of Animal Biotechnology, and †Main Station Field Laboratory, University of Nevada, Reno 89557
ABSTRACT: Grazing ability is difficult to record in
animals under free-ranging conditions without sophisticated methods. Alternatively, grazing ability may be
indirectly inferred from changes in BW and production
characteristics during the grazing period. The present
study investigated the effect of grazing on resourcelimited rangelands on BW, wool characteristics, and
offspring weaning weights in nine hundred five 5/8, 7/8,
and fullblood Merino ewes of 2 to 7 yr of age during a
grazing period of approximately 2.5 mo (between January and March). A total of 469 ewes gave birth to a
single lamb, 248 to twin lambs, and 188 did not give
birth. Body weights were measured and wool samples
taken before and after the ewes were allowed to graze
freely on the rangelands; absolute change in BW and
change in BW as a percentage of initial BW were estimated. On average, grazing on resource-poor rangelands resulted in BW loss, a reduction in fiber diameter
and its CV, and increased staple length. Animals with
finer wool at the start of the grazing period lost phenotypically (r = −0.07, P < 0.05) and genetically (r =
−0.23, P < 0.05) less BW during the grazing period
and had a greater probability to carry 1 lamb (or 2) to
term (P < 0.05). Animals that lost less BW produced
more greasy fleece (r = 0.09, P < 0.01). Body weight
change did not significantly influence offspring weaning
weights. Change in BW was moderately heritable at h2
= 0.29; fiber diameter was strongly heritable at h2 =
0.51. Animals with the least inclusion of Merino genetics lost more BW (P < 0.01) during the grazing period
and had a more uniform fiber diameter (P < 0.05) but
shorter staples (P < 0.05) and less fleece (P < 0.0001)
than animals with a greater level of Merino genetics.
Our results indicate that animals with finer wool appeared to be better adapted to the cold Nevada desert.
Thus, selection for finer wool may positively influence
adaptability to resource-limited cold climate conditions;
alternatively, BW change may be selected for directly.
Because nutritional deficiencies during pregnancy can
have adverse consequences for the offspring, indirect
selection for grazing ability would foremost result in
healthier ewes that can produce lambs and wool without compromising their welfare.
Key words: adaptability, body weight gain, fiber diameter, selection, sheep
©2010 American Society of Animal Science. All rights reserved.
J. Anim. Sci. 2010. 88:860–870
doi:10.2527/jas.2009-2221
INTRODUCTION
In extensive production systems, animals (seasonally)
graze on rangelands. Livestock grazing reduces production costs because animals do not need to be fed. However, grazing animals are subject to recurrent periods
of undernutrition during droughts and the winter in
which large amounts of body tissue may be catabolized
(Farrell et al., 1972). Sheep have a greater ability to selectively harvest leaves and current annual plants than
do cattle or horses (Hanley and Hanley, 1982). Also,
within species, there is variation in the ability to graze
selectively (Walker et al., 1992; Kronberg and Malechek,
1997). Therefore, selection for within-species variation
in grazing ability may offer the opportunity to breed for
better adaptation to poor-quality rangelands, resulting
in healthier animals and improved production.
1
This research was financed by the Nevada Agricultural Experiment Station, project number 5328: “The Genetic Architecture of
Grazing Efficiency of Sheep on Nevada Rangelands” and project
number 5325: “Molecular and Phenotypic Genetic Parameters of
Performance in the Rafter 7 Merino Sheep Flock.” We are grateful
to R. C. Avansino Jr. of the Edwin L. Wiegand Trust (Reno, NV),
H. A. Glimp (University of Nevada, Department of Animal Biotechnology, Reno), the Edwin L. Wiegand Trust, and to all technicians
(Rafter 7 Ranch, Yerington, NV) for their help and cooperation. The
reviewers are greatly thanked for their constructive comments, in
particular for suggesting including previous number of lambs in the
analysis.
2
Corresponding author: [email protected]
Received June 16, 2009.
Accepted November 11, 2009.
860
Adaptability of Merino ewes to the cold desert climate
Grazing ability, however, is difficult to record in individual animals under free-ranging conditions because
feed intake cannot be accurately recorded without timeconsuming methods. Bailey et al. (1996) investigated
grazing behavior as a way of understanding grazing
patterns of large herbivores. Lee et al. (1995) and Fogarty et al. (2006) investigated pasture intake of grazing
Merino ewes by means of fecal markers. However, these
methods are unlikely to be used as a selection criterion
in practical breeding. Alternatively, grazing ability may
be indirectly inferred from changes in BW and production characteristics during the grazing period (Rauw,
2008).
The objectives of this study are to investigate the effect of grazing on resource-limited rangelands on BW,
wool characteristics, and offspring weaning weights in
pregnant ewes. Heritabilities are estimated and selection opportunities investigated.
MATERIALS AND METHODS
Protocols for handling of animals and recording of
traits were approved by the Institutional Animal Care
and Use Committee of the University of Nevada–Reno.
Animals and Their Management
A Merino flock was initiated in Nevada near the Californian border in 1990 with the purchase of 500 purebred Rambouillet ewes from 2 prominent breeders in
the United States. These ewes were mated via AI with
imported semen from Australian Merino rams. The
initial breeding objective was to develop a purebred
Merino flock with Australian genetics that would be
adapted to the rangeland environment of the western
United States. Because females or embryos could not
be imported, a grade-up program (1/2, 3/4, 7/8, 15/16,
and greater Merino breeding, with 15/16 Merino being
considered purebred by the world animal breeding community; Canadian Sheep Breeders Organization, 2009)
was implemented utilizing semen and imported rams.
However, early in the breeding program it was observed
that crossbred Merino × Rambouillet offspring produced approximately 70% more clean wool than the
purebred Rambouillet (unpublished data), and it was
decided to develop a selection line from the best 1/2
Merino ewes and a limited number of 3/4 Merino ewes
and rams. This line is therefore approximately 5/8 Merino and 3/8 Rambouillet and has been a closed line for
almost 10 yr.
The ranch includes approximately 3,400 acres of private property plus grazing permits on approximately
85,000 acres of Bureau of Land Management land, and
4,500 acres of US Forest Service land owned by the government (Rauw et al., 2007)]. Approximately 250 acres
of the private land are in irrigated pasture for the sheep
flock and hay production. Under normal management,
861
ewes are mated via AI or in single sire pastures from
mid-November to late December for approximately 40
d, with hay fed only when snow cover prevents grazing
or available forage is inadequate. The ewes are treated
for internal parasites (lice treatment, Permethrin topical
Boss Pour-On, Intervet Schering-Plough Animal Health,
Omaha, NE; gastrointestinal helminth treatment, Ivermectin oral drench, Ivomec Drench for Sheep, Merial
Ltd., Duluth, GA) at the onset and end of the mating
season and are then managed under herder supervision
on the desert rangelands until mid-March. Animals are
shorn in March. After shearing, the ewes are treated for
internal and external parasites and vaccinated with an
8-way Clostridium spp. vaccine (Covexin 8 for Clostridium
chauvoei-septicum-haemolyticum-novyi-tetaniperfringens types C and D Bacterin-Toxoid, Intervet
Schering-Plough Animal Health) to provide passive immunity to their lambs through colostrum. The lambing
program is based on a modified set-stocking program
with up to 250 prelambing ewes within a line group in a
pasture that is adjacent to 3 vacant pastures. The ewes
are not fed any supplemental feed other than an appropriate trace mineral salt pre- or postlambing. The ewes
and their lamb(s) are then moved to 1 of the adjacent
vacant pastures until a pasture is determined to contain enough ewes and lambs and then another pasture
is used. When the smaller pasture groups of lambs are
approximately 2 wk old, the 2 pasture groups are mixed
together. This process is continued until a maximum of
3 or 4 groups are managed from 4 to 5 wk of age in an
intensive pasture rotation system of 4 to 6 pastures per
group until weaning. At 4 to 8 wk of age, the lambs and
ewes are treated for internal parasites, and the lambs
are vaccinated with an 8-way Clostridium spp. vaccine.
Lambs are weaned and weaning weights are recorded in
mid-August when the lambs are between 3 and 4 mo of
age (Rauw et al., 2007).
Data were obtained for four hundred fifty 5/8 Merino
× 3/8 Rambouillet ewes, one hundred sixty 7/8 Merino × 1/8 Rambouillet ewes, and 295 fullblood Merino
ewes (a total of 905 ewes) from 50 sires. About 150 animals originated from AI, and all others originated from
flock matings. One hundred fifty-eight animals originated from flock matings with several rams; therefore,
for this group of ewes the sire was considered unknown.
In addition, the sires of 11 ewes were unknown. Sires
represented between 1 and 42 (average of 14.7) progeny
per sire. Ewes were 2 to 7 yr of age. Pregnant ewes
were 76 to 119 d in gestation at the end of the grazing
period.
The current number of offspring (in 2005) is indicated as NrLambcurrent, whereas the number of offspring in the previous year (2004) is indicated as
NrLambprevious. A total of 469 ewes gave birth (NrLambcurrent) to a single lamb and 248 ewes to a twin;
188 ewes did not give birth. Offspring of the current
lambing were weaned on August 20, 2005, at 83 to 127
d of age. Of all ewes with 0 NrLambcurrent, 82 ewes had
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Rauw et al.
0, 48 ewes had 1, and 56 ewes had 2 NrLambprevious; of
all ewes with 1 NrLambcurrent, 172 ewes had 0, 191 ewes
had 1, and 103 ewes had 2 NrLambprevious; of all ewes
with 2 NrLambcurrent, 55 ewes had 0, 81 ewes had 1, and
108 ewes had 2 NrLambprevious. Nine animals had no
data on NrLambprevious. Offspring of the previous lambing were weaned on August 18, 2004, at 80 to 181 d of
age.
BW, Wool Traits, and Offspring
Weaning Weight
Animals were weighed in 2005 right before grazing
the rangelands on January 2 (BWstart) and right after
returning from the rangelands on March 18 (BWend);
that is, after a period of 75 d. Change in BW (ΔBW)
was measured as the difference between BWend and
BWstart. Change in BW as a percentage of initial BW
(ΔBW%) was estimated as (BWend/ BWstart) × 100.
Wool samples were collected for each individual on
the same dates (start and end). Samples were analyzed
with the OFDA2000 instrument (Baxter, 2001) for
mean fiber diameter (FD), CV of diameter (CVFD),
and staple length (SL).
Ewes were shorn on March 22 and 23, 2005. Wool
was weighed individually (greasy fleece weight; GFW);
wool on head, legs, belly, and tail was not included.
Statistical Analysis
The mixed model procedure (SAS Inst. Inc., Cary,
NC) was used to investigate the effect of days in gestation at BWend (from a total of 147 d) on ΔBW and
ΔBW% in pregnant ewes only:
ΔBW(%)ijklmno = μ + Linei + NrLambcurrent j
+ NrLambprevious k + Agel + Sire(Line)mi
+ GESTn + eijklmno,
[1]
where μ = population intercept, Linei = effect of line
i (5/8, 7/8, and fullblood Merino; fixed variable), NrLambcurrent j = effect of number of offspring in the current pregnancy j (1, 2; fixed variable), NrLambprevious k
= effect of number of offspring k in the previous lactation period (0, 1, 2), Agel = the effect of age l (2 to 7
yr; fixed variable), Sire(Line)mi = effect of sire m nested
within line i (1 to 50; random variable), and GESTn =
the effect of days in gestation n at BWend (covariate),
eijklmno = error term of animal o, eijklmno~NID(0, σ2e),
and ΔBW(%)ijklmno = ΔBW and ΔBW% for the grazing period of 75 d, measured on animal o, of line i, carrying number of offspring j, with number of offspring
k in the previous lactation period, at age l, with sire
m, at days in gestation n. With this model, ΔBW and
ΔBW% in pregnant ewes were adjusted to 0 d in gestation according to
ΔBWadjusted = ΔBW – (0.05979 × GEST), and [2]
ΔBW%adjusted = ΔBW% – (0.1104 × GEST). [3]
The SAS mixed model procedure was then used to
investigate the data on ΔBW and ΔBW% (adjusted)
including all individuals:
ΔBW(%)ijklmn = μ + Linei + NrLambcurrent j
+ NrLambprevious k + Agel + Sire(Line)mi + eijklmn, [4]
where all variables are as in model [1], with the exception that NrLambcurrent now had a range of 0, 1, and 2.
Preliminary analysis indicated that wool traits and
fleece weights were not significantly influenced by days
in gestation or NrLambcurrent. Therefore, wool samples
were investigated with model 4, but excluding the effect of NrLambcurrent j. Fleece weights were adjusted, in
addition, for size:
GFWijklmn = μ + Linei + BWstart j + NrLambprevious k
+ Agel + Sire(Line)mi + eijklmn,
[5]
where BWstart j = BW j at the start of the grazing period (covariate), and all other variables are as in model
[1].
Phenotypic correlations were estimated after adjusting the traits for line, age, NrLambcurrent, NrLambprevious,
and days in gestation (ΔBW and ΔBW%), line, NrLambprevious and age (wool traits), and line, NrLambprevious,
age, and initial BW (GFW), with the GLM procedure
of SAS. This analysis included ewes with unknown sire.
The SAS logistic procedure was used to investigate the
effect of FDstart and SLstart on the incidence of delivering
a lamb (single or twin) to term or not.
To investigate whether change in BW of the mother
influenced offspring weaning weights, weaning weights
were analyzed with the following model (according to
Rauw et al., 2007):
Yijklmnop = μ + Linei + Sexj + BRTypek + AgeDaml
+ WnAgem + Damn + ΔBWo + eijklmnop, [6]
where μ = overall mean, Linei = effect of line i (5/8,
7/8, and fullblood Merino), Sexj = effect of sex j (ewe,
ram), BRTypek = effect of birth-rearing type k (singlesingle, twin-twin, twin-single, single-twin), AgeDaml =
effect of the age of dam l (2 to 7 yr), WnAgem = effect of age at weaning m, Damn = the effect of dam n,
ΔBWo = effect of change in BW of the dam during the
grazing period o, and eijklmnop = error term of animal
p, eijklmnop~NID(0, σ2e). Weaning weight measured on
animal p of line i, sex j, birth-rearing type k, age of
the dam l, age at weaning m, with dam n that had a
change in BW o is denoted by Yijklmnop. All effects were
considered fixed class effects, with the exception of the
effects of age at weaning and change in BW, which were
included as covariate effects, and the effect of dam,
which was considered random (Rauw et al., 2007). The
Adaptability of Merino ewes to the cold desert climate
863
Figure 1. Distribution of change in BW.
distribution of ΔBW was tested for normality with the
CAPABILITY procedure of SAS.
Heritabilities
A multitrait animal model was used to estimate variance components of ΔBW, FDstart, CVFDstart, SLstart,
and GFW. The model used for ΔBW was
ΔBWijklm = Linei + NrLambcurrent j + NrLambprevious k
+ Agel + aijklm + eijklm, [7]
where aijklm = effect of animal l; eijklm = error term of
animal l; ΔBWijklm = ΔBW of animal m of line i, carrying number of offspring j, with number of offspring k
in the previous lactation period, at age l; and Linei, NrLambcurrent j, NrLambprevious k, and Agel are as defined in
model [1]. The variables FDstart, CVFDstart, and SLstart
were analyzed with the same model, but excluding the
effect of NrLambcurrent. The model used for GFW was
GFWijklm = Linei + BWstart j + NrLambprevious k
+ Agel + aijklm + eijklm, [8]
where BWstart j = initial BW j at the start of the grazing
period and other traits are as in model [7]. The software VCE4 was used to estimate genetic parameters
and their approximate SE with REML (Groeneveld
and Garcia-Cortes, 1998).
RESULTS
ΔBW and ΔBW%
Body weight was on average 63.1 kg (±0.264 kg) at
the first measurement (BWstart) and 56.7 kg (±0.220
kg) at the second measurement (BWend). Of all animals, 93.6% lost BW during the grazing period, 5.4%
gained BW, and 1.0% neither lost nor gained BW. The
distribution of ΔBW is given in Figure 1. The ShapiroWilk test (P < 0.05), but not the Kolmogorov-Smirnov,
Cramer-von Mises, and Anderson-Darling tests, rejected the hypothesis of normality. Animals that were
further into gestation had lost less BW than animals
that were less far into gestation (r = 0.13, P < 0.001,
adjusted for the effect of age, line, NrLambcurrent, and
NrLambprevious). Therefore, ΔBW and ΔBW% were
subsequently adjusted to 0 d in gestation.
Least squares means of ΔBW and ΔBW%, adjusted
for the effects of model [1] are presented in Figure 2
for line, NrLambcurrent, NrLambprevious, and age. The 5/8
Merino ewes lost more BW and a larger percentage of
their BW than 7/8 and fullblood Merinos (P < 0.01).
Nonpregnant ewes in the current pregnancy lost less
BW and a smaller percentage of their BW than pregnant ewes (P < 0.001). Ewes carrying 1 lamb in the current pregnancy tended to lose more BW (P = 0.06) and
lost a larger percentage of their BW (P < 0.05) than
ewes carrying twins. Ewes with 0 NrLambprevious lost
more BW (P < 0.01) and a larger percentage of their
BW (P < 0.05) than ewes with 1 or 2 NrLambprevious.
Of all ewes with 0 NrLambprevious, 82 ewes had 0 NrLambcurrent, whereas 172 ewes had 1, and 55 ewes had
2 NrLambcurrent.
Body weight loss increased with age but was not significantly different between 4, 5, 6, and 7 yr of age;
trends were very similar for ΔBW% (Figure 2). When
the age effect was included in the model as a covariate
(regression) effect, animals were estimated to lose 1.12
kg of BW and 1.40% of their BW with each year of age.
The effect of sire was significant for ΔBW and ΔBW%
(P < 0.05).
Wool Traits
Least squares means of FD, CVFD, and SL, adjusted
for the effects of line, NrLambprevious, age and sire, are
864
Rauw et al.
Figure 2. Least squares means (with SE) of change in BW (DBW) and change in BW as a percentage of initial BW (DBW%) by line (5/8,
7/8, and fullblood Merino), number of offspring in the current (NrLamb current) and in the previous reproductive cycle (NrLamb previous), and
age.
given in Figure 3 (a to c) for both the measurements at
the start and at the end of the grazing period. On average, FD decreased during the grazing period (Figure
3a). The FDstart and FDend did not significantly differ
between lines, but 5/8 Merinos had a smaller decrease
in FD than fullblood Merino ewes (P < 0.05) and tended to have a smaller decrease in FD than 7/8 Merinos
(P = 0.0696). Both FDstart and FDend were larger in
ewes with 0 than for ewes with 1 or 2 NrLambprevious
(P < 0.001), and larger for ewes with 1 than for ewes
with 2 NrLambprevious (P < 0.01). The decrease in FD
was larger for ewes with 0 than for ewes with 2 NrLambprevious (P < 0.01) and tended to be larger than for
ewes with 1 NrLambprevious (P = 0.07). Fiber diameter
increased with age (FD blowout) for FDstart and FDend,
but this was not significant (P > 0.10) between 4, 5,
and 7 yr of age; FD was greater in 6-yr-old ewes than
in 4- and 5-yr-old ewes (P < 0.05). The difference in
FD between the start and the end of the grazing period
was larger for 3-yr-old ewes than for 2-yr-old ewes (P
< 0.05) and tended to be larger for 6-yr-old ewes than
for 2- and 7-yr-old ewes (P = 0.06). The effect of sire
was significant for FDstart (P < 0.01) and FDend (P <
0.001).
On average, the CVFD decreased during the grazing
period (Figure 3b). The CVFDstart was less in 5/8 Merino ewes than in 7/8 and fullblood Merino ewes (P <
0.05). The CVFDend was less in 5/8 Merino ewes than
in 7/8 Merino ewes (P < 0.05) and tended to be less
than in fullblood Merino ewes (P = 0.08). The decrease
in CVFD was similar for all lines. The CVFDstart was
greater in ewes with 2 NrLambprevious than in ewes with
1 NrLambprevious (P < 0.05). The CVFDend was greater
in ewes with 0 NrLambprevious than in ewes with 1 or 2
NrLambprevious (P < 0.05). The difference in CVFD was
greater for ewes with zero than for ewes with 1 or 2
NrLambprevious (P < 0.01) and tended to be greater for
ewes with 1 than for ewes with 2 NrLambprevious (P =
0.07). The CVFD did not show a particular trend over
age groups. The CVFDstart was less in 5-yr-old ewes
than in 2-, 4-, 6-, and 7-yr-old ewes (P < 0.05). The
CVFDend was less in 5-yr-old ewes than in 6- and 7-yrold ewes (P < 0.05) and tended to be less than in 4-yrold ewes (P = 0.09). The decrease in CVFD was larger
in 2-yr-old ewes than in 3-yr-old ewes (P < 0.05) and
tended to be larger than in 4- and 5-yr-old ewes (P =
0.07). The effect of sire was significant for CVFDstart (P
< 0.001) and CVFDend (P < 0.01).
Staple length increased on average during the grazing
period (Figure 3c). The 5/8 Merino ewes had shorter
staples than 7/8 (P < 0.05) and fullblood Merino ewes
(P < 0.01) at the start of the grazing period and had
shorter staples than fullblood Merino ewes (P < 0.05)
and tended to have shorter staples than 5/8 Merino
ewes (P = 0.08) at the end of the grazing period. The
difference between SLstart and SLend was similar for all
lines. The SLstart was shorter for ewes with 2 than for
ewes with 0 or 1 NrLambprevious (P < 0.001) and tended to be less for ewes with 1 than for ewes with 0
NrLambprevious (P = 0.08). The SLend was greater for
ewes with 0 than for ewes with 1 or 2 NrLambprevious (P
< 0.05). The difference between SLstart and SLend was
similar for all NrLambprevious. Staple length decreased
with age. The SLstart and SLend were larger in 2-yr-olds
than in all other age groups (P < 0.0001). The SLstart
was larger in 3-yr-olds than in 5-, 6-, and 7-yr-olds (P
< 0.01), larger in 4-yr-olds than in 5-, 6-, and 7-yr-olds
(P < 0.05), and tended to be larger in 5-yr-olds than in
7-yr-olds (P = 0.08). The SLend was larger in 3-yr-olds
than in 6- and 7-yr-olds (P < 0.01), larger in 4-yr-olds
than in 6- and 7-yr-olds (P < 0.01), and larger in 5-yrolds than in 6- and 7-yr-olds (P < 0.05). During the
grazing period, SL grew more in 5-yr-old ewes than in
3- and 6-yr-old ewes (P < 0.05) and tended to grow
more than in 2-yr-old ewes (P = 0.08). The effect of sire
was significant for SLstart and SLend (P < 0.01).
Least squares means of fleece weight at the end of
the grazing period, adjusted for the effects of line, NrLambprevious, age, BWstart, and sire, are given in Figure
3d. Fleece weight was smaller for 5/8 Merino ewes than
for 7/8 and fullblood Merino ewes (P < 0.0001). Fleece
weight was greater in ewes with 0 than in ewes with 1
Adaptability of Merino ewes to the cold desert climate
865
Figure 3. Least squares means (with SE) of fiber diameter (a), CV of fiber diameter (CV of FD; b), and staple length (c), measured at the
start and at the end of the grazing period, and greasy fleece weight (d) measured at the end of the grazing period, by line (5/8, 7/8, and fullblood
Merino), number of offspring in the previous reproductive cycle (NrLamb previous), and age.
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Rauw et al.
Table 1. Heritabilities (diagonal) (SE) and genetic (upper diagonal) correlations for
change in BW (ΔBW), fiber diameter (FD), CV of fiber diameter (CVFD), and staple
length (SL) at the start of the grazing period, and greasy fleece weight (GFW) at the
end of the grazing period
Item
ΔBW
FDstart
CVFDstart
SLstart
GFW
ΔBW
FDstart
CVFDstart
0.29 (0.05)
−0.23 (0.10)
0.51 (0.05)
−0.21 (0.10)
0.49 (0.09)
0.50 (0.05)
or 2 NrLambprevious (P < 0.01) and tended to be greater
in ewes with 1 than with 2 NrLambprevious (P = 0.09).
Fleece weights decreased with age, with the exception of
4-yr-old ewes. Two-year-old ewes produced more wool
relative to their body size than 5-, 6-, and 7-yr-old ewes
(P < 0.01). Three-year-old ewes produced more wool
than 5-, 6-, and 7-yr-old ewes (P < 0.01) but less than
4-yr-old ewes (P < 0.05). Four-year-old ewes produced
more wool than 5-, 6-, and 7-yr-old ewes (P < 0.0001),
and 7-yr-old ewes produced less wool than 5- and 6-yrold ewes (P < 0.0001). Sire had a significant effect on
fleece weight (P < 0.01).
Phenotypic Correlations, Logistic Procedure,
and Offspring Weaning Weights
Ewes with a larger FD had a larger CVFD (r = 0.14,
P < 0.0001 at the start and r = 0.08, P < 0.05 at the
end) and longer staples (r = 0.18, P < 0.0001 at the
start and at the end). Animals with larger FD at the
start also had larger FD at the end (r = 0.89, P <
0.0001), animals with greater CVFD at the start also
had greater CVFD at the end (r = 0.76, P < 0.0001),
and animals with longer staples at the start had longer
SL at the end (r = 0.61, P < 0.0001).
Animals with a larger FD at the start had a larger
drop in FD between the start and the end (r = −0.34,
P < 0.0001). Animals with a larger CVFD had a larger
drop in CVFD between the start and the end (r =
−0.36, P < 0.0001). Animals with longer staples had a
smaller increase in SL between the start and the end (r
= −0.35, P < 0.0001).
At the end of the grazing period, animals with heavier
fleeces had a larger FD (r = 0.34, P < 0.0001), greater
CVFD (r = 0.11, P < 0.001) and longer staples (r =
0.28, P < 0.0001). Animals with the largest reduction
in FD and the largest difference in SL (those with the
largest wool growth) between the start and end of the
grazing period had the heaviest fleeces (r = −0.07, P <
0.05, and r = 0.11, P < 0.01, respectively).
Animals with smaller FD at the start lost less BW
during the grazing period (r = −0.07, P < 0.05). Animals that lost less BW produced more wool (r = 0.09,
P < 0.01).
To test whether FD or SL at the start of the grazing
period would influence the chance of producing a lamb
SLstart
0.17
0.37
−0.05
0.39
(0.07)
(0.06)
(0.04)
(0.04)
GFW
−0.21
0.21
0.29
0.34
0.36
(0.08)
(0.05)
(0.07)
(0.09)
(0.05)
(or 2 lambs), a logistic procedure was used. The chisquared test was significant for FDstart (P < 0.05), but
not for SLstart and indicated that animals with smaller
fibers had a greater probability to carry a lamb to term
(P < 0.05). Analyzing offspring weaning weights with
model [7] indicated that BW lost during the grazing
period did not significantly influence offspring weaning
weights in the present study (P = 0.6340).
Heritabilities and Genetic Correlations
Heritabilities and genetic correlations are given in
Table 1. Wool traits were moderately to highly heritable; change in BW was moderately heritable. The
change in BW was negatively correlated with FD and
GFW and positively correlated with SL. Fiber diameter was positively correlated with CVFD, SL, and
GFW. Greasy fleece weight was positively correlated
with CVFD and SL.
DISCUSSION
Although the importance of grazing efficiency has
been recognized (Fogarty et al., 2006), a practical
method to record feed intake in grazing animals is unavailable today and is therefore not considered a trait
that can be selected for under grazing conditions. However, grazing ability may be indirectly inferred from the
response in BW during the grazing period. Changes in
BW may then be used in a selection program that is
aimed at improving adaptability to resource-poor environments. The aim of this study was to investigate the
response in BW and wool traits in pregnant ewes grazing on the resource-limited cold Nevada desert.
The results show that grazing during the winter
conditions in the Nevada desert resulted in BW loss,
whereas pregnant animals in particular need to gain
BW. These results are supported by Thomas and Kott
(1995), who concluded that unsupplemented ewes on
rangeland in Montana lose a significant amount of BW
from early to mid-gestation. Prolonged bouts in which
more than 50% of the NRC requirements for gestation
are not met are common for ewes grazed on rangelands
(Thomas and Kott, 1995). In situations in which energy
availability from grazed forage is too small to meet production demands, some form of energy supplementation
Adaptability of Merino ewes to the cold desert climate
is often practiced (Caton and Dhuyvetter, 1997). The
observation that ewes that were further into gestation
had lost less BW than animals that were less far into
gestation can be explained by the fact that the fetus is
growing and is putting on overall maternal BW. However, the observation that nonpregnant ewes lost less
BW than pregnant ewes (when the change in BW was
adjusted to 0 d in gestation) indicates that fetus growth
was at the expense of the body reserves of the mother.
Because benefits to offspring have an associated maternal cost, trade-offs and conflicts occur during pregnancy
and lactation resulting from a limit to food assimilation
and sustained metabolic rate (Weiner, 1992; Rogowitz,
1996). If the dam allocates too many of her resources to
her offspring, she may lose excessive BW, increase her
risk of mortality, and compromise future reproductive
potential, whereas an insufficient rate of energy export
to young may decrease postnatal growth or cause offspring mortality (Rogowitz, 1996). In our data, ewes
carrying 1 lamb tended to lose more BW and lost a significantly larger percentage of their BW than ewes carrying twins. From a resource allocation perspective, the
opposite would be expected. Although 2-yr-old dams,
which are simultaneously challenged with the drive to
grow and support pregnancy, gave birth to the largest
number of singletons (140 lambs), 469 singletons were
born from 3- to 7-yr-old dams. Because animals in the
present study were not scanned for pregnancy, the results may not be a true reflection of pregnancy status
during the grazing period.
Our results indicate that animals that had given birth
the year before were better adapted to the cold Nevada
desert than animals that had not given birth the year
before. Ewes that did not give birth the previous year
but did give birth to 1 or 2 lambs in the current year
lost about 1.5 kg more BW than ewes that gave birth
to 1 or 2 lambs both in the previous and in the current
year. These results suggest that animals that had previously dealt with the challenge of supporting pregnancy
in a resource-poor environment were better adapted to
deal with the same situation again.
Clean wool is pure protein with a particularly elevated content of the AA Cys. Therefore, quantitative and
qualitative aspects of protein nutrition, but also the
amount of energy in the diet, can be limiting factors
to wool growth, in particular when sheep have been
exposed to lengthy periods of undernutrition (Hutchinson, 1961). Sheep fed a submaintenance ration resulting in a slight loss of BW resulted in a reduced average FD in the study of Weber (1932), who concluded
that wool growth is materially retarded when sheep are
subjected to a low plane of nutrition. Robertson et al.
(2000a) indicated that FD increased with pasture availability. In our experiment, FD and its CV decreased
significantly in response to grazing in a resource-limited
environment. In addition, wool growth is affected by
environmental and physiological stresses, such as lambing, disease, season, and transport (Hunter et al., 1990;
Reis, 1992). Hunter et al. (1990) indicated that feeding
867
stress caused up to 40% reduction in FD with more
severe stress conditions causing a larger decrease. Wool
growth and FD are usually depressed during pregnancy, and the effects may be larger in ewes bearing and
rearing twins (Reis, 1992; Robertson et al., 2000b). The
effects of pregnancy and lactation are partially due to
competition between tissues for essential nutrients, but
hormonal factors may also be involved (Reis, 1992;
Robertson et al., 2000b). A reduction in FD resulting
from a large reduction in the supply or allocation of
nutrients will result in a weaker region of the staple and
a reduced staple strength (Reis, 1992). In the present
study, ewes were subjected both to a resource-limiting
environment and pregnancy. Fiber diameter was smaller
in pregnant ewes than in nonpregnant ewes and smaller
in ewes carrying 1 lamb than in ewes carrying twins,
but none of this was significant (data not presented).
For this reason, number of offspring was not included
in the analysis of the wool traits. However, in our data,
FD and SL decreased with number of lambs (0, 1, or 2)
delivered in the previous grazing period, which includes
the effect of the previous lactation. Because lactation
is the period of peak energy demand in female mammals (Oftedal, 1984), the effect of number of offspring
in the current grazing period may have become significant during the lactation period, but this was not
investigated. Indeed, Robertson et al. (2000a,b) indicated that staple strength of reproducing ewes is more
sensitive to nutritional change around parturition and
early lactation.
Undernutrition during pregnancy may have adverse
consequences for offspring (Gopalakrishnan et al., 2004;
Wu et al., 2004), considering that maternal nutrition in
pregnancy affects lamb birth weight and the incidence
of lamb mortality. Maternal undernutrition is associated with a reduction in udder weight and mammary
development, resulting in a reduced colostrum yield
and total milk production, and with a delayed onset of
lactation and a reduced milk secretion rate (Dwyer et
al., 2003). Better-fed ewes produce progeny that will
produce more and finer wool throughout their lives,
and the effects on farm profitability of improving ewe
nutrition during pregnancy are potentially very large
(Thompson and Young, 2002). Dwyer et al. (2003) concluded that even a moderate level of undernutrition
impairs the attachment between ewes and their lambs
by affecting maternal behaviors expressed at birth. In
addition, they suggest that levels of nutrition that result in a decreased birth weight will affect neonatal
lamb behavioral progress. Reduced availability of AA
in the conceptus plays a major nutritional role in fetal
growth restriction in nutrient-restricted ewes (Kwon et
al., 2004). In the present study, birth weights were not
recorded. The relation between the amount of BW lost
during grazing and offspring weaning weights were not
significant. This may be explained by the fact that ewes
in the present study spent the last several weeks of gestation and their lactation period on pasture feeding after returning from the rangeland, which increased their
868
Rauw et al.
nutritional resources. Birth weights need to be recorded
to investigate this further.
An interesting observation is that, although the relationship was quite weak, ewes with finer wool at the
start of the grazing period lost significantly less BW
during the grazing period. Lee et al. (2002) observed a
genetic correlation of 0.40 between FD and digestible
OM intake under field conditions and suggested that
selection for reduced FD could decrease feed intake (requirements); this relationship may have been related to
differences in body composition (Adams and Cronje,
2003). Also, wild sheep evolved an outer coat that is
stiff and hairy, which covers a woolly undercoat that
protects it from the cold in the coldest period of the
year and is shed before the hottest period of the year.
Fleece of domestic sheep consists entirely of the woolly
undercoat that grows all year round (Clutton-Brock,
1999). Because finer fibers provide a greater thermal
insulation, our results suggest that animals with finer
wool were better adapted to the cold desert climate
in Nevada. In addition, our results indicate that ewes
with finer wool (which lost less BW) had a greater
probability to carry a lamb to term. Because animals
were not detected for pregnancy, it is not possible to
know whether animals that did not deliver offspring
terminated their pregnancy during gestation or were
never pregnant to begin with. Also, animals with multiple embryos or fetuses may lose potential offspring
without a total loss of pregnancy (Schrick and Inskeep,
1993). Although poor levels of nutrition can result in
increased rates of embryo loss (Abecia et al., 2006),
Vonnahme et al. (2003) demonstrated no fetal loss in
response to nutrient restriction. Embryo or fetal loss
were not detected in the present study. However, it may
be more likely that the interaction between wool type
and environmental adaptation during gestation influenced lambing rate than that an interaction between
wool type and conception rate would have influenced
pregnancy rate.
Heritability estimates were moderate to high for the
wool traits. Heritabilities of wool traits are a little under the values reported by Fogarty (1995) and Safari et
al. (2005), who reported heritabilities of 0.34 and 0.37
for GFW, 0.51 and 0.59 for FD, 0.52 for CVFD, and
0.46 for SL. Genetic correlations are more or less in line
with those published by Safari et al. (2005) with the exception of the genetic correlation between CVFD with
FD, SL, and GFW. In their study, the genetic correlation between CVFD and FD was −0.10 and the phenotypic correlation was −0.09, whereas we estimated
a genetic and phenotypic correlation of 0.49 and 0.14,
respectively. Lunney (1983) also indicated a positive
phenotypic relationship between FD and its CV. Andrews and Bow (1980) indicated that an increase of 1%
in CV has an influence equivalent to an increase of 0.1
to 0.2 μm in average FD.
In the present study, change in BW during the grazing period was moderately heritable at an estimate
of 0.29. This indicates that selection for BW change
will result in a positive selection response. In addition,
FD was heritable at an estimate of 0.51. Although the
phenotypic correlation was rather weak, animals with
smaller FD lost both phenotypically and genetically
less BW during the grazing period. This indicates that
selection for finer wool may (eventually) result in animals that are better adapted to the cold desert climate
as a correlated effect. Selection for greater adaptability
to the resource-limited cold desert climate will aim at
selecting animals that can produce wool at acceptable
levels while their health and welfare is not being compromised.
Our results indicate that animals with the least inclusion of Merino genetics (5/8 vs. 7/8 and 8/8) lost more
BW during the grazing period and had a more uniform
FD but shorter staples and less fleece than animals
with a greater level of Merino genetics. Thus, in our
study, Merino genetics appears to be better adapted to
the cold desert climate.
To summarize, insufficient nutrient intake resulting
from grazing on resource-poor rangelands in the cold
Nevada desert resulted in BW loss and reduced FD.
Animals with finer wool at the start of the grazing period appeared to be better adapted to the cold Nevada
desert, resulting in less BW loss and a greater chance
to carry a lamb (or 2) to term. Change in BW and FD
are heritable traits that are negatively correlated both
phenotypically and genetically. Therefore, selection
for finer wool may positively influence adaptability to
resource-limited cold climate conditions. Alternatively,
BW change may be selected for directly, resulting in
longer wool growth and possibly finer wool.
Rauw (2008) proposed a model to estimate grazing
intake in terms of estimated energy units consumed:
EGIi = FIi − b0 − ei = (b1 × BWi0.75) + (b2 × ΔBWi)
+ (b3 × PRODi), where EGIi = estimated grazing intake of individual i; FIi = actual feed intake of individual i; BWi0.75 = metabolic BW; ΔBWi = change in
BW; PRODi = level of production (kg of milk or wool,
number of eggs, and so on); b0 = population intercept;
b1, b2, and b3 = partial regression coefficients representing maintenance requirements, feed requirements
for growth, and feed requirements for (re)production,
respectively; and ei = the error term, representing residual feed intake. Regression coefficients represent the
average cost of body maintenance, growth, and production (such as number of offspring), based on the population on which the model is formed. This model shows
that the estimated grazing intake is confounded with
the efficiency of the resources allocated. However, when
animals are grazing in resource-poor environments it is
more important that the animal has been able to ingest
a sufficient amount of resources than if the animal is
more or less efficient in allocating those. This model
can be finalized by including estimates of the b-values.
When these estimates are available, BW can be recorded before and after animals are allowed to graze freely
on the rangelands, and metabolic BW and change in
BW can be estimated.
Adaptability of Merino ewes to the cold desert climate
Body weight changes as presented in this paper,
or grazing intake estimated from the aforementioned
model, can be used by sheep ranchers to assess the
nutritional adequacy of their range and determine if additional supplementation is necessary. The majority of
Great Basin range flock operations will spend the entire
year grazing public lands as opposed to several months
each year only. In that case, animals can be brought
in and weighed to mark the start and end of specific
management or physiological periods in which grazing
intake can be estimated. In addition, BCS may improve
the estimates. When grazing intake is expressed in ME
consumed units, comparison of grazing efficiency can be
made not only for animals sharing a given environment,
but also for animals living in different environments
or even different species living in the same environment. The methods can be used to evaluate the grazing
potential or the load of a flock on a given ecosystem
(Rauw, 2008). Estimation of grazing intake can be used
to evaluate the grazing potential of different ecosystems
in different periods of the year. Furthermore, grazing of
several species of herbivores on the same area typically
results in more efficient utilization of forage resources
and increases sustainable production (Walker, 1994).
Because of the uniform unit in which estimated grazing intake can be estimated, grazing efficiency of different ruminant species can be compared, and the grazing
load on the rangeland can be determined for different
combinations of ruminant species. Information on grazing potential could be used to develop more accurate
methods for managing rangelands such as the granting
of grazing permits for a given resource (land type, ruminant species, and number of animals) by government
agencies. Further research should be devoted to optimizing a grazing efficiency model and implementing
grazing efficiency into a breeding goal.
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