INTRODUCTION
Ewe reproduction and lamb survival affect economic and biological efficiency more than other production traits (Wang and Dickerson, 1991; Borg et al., 2007). Crossing super-prolific (e.g., Finnsheep and Romanov) and domestic breeds has greatly enhanced ewe reproductive performance in shed-lambing systems (Thomas, 2010). However, most lambs in Intermountain West and Great Plains states are born on open range (28%) or fenced pasture (31%; USDA APHIS, 2014) and reports of ewe productivity and lamb survival from these prolific breed types in extensive systems are scarce. Eliminating the cost of shearing may also be advantageous in environments that do not favor the production of high-quality wool. The Composite IV is a ½ Romanov, ¼ Katahdin, ¼ White Dorper hair sheep developed at the U.S. Meat Animal Research Center and has since been managed and selected in a forage-based system with limited human intervention from lambing through weaning. The objective of the present work was to provide a discussion of the development, selection criteria, and performance of Composite IV ewes and lambs.
MATERIALS AND METHODS
Composite IV Development
Matings that contributed to the development of the Composite IV are displayed in Table 1. An earlier experiment conducted at the U.S. Meat Animal Research Center (USMARC; Clay Center, NE) generated F1 lambs that were born to Romanov dams and sired by wool or hair breed rams. Unpublished results indicated that total weight of lamb weaned was greatest for White Dorper × Romanov (WD-R) ewes in both shed- and pasture-lambing systems. Katahdin × Romanov (K-R) ewe productivity was numerically lower than most other F1, but unquantified Katahdin attributes such as coat shedding, enhanced internal parasite tolerance, and foot soundness are favorable for low-input, forage-based systems.
Table 1.
Contributing breeds, their crosses, and corresponding lamb birth year of the Composite IV
| Breed composition1 | |||
|---|---|---|---|
| Sire (n) | Dam (n) | Lamb | Birth year |
| WD (19) | R (226) | WD-R | 2001 to 2004, 2007, 2011 |
| K (22) | K-R | 2001 to 2003, 2008, 2011 | |
| WD-R | WD-R | WD-R | 2005 to 2008 |
| WD-R | K-R | C-IV1 | 2008 to 2012 |
| K-R | WD-R | 2009 to 2012 | |
| C-IV1 | C-IV1 | C-IV2 | 2010 to 2018 |
1K = Katahdin; WD = White Dorper; R = Romanov; C-IV1 = first-cross Composite IV; C-IV2 = second-cross Composite IV.
Twenty-two registered Katahdin and 19 White Dorper rams were sourced from industry flocks and bred to 226 USMARC Romanov ewes to initiate the Composite IV. Some WD-R and K-R ewes were retained from the initial experiment, but most were re-generated over the next few years. Initial crosses focused on developing a pasture lambing, WD-R composite flock. These F2 WD-R and F1 WD-R were reciprocally mated to F1 K-R to produce first-cross Composite IV lambs. Inter-se matings between first-cross Composite IV ewes and rams produced the final cross (i.e., second-cross Composite IV) born in 2010, and this flock has since remained closed while maintaining the genetic diversity of foundational sires.
Flock Management and Selection Criteria
The Composite IV mating season commenced each year in mid-December for 35 d. Ewes were exposed to rams in single-sire mating pens from the 2010 to 2015 mating seasons and have since been group mated with 3 or 4 rams (approximately 20 ewes per ram). Ewes were fed a corn-silage based ration while in mating but were managed on pasture at all other times of the year. Mature ewes were managed on stockpiled forage throughout the winter and supplemented alfalfa hay and whole shelled corn. Approximately 3 wk before expected parturition, groups of ~75 ewes were assigned to 4 ha lambing paddocks.
From 2010 to 2016, lambs were tagged near birth, but periparturient ewes and lambs have since been undisturbed and lambs are not given unique identification until they die, enter the nursery, or are weaned. Dam assignment was through observation (2010 only) or DNA (2011 to present). Lambs were weaned and weighed at approximately 10 wk then entered the drylot for finishing or until selection decisions were made.
The flock is currently at ~650 ewes with a targeted goal of 800 ewes. Independent culling levels are in place for white color, ability to shed, polled, type of rearing, and genotypes at two loci: scrapie prion (PRNP) and susceptibility to Ovine Progressive Pneumonia (TMEM154). Selection of twin- and triplet-reared rams over singles should gradually improve lamb survival without the need for human intervention. However, as several traits are incorporated into the selection strategy, improvement of individual traits will be relatively slow.
Lamb and Ewe Traits
Lamb traits included survival from birth to weaning (n = 7,851) and body weight (BW) at weaning (BWW; n = 5,995). For the purposes of this study, 130 nursery-reared lambs were considered a preweaning mortality. Ewe BW was recorded prior to mating (BWM) each year, and fertility was calculated as whether a ewe present at mating lambed the following spring (n = 3,153). Number of lambs accounted for near parturition (NLB; n = 2,801) was calculated on a per ewe lambing basis and included all live and dead lambs for which parentage could be determined. Number of lambs weaned (NLW) or total litter weaning weight (LWW) per ewe lambing did not include lambs that died or were transferred to the nursery.
Analyses
Only records from second-cross Composite IV lambs and ewes were considered in the following analyses. Birth date and birth BW of most lambs were unknown, so could not be used to adjust lamb or ewe traits. Lamb traits were analyzed with fixed effects of birth type (1, 2, or 3+), sex (ewe or ram), dam age (1 to 5 yr), and birth year (2010 to 2018) and the random effect of sire. Ewe traits were analyzed as repeated measures with fixed effects of ewe age (1 to 5 yr) and ewe birth year (2010 to 2017). Additionally, a random effect of sire was fit and a compound symmetric covariance structure with heterogenous variance across age was assumed for the ewe effect. To assess the impact of litter size at birth (1, 2, or 3) on mature ewe (2 to 5 yr) productivity at weaning, NLW and LWW (n = 1,769) were also analyzed in similar models above with the additional fixed effect of NLB. Lamb survival and ewe fertility were analyzed as binary variables in the GLIMIIX procedure of SAS (v. 9.4; SAS Institute Inc., Cary, NC), and all other traits were analyzed in the MIXED procedure. All cross-classified two-way interactions were fit and subsequently removed if they were not significant (P < 0.05). Birth year effects and interactions are not discussed.
RESULTS
Lamb Traits
Birth type x sex interaction was significant in the analysis of lamb survival to weaning (P < 0.01). Within single and twin born lambs, survival was similar between ewe and ram lambs (P ≥ 0.53). However, within triplet and larger litters, survival was greater for ram than ewe lambs (0.69 ± 0.02 vs. 0.61 ± 0.02; P < 0.01). Least-squares means for the main effects on lamb traits are displayed in Table 2. As main effects, lamb survival was not influenced by sex (P = 0.35) but decreased with increasing birth type (P < 0.01). Lambs born to 1- and 5-yr-old dams had lower survival than other ages (P ≤ 0.04). Lambs reared by 1-yr-old dams had the lightest BWW (P < 0.01). As expected, BWW was lighter for ewe than ram lambs (P < 0.01) and decreased with increasing birth type (P < 0.01).
Table 2.
Least-squares means for the main effects of dam age, birth type, and sex on Composite IV lamb traits
| Trait1 | |||
|---|---|---|---|
| Effect | Level | Survival | BWW, kg |
| Dam age, yr | 1 | 0.78 ± 0.01b | 15.2 ± 0.11c |
| 2 | 0.82 ± 0.01a | 16.7 ± 0.11b | |
| 3 | 0.84 ± 0.01a | 17.4 ± 0.13a | |
| 4 | 0.83 ± 0.01a | 17.3 ± 0.15a | |
| 5 | 0.78 ± 0.02b | 17.3 ± 0.21a,b | |
| Birth type, n | 1 | 0.90 ± 0.01a | 19.9 ± 0.14a |
| 2 | 0.82 ± 0.01b | 15.9 ± 0.09b | |
| 3+ | 0.65 ± 0.01c | 14.6 ± 0.18c | |
| Sex | Ewe | 0.80 ± 0.01 | 16.3 ± 0.10b |
| Ram | 0.81 ± 0.01 | 17.2 ± 0.10a |
1Survival = lamb survival from birth to weaning (0 or 1); BWW = lamb body weight at weaning (~10 wk of age).
a–cMeans within a column and effect with no common superscript are different (P ≤ 0.04).
Ewe Traits
Least-squares means for the main effect of age on ewe performance are displayed in Table 3. Ewe BWM increased with age and was different between every class (P ≤ 0.02). Fertility of 1-yr-old ewes was lower than all other ages (P < 0.01). One-year-old ewes had the fewest lambs born and NLW (P < 0.01), and 2-yr-old ewes had fewer than older age classes (P ≤ 0.02). Similarly, LWW was least for 1-yr-old ewes (P < 0.01), lower for 2-yr-old than 3- or 4-yr-old ewes (P ≤ 0.02), but not different between 2- and 5-yr-old ewes (P = 0.21).
Table 3.
Least-squares means (± SE) for the main effect of age on Composite IV ewe traits
| Trait1 | |||||
|---|---|---|---|---|---|
| Age, yr | BWM, kg | Fertility | NLB, n | NLW, n | LWW, kg |
| 1 | 40.8 ± 0.24e | 0.84 ± 0.01b | 1.55 ± 0.02c | 1.24 ± 0.02c | 19.7 ± 0.31c |
| 2 | 50.7 ± 0.25d | 0.93 ± 0.01a | 1.99 ± 0.03b | 1.57 ± 0.03b | 24.3 ± 0.39b |
| 3 | 57.8 ± 0.31c | 0.92 ± 0.01a | 2.20 ± 0.03a | 1.78 ± 0.03a | 27.5 ± 0.53a |
| 4 | 60.9 ± 0.35b | 0.92 ± 0.01a | 2.23 ± 0.04a | 1.83 ± 0.04a | 26.4 ± 0.63a |
| 5 | 62.2 ± 0.45a | 0.92 ± 0.02a | 2.23 ± 0.05a | 1.75 ± 0.06a | 26.3 ± 0.90a,b |
1BWM = ewe body weight at mating; fertility = whether a ewe present at mating lambed the following spring (0 or 1); NLB/NLW = number of lambs born/weaned per ewe lambing; LWW = total litter weaning weight per ewe lambing.
a–eMeans within a column with no common superscript are different (P ≤ 0.02).
The frequency of litter size class at birth and weaning and least-squares means for the main effect of NLB on NLW and LWW are displayed in Table 4. Most mature Composite IV ewes that gave birth to single or twin lambs reared their entire litter through weaning, while most ewes that gestated triplets reared twins. The ewe age × NLB interaction was significant in the analyses of NLW and LWW (P ≤ 0.03) due to relative differences between but not a re-ranking among NLB classes within ewe age. On average, both NLW and LWW increased with increasing NLB (P < 0.01).
Table 4.
Descriptive statistics for litter size at weaning in relation to number of lambs born (NLB) and least-squares means (± SE) for the main effect of NLB on number of lambs weaned (NLW) and total litter weaning weight (LWW) of Composite IV ewes
| No. weaned, % | ||||||
|---|---|---|---|---|---|---|
| NLB (%)1 | 0 | 1 | 2 | 3 | NLW, n | LWW, kg |
| 1 (16.6) | 12.6 | 87.4 | — | — | 0.87 ± 0.05c | 16.4 ± 0.74c |
| 2 (58.7) | 4.1 | 24.8 | 71.1 | — | 1.70 ± 0.03b | 26.0 ± 0.44b |
| 3 (24.7) | 5.9 | 16.0 | 45.3 | 32.8 | 2.06 ± 0.03a | 28.9 ± 0.55a |
1Based on a total of 293 single, 1,039 twin, and 437 triplet litters from mature ewes (2 to 5 yr old at lambing).
a–cMeans within a column with no common superscript are different (P < 0.01).
DISCUSSION
Component breeds and selection pressure of the Composite IV has resulted in a white, polled, maternal composite with predicted 62.5% individual and maternal heterosis. Composite IV sheep do not require docking or shearing and have been managed in a forage-based, pasture-lambing system which drastically reduces costs of production. Input costs were not evaluated in the present study, but Ali et al. (2005) estimated that annual ewe feed costs in a pasture-lambing system in Iowa were 54% lower than expected from a typical shed-lambing system. Inclement weather, predation, and internal parasitism can certainly limit performance in range- or pasture-lambing systems and this should be considered jointly when evaluating profitability.
Burfening and Van Horn (1993) compared the productivity of Western white-faced ewes under shed- or range-lambing in Montana. While shed-lambed ewes had greater NLW and LWW per ewe exposed (0.98 lambs and 39.2 kg) than range-lambed ewes (0.88 lambs and 32.1 kg), economic simulation of these performance levels generally estimated greater returns for range-lambed ewes. Performance in extensive rangeland operations has been greatly enhanced by infusing a proportion of prolific genetics into typical Western white-faced flocks (e.g., Rambouillet and Targhee). Walker et al. (1993) evaluated mature ¾ Targhee × ¼ Finnsheep ewes in range- or shed-lambing treatments in Idaho (NLB = 1.93 lambs). Litter size at 25 d was greater for shed- than range-lambed ewes (1.63 vs. 1.42 lambs) but differences were less pronounced by weaning (1.13 vs. 1.08 lambs). However, BWW was lighter for shed- than range-born lambs (30.9 vs. 32.3 kg), so that ewe LWW was similar between treatments. Terminally mated, pasture-lambed Polypay × Dorset ewes evaluated in the Midwest by Ali et al. (2005) had similar NLB (1.65 to 1.74 lambs) and LWW (32.1 to 36.2 kg) to the range-lambed ewes above.
Range- and pasture-lambed ewes in previous studies had lower NLW but greater LWW than straight-bred Composite IV in the present study. Notter et al. (2017) evaluated terminally mated Rambouillet, Polypay, and Romanov-White Dorper × Rambouillet (R-WD × R) ewes through four parities in Idaho. Cumulative NLW and LWW was much greater for R-WD × R (4.8 lambs and 153 kg) than Polypay (3.8 lambs and 123 kg) and Rambouillet ewes (2.9 lambs and 99 kg). Therefore, ewe productivity was greatly enhanced with the inclusion of ¼ Romanov breeding and may be expected to be greater in ½ Romanov ewes. Although no direct comparisons of Composite IV ewes have been published, a recently completed USMARC experiment evaluated Composite IV, Katahdin, and Polypay ewes through four parities of pasture lambing. Preliminary results indicate productivity and longevity were greatest for Composite IV ewes especially when mated to Texel rams.
Genetic improvement of NLB increases the frequency of triplet or larger litters which may reduce lamb survival (Borg et al., 2007). Shed-lambing systems commonly reduce triplet litters by cross-fostering or artificially rearing lambs. While this may improve survival, it requires additional labor or milk replacer and biases phenotypes for selection. Pasture-born lamb survival was greatest for singles (81% to 99%), intermediate for twins (59% to 77%), and lowest for triplets (41% to 91%) in the study of Ali et al. (2005). Notter et al. (2018) evaluated NLB effects in shed-lambed Polypay and R-WD × R ewes and reported that NLW was 0.2 lambs greater for triplet- than twin-bearing ewes. However, triplet-bearing ewes also lost an additional 0.75 lambs which corresponded to 3.75 dead lambs per additional lamb weaned. Triplet-bearing Composite IV ewes in the present study weaned 0.36 but lost 0.64 more lambs than twin-bearing ewes (i.e., 1.78 dead lambs/lamb weaned). Ewe and lamb supplementation would likely improve triplet survival and growth through weaning, but results suggest an intermediate optimum for ewe NLB in extensive production systems.
Composite IV breeding stock have been disseminated and is gaining regional popularity though no breed society has been formed. Phenotypic selection of this composite has favored multiple-rearing ability in a forage-based system with reduced labor inputs. Since lamb survival and ewe prolificacy, maternal ability, and longevity are lowly to moderately heritable traits, annual response in the present selection scheme is expected to be low. Composite IV lamb BW at weaning is lighter than most maternal or dual-purpose breeds and, if this remains through finishing, may present challenges in meeting carcass expectations of traditional U.S. markets. Direct selection for lamb BW and/or use of terminal sires could improve lamb survival and growth. Planned research in this flock will evaluate terminal sire breeds, ewe productivity in shed and pasture lambing, and pedigree-based or genomic-enhanced estimated breeding values for use in selection.
Footnotes
USDA is an equal opportunity provider and employer. The mention of trade names of commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA.
The authors acknowledge Kreg A. Leymaster (retired) who provided the primary leadership for conceiving and designing this composite and USMARC sheep operations staff for the care and management of animals.
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