Progress 04/01/03 to 03/31/09
Outputs
OUTPUTS: This project focuses on two specific genetic resources: 1) a line of sheep developed by 11 years (approximately 3 sheep generations)of selection for reduced seasonality of breeding and 2)recently derived composite hair sheep breeds such as the Katahdin and Dorper. In terms of selection for reduced seasonality of breeding, outputs include: 1) the response to selection has been positive and substantial; 2) ewes from the selected line are reproductively inactive for at most 3 to 6 weeks in late summer ; 3) rebreeding of lactating selected ewes in spring has been successful at 60 to 90 days following lambing, with pregnancy rates of 60 to 80 percent; 4) lactating ewes that were exposed to rams in spring at less than 50 days after lambing had reasonable pregnancy rates (38 percent) but exhibited increased fetal lamb losses and had difficulty maintaining pregnancy; 5) short-term lamb separation prior to ram introduction in the spring did not enhance spring fertility in this line, perhaps because of the very high baseline fertility level; 4) a genetic polymorphism in the melatonin receptor 1a gene is associated with genetic merit for spring breeding in the line, and could serve as a genetic marker for reduced seasonality of breeding. This population currently has one of the longest breeding seasons ever reported for temperate sheep breeds. High levels of spring fertility for lactating ewes are likewise unprecedented in the literature. Studies of composite hair sheep breeds have focused on both overall animal performance and resistance to gastrointestinal roundworms. Result show that: 1) Dorper crossbred lambs are superior to Katahdin lambs in muscling and conformation but do not grow faster and are somewhat fatter; 2) Katadin ewes have longer productive lives than Dorper crossbred ewes or the ewes from the selection line described above; 3) composite breeds developed from Caribbean hair sheep breeds such as the St. Croix have higher levels of worm resistance than traditional temperate wool breeds; 4) the South African Dorper breed, developed from Middle Eastern hair sheep breeds, is less resistant to worms than breeds of Caribbean ancestry and not different from traditional wool breeds; 5) however, Dorper sheep have higher hemoglobin levels and are superior to wool breeds in their ability to cope with the blood losses associated with parasitism; 6) high levels of genetic variation in parasite resistance have been documented in the Katahdin breed, and selection for increased resistance is expected to be effective in further enhancing resistance. Results of this study have been disseminated to the scientific community through participation in the NCERA-190 Regional (de facto national) Coordinating Committee. Dissemination to sheep producers has occurred in association with activities of the National Sheep Improvement Program Genetic Evaluation Center at Virginia Tech, through interactions with Katahdin Hair Sheep International, via presentations to the American Sheep Industry Association, and in the agricultural press. PARTICIPANTS: Scott Bowdridge and Joe Emenheiser were graduate students at Virginia Tech who worked with and received training based on this project. Garrett Smith used animals from this project for an undergraduate Honor's Project at Virginia Tech. Collaborations with Dr. Fred Karsh at the University of Michigan and Drs. Keith Inskeep and Robert Daily at West Virginia University contributed to the conduct of these studies. At Virginia Tech, collaborations with Dr. Anne Zajac, a parasitologist from the Virgnia-Maryland Regional College of Veterinary Medicine, and Dr. Robert Akers in the Department of Dairy Science were likewise essential. TARGET AUDIENCES: This research was mainly anticipated to benefit sheep producers and breeders in Virginia and nationally. Our target audience also includes scientists interested in the genetic mechanisms controlling seasonal breeding and in the genetic and immunological mechanisms associated with genetic control of parasite resistance. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Changes in knowledge associated with this project include: 1) clear evidence that the timing and duration of the breeding season in sheep is under additive genetic control and can be improved by selection; 2) documentation that rebreeding of lactating ewes in spring is feasibly in selected lines, thereby removing one of the most significant impediments to increasing the frequency of lambing in commercial sheep production; 3) identification of a potential genetic marker associated with seasonal breeding; 4) documentation that the extent of parasite resistance in composite hair sheep breeds is associated with the geographic origin of the founder breeds; 5) clear evidence that resistance to gastrointestinal roundworms is under additive genetic control and can be improved by selection; 6) documentation of the longer productive life of Katahdin ewes relative to several other breeds; 7) increased understanding of the differences and relative importance of parasite resistance (i.e., the ability to limit establishment of worm populations in the gut, as in the St. Croix and Katahdin) and parasite resilience (i.e., the ability to maintain health and performance in the presence of parasite infection, as exemplified by the high hemoglobin levels of the Dorper). Changes in actions associated with this project include: 1) development and implementation of methods for genetic evaluation of parasite resistance in the sheep industry through the National Sheep Improvement Program (NSIP) and including the calculation and publication of estimates of genetic merit for this trait in Katahdin sheep; 2) two separate Sustainable Agricultural Research and Extension farmer-led projects that were motivated in part by these studies and whose results have been integrated with those of the current project; 3)alternatives for genetic evaluation of ewe reproductive performance in out-of-season and accelerated lambing systems in NSIP have been developed and are available for implementation; 4) widespread global consideration of the potential of the melatonin receptor 1a genetic polymorphism as a genetic marker for reduced seasonality in sheep has occurred, resulting in an emerging consensus that the marker may be effective in some breeds but is not effective in all sheep populations. The most significant changes in conditions that have been facilitated by this project are the ongoing expansion hair sheep composite breeds in the sheep industry and the recognition that genetic improvement in parasite resistance is clearly possible.
Publications
- MacKinnon, K. M., J. L. Burton, A. M. Zajac, and D. R. Notter. 2009. Microarray analysis reveals differences in gene expression profiles of hair and wool sheep infected with Haemonchus contortus. Vet. Immunol. Immunopath. 130:210-220.
- Kuehn, L. K., R. M. Lewis, and D. R. Notter. 2009. Connectedness in Targhee and Suffolk flocks participating in the U.S. National Sheep Improvement Program. J. Anim. Sci. 87:507-515.
- Bowdridge, S. A. 2009. Characterizing physiological and genetic differences in the early immune response to Haemonchus contortus in resistant and susceptible sheep. Ph. D. Dissertation, Virgnia Polytechnic Institute and State University.
- Notter, D. R., D. L. Thomas, and D. F. Waldron. 2009. Biological and economic consequences of the FecB mutation in the U.S.A. In: Walkden-Brown, S. W., J. H. J. van der Werf, C. Nimbkar, and V. S. Gupta (Eds.), Use of the FecB (Booroola) Gene in Sheep Breeding Programs, ACIAR Proc. No. 133. Australian Centre for International Agricultural Research, Canberra.
- Notter, D. 2009. Genetic selection to improve fertility in spring matings: selection response, impact on duration of the seasonal anestrus, potential genetic markers, and fertility of lactating ewes in spring matings. Proc. U.S. Sheep Res. Programs, Amer. Sheep Industry Convention, San Diego, CA, January 21-24, pp. 12-14.
- Bowdridge, S. A., G. P. Smith, S. P. Greiner, A. M. Zajac, and D. R. Notter. 2009. Performance of Katahdin lambs is maximized by strategic de-worming followed by grazing pastures with lower nematode parasite contamination. J. Anim. Sci. 87(E-Suppl. 3):10-11 (Abstr.).
- Jordan, K. M., J. W. Knight, and D. R. Notter. 2009. Annual hormonal patterns underlying differences in timing and duration of the breeding season in three breeds of ewes. Biol. Reprod. 81(Suppl. 1):256 (Abstr.).
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Progress 10/01/07 to 09/30/08
Outputs
OUTPUTS: As described in previous reports for this project, selection for ability of ewes to mate and conceive in spring (May and June) led to development of the Virginia Tech out-of-season breeding (OOS) line, a population of sheep with a markedly extended annual breeding season. To further document the breeding behavior of these animals, a study was conducted to characterize the breeding season of 3 types of sheep, the Suffolk, St. Croix, and OOS selection line. The study used 18 ewes of each type. Half of the ewes in each group were ovariectomized (OVX) and received a long-acting subcutaneous estradiol implant. The other half remained intact. Circulating levels of progesterone (P4) were used to assess ovarian status and determine the incidence of ovulation in intact ewes. For estradiol-implanted OVX ewes, circulating levels of luteinizing hormone (LH) from the hypothalamus were used to track the period of potential reproductive activity, under the assumption that seasonal changes in responsiveness of the hypothalamus to a constant level of estradiol are diagnostic of changes in reproductive status. Blood samples were taken twice weekly from all ewes for 12 months starting in November 2006 and assayed for circulating P4 in intact ewes and circulating LH in OVX ewes to determine the timing and duration of the breeding season. In intact ewes, the seasonal anestrus was much shorter for OOS ewes (57 days) than for St. Croix (133 days) or Suffolk ewes (140 days). The OOS ewes had both a later onset of anestrus (May 19) and an earlier end to anestrus (August 2) than ewes of the other 2 types. Similar results were obtained for OVX ewes; the period of neuroendocrine inactivity (corresponding to baseline LH levels and the seasonal period of sensitivity to negative feedback effects of estradiol) was only 61 days for OOS ewes, but 121 days for St. Croix ewes and 164 days for Suffolk ewes. Another critical characteristic of sheep breeds is their sensitivity to gastrointestinal parasites. To better understand genetic control of resistance to the abomasal parasite Haemonchus contortus in St. Croix hair sheep, we characterized the early cellular immune response to parasite infection using 24 OOS and 24 St. Croix lambs sacrificed at 0, 3, 5 and 7 d after infection with 10,000 H. contortus larvae. Peripheral blood samples taken immediately before harvest revealed that St. Croix lambs had significantly higher levels of circulating neutrophils (3,018 cells/microliter) than OOS lambs (1,818 cells/microliter). Neutrophil infiltration into the abomasal lining was also significantly higher St. Croix lambs (831cells/square mm of abomasal surface) than in OOS lambs (561 cells/square mm) at all time points. Total abomasal lymph node (ALN) weight increased exponentially from 2.60 g at day 0 to 6.57 g at day 7 (a 2.5-fold increase) in St. Croix lambs, but ALN weight only increased from 1.88 to 2.67 g (only a 1.4-fold increase) over this period in OOS lambs, suggesting a greater influx of lymphocytes to ALN, where they are made antigen specific, in St. Croix lambs. PARTICIPANTS: Kathryn MacKinnon, Katherine Jordan, and Scott Bowdridge were doctoral students at Virginia Polytechnic Institute and State University (Virginia Tech) who worked on these projects. Mr. Garrett Smith also used a portion of this work as an undergraduate Honors Project at Virginia Tech. Mr. Josh McCann, a student at Giles County High School, used animals from this project for a science fair project; the project, entitled "Does susceptibility to Haemonchus contortus affect parasite burden, packed cell volume, and serum IgA levels in Ovis aries during gestation" received first place in the Animal Science Division and the Grand Award at the Southwest Virginia Regional Science Fair, first place in the Zoology Section of the Virginia State Science Fair, and was subsequently presented at the International Science and Engineering Fair in Albequerque, NM. Collaboration with Dr. Fred Karsh at the University of Michigan and Dr. Jeanne Burton at the Michigan State University Center for Functional Genomics was essential to the conduct of these studies, as was the input of Dr. Anne Zajac, a parasitologist from the Virginia-Maryland Regional College of Veterinary Medicine and access to the Meats Laboratory in the Department of Food Science at Virginia Tech for lamb sacrifice and tissue collection. TARGET AUDIENCES: Research involving the development and characterization of the Virginia Tech OOS Selection line is mainly directed at sheep producers and breeders in Virginia and elsewhere. This audience also is expected to benefit from research into the genetic control of parasite resistance, but our target audience for this research also includes researchers striving to understand the genetic mechanisms that control genetic resistance to gastrointestinal parasites. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Results of these experiments confirm that selection of ewes for ability to mate and conceive in spring has been accompanied by an extension of the duration of the breeding season and a corresponding reduction in the duration of the seasonal anestrus. The duration of the seasonal anestrus in intact OOS ewes was 57 days. This figure is longer than that observed in previous studies where OOS ewes were continuously cohabited with rams, suggesting that OOS ewes may also be more sensitive than unselected ewes to behavioral interactions with males. However, even under conditions of ram isolation, the duration of the seasonal anestrus in OOS ewes is one of the shortest ever reported for a temperate sheep breed. Results of this study thus clearly demonstrate that the timing and duration of the breeding season of the sheep is under genetic control and that the breeding season can be extended by selection. This is an important result, in that it provides evidence for the importance of including measures of seasonal breeding in practical sheep breeding and selection programs. The observed differences in immune function in response to parasitic infection in OOS and St. Croix hair sheep likewise has extended understanding of the mechanisms that underlie genetic resistance to gastrointestinal parasites. In particular, the much higher levels of neutrophils in circulation and in abomasal mucosa of St. Croix lambs in the first few days following infection are characteristic of a more rapid and robust immune response in these lambs. Early infiltration of neutrophils to gut tissues during nematode infection aides in development of a robust TH2 immune response and may also play a role in presentation of antigen to other immune effector cells. Greater numbers of local antigen-specific lymphocytes at an earlier stage of infection can limit establishment of adult parasites and thereby shorten duration and/or severity of infection. Understanding of the mechanisms that control breed differences in parasite resistance may both allow for selective breeding of more resistant strains of sheep and also provide information that can be used in development of more effective parasite treatments.
Publications
- Kuehn, L. A., R. M. Lewis, and D. R. Notter. 2007. Managing the risk of comparing estimated breeding values across flocks or herds through connectedness: a review and application. Genet. Selec. Evol. 39:225-247.
- Borg, R. C., D. R. Notter, L. A. Kuehn, and R. W. Kott. 2007. Breeding objectives for Targhee sheep. J. Anim. Sci. 85:2815-2829.
- MacKinnon, K. M. 2007. Differential expression and immune regulatory mechanisms in parasite-resistant hair and susceptible wool sheep infected with the parasitic nematode, Haemonchus contortus. Ph. D. Dissertation, Virginia Polytechnic Institute and State University.
- Kuehn, L. A., D. R. Notter, and R. M. Lewis. 2008. Assessing genetic gain, inbreeding, and bias due to different flock genetic means in alternative sheep sire referencing schemes. J. Anim. Sci. 86:526-535.
- Jordan, K. M., J. W. Knight, and D. R. Notter. 2008. Ability of ewes to rebreed while lactating in spring. J. Anim. Sci. 86(E-Suppl. 2):395(Abstr.).
- Bowdridge, S. A., G. P. Smith, A. M. Zajac, and D. R. Notter. 2008. The early cellular immune response to Haemonchus contortus in resistant and susceptible sheep. Proc. Annu. Meeting Amer. Assoc. Vet. Parasitologists, New Orleans, LA, July 19-22, pp 39-40 (Abstr.).
- Bowdridge, S. A., K. M. MacKinnon, J. McCann, A. M. Zajac, and D. R. Notter. 2008. Humoral immune response to nematode parasite infection is accelerated and longer-lived in hair-type sheep. Proc. Annu. Meeting Amer. Assoc. Vet. Parasitologists, New Orleans, LA, July 19-22, p76 (Abstr).
- Kuehn, L. A., R. M. Lewis, D. R. Notter, and G. Nieuwhof. 2008. Changes in connectedness over time in alternative sheep sire referencing schemes. J. Anim. Sci. 86:536-544.
- Notter, D. R. 2008. Genetic aspects of reproduction in sheep. Reprod. Domes. Anim. 43(Suppl. 2): 122-128
- Jordan, K. M. 2008. Characterization of seasonal reproduction in Virginia Tech Selection Line, St. Croix, and Suffolk ewes. Ph. D. Dissertation, Virginia Polytechnic Institute and State University.
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Progress 10/01/06 to 09/30/07
Outputs
OUTPUTS: In spring 2006, 24 ewes from the Virginia Tech out-of-season lambing flock and 23 St. Croix hair sheep ewes were evaluated for ability to cycle while lactating in March and April. As reported in the 2005-06 AD-421 Report, 71 percent of selected ewes but only 30 percent of St. Croix ewes mated. The percentage diagnosed as pregnant in late summer, 2006 was correspondingly 67 percent for selected ewes and 39% for St. Croix ewes. All 39 percent of the St. Croix ewes subsequently lambed in fall 2006, but only 43 percent of selected ewes lambed. The very early mating by selected ewes (42 percent mated within 21 days of ram introduction) was apparently accompanied by substantial embryonic losses. In contrast, later matings by St. Croix ewes (0 percent mated at 21 days but 35 percent mated within 39 days) resulted in more maintained pregnancies. A similar study was conducted in spring of 2007; 34 ewes from the Virginia Tech out-of-season lambing flock with an average lambing date of
March 20 were evaluated for ability to cycle and conceive while lactating in May and June. Rams with marking harnesses were introduced on May 3, when ewes were an average of 44 days postpartum, and marks were recorded twice weekly. By 28 days following ram introduction, and at an average of 72 days postpartum, 47% of the ewes had mated. However, only 12 of these 16 ewes subsequently lambed, and a high proportion of the lambs had lower than expected birth weights and perinatal survival. A final study of patterns of anestrus in selected ewes involved monitoring of estrous behavior during summer, 2007 using 17 ewes selected ewes and 11 St. Croix ewes. Ewes were isolated from rams, and reproductive activity was monitored by assaying twice-weekly blood samples for circulating progesterone levels to determine the incidence of ovulation. Ewes were monitored for 2 weeks beginning April 17, 2 weeks beginning May 14, and 8 wk beginning in early July. The first two periods are times when ewes
from this flock were shown to cycle with high frequency, but the last period represents the time of a hypothetical late-summer anestrus. Analyses of circulating hormone levels will be used to determine frequencies of ovulation in these ewes at the different times. Susceptibility to the parasitic nematode Haemonchus contortus is also an important characteristic in commercial sheep production. We therefore characterized differences in antibody production and gene expression in 12 St. Croix hair sheep lambs and 12 lambs from the line selected for reduced seasonality of breeding. Lambs were infected with 10,000 H. contortus larvae and an additional 14 lambs per breed were left as uninfected controls. Infected hair sheep had significantly larger lymph nodes and higher levels of immunoglobulin E (IgE) in lymph nodes at 27 days after infection. At this time, resistant hair lambs also had lower expression of interferon-gamma and higher (P less than 0.10) expression of IgE and the Th2 cytokine
interleukin-13 in both lymph nodes and the abomasal lining.
PARTICIPANTS: Kathryn MacKinnon, Katherine Jordan, and Hima-Bindu Vanimisetti were PhD students at Virginia Polytechnic Institute and State University who worked on this study. Dr. Jeanne Burton, Department of Animal Science, Michigan State University collaborated with us on the microarray and PCR analyses of gene expression in lymph nodes and abomasal tissue. Dr. Isis Mullarky, Department of Dairy Science, Virginia Polytechnic Institute and State University, provided laboratory facilities and assistance for PCR analyses. Dr. Anne Zajac, College of Veterinary Medicine, Virginia Polytechnic Institute and State University, collaborated on all aspects of parasite susceptability.
TARGET AUDIENCES: The main target audience is sheep producers and breeders in Virginia and elsewhere in the US.
Impacts
A response to selection for ability to conceive in spring and lamb in fall has been demonstrated in the Virginia Tech selection line. The present studies were designed to assess the ability of the selected ewes to lamb more frequently than once per year. Results of the 2006 and 2007 studies involving rapid spring rebreeding of lactating ewes demonstrates that rapid rebreeding with establishment of pregnancy is possible in spring-lambing, lactating ewes of the selected line. However, these very early matings are accompanied by relatively high levels of embryonic and fetal losses. Optimal spring fertility would likely be achieved by delaying matings to an average of 70 to 90 days postpartum, a time schedule that is still consistent with average lambing intervals of 7.5 months. A lambing interval of 7 to 8 months is consistent with most programs developed for accelerated lambings, so the performance of the selected ewes documents that selection can be used to develop
sheep that are adapted to frequent lambing programs, provided that a resonable period is allowed between lambing and first mating. Results regarding resistance to gastrointestinal nematode infection in hair sheep demonstrates taht resistant animals possess a rapid and robust Th2-type immune response, characterized by decreased IFN, and increased expression of interleukin-13 and immunoblobulin E. Differential regulation of Th2 cytokines between breeds may be partially responsible for differences in parasite resistance. These differentially expressed genes in infected animals are candidates for further study. Identification of favorable mutations in these genes and their incorporation into breeding lines may hasten development of parasite-resistant lines of sheep.
Publications
- Borg, R. C., D. R. Notter, and R. W. Kott. 2007. Genetic analysis of ewe stayability and its association with lamb growth and adult body weight. Journal of Animal Science 85(Suppl. 1):624-625 (Abstract).
- MacKinnon K. M., F. N. Kooyman, A. M. Zajac, I. K. Mullarky, and D. R. Notter. 2007. Differences in antibody production and local gene expression between parasite resistant hair sheep and susceptible wool sheep infected with Haemonchus contortus. Proceedings of the Annual Meeting of the American Association of Veterinary Parasitologists, Washington, DC, July 14-17, pp 51-52 (Abstr).
- MacKinnon, K. M., I. K. Mullarky, and D. R. Notter. 2007. Effects of Haemonchus contortus on the humoral and cellular immune response of parasite-resistant hair sheep. Proceedings of the 8th International Veternary Immunology Symposium, Ouro Preto, Brazil, August 15-19, pp 106-107 (Abstr).
- Vanimisetti, H. B. 2006. Genetic evaluation of ewe productivity and its component traits in Katahdin and Polypay sheep. Ph.D. Dissertation. Virginia Polytechnic Institute and State University.
- Notter, D. R., L. A. Kuehn, and R. M. Kott. 2007. Genetic analysis of fibre characteristics in adult Targhee ewes and their relationship to breeding value estimates derived from yearling fleeces. Small Ruminant Research 67:164-172.
- Vanimisetti, H. B., D. R. Notter, and L. A. Kuehn. 2007. Genetic (co)variance components for ewe productivity traits in Katahdin sheep. Journal of Animal Science 85:60-68.
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Progress 10/01/05 to 09/30/06
Outputs
Annual rates of lamb production can be increased by increasing frequency of lambing. Under ideal conditions, ewes can lamb every 7 to 9 months, but in most production systems lamb only once per year. This pattern reflects the seasonal anestrus of the ewe, with very low ewe fertility in spring and summer. A line of sheep with high fertility during the normal anestrous period in May and June and a generally extended breeding season was developed at Virginia Tech. The objective of the current research was to test their ability to rapidly rebreed while lactating in spring. To that end, 24 ewes from the Virginia Tech out-of-season lambing flock and 23 St. Croix hair sheep ewes that lambed in January 2006 were evaluated for ability to cycle in while lactating in late March and April. The St. Croix was used as a control because this breed is also generally considered to have an extended breeding season. Rams were introduced on March 24 and removed on May 2. Rams were
equipped with marking harnesses, and marks were recorded at each bleeding date. Ewes were an average of 60 d postpartum at the start of breeding and 99 d postpartum at the end of breeding. Forty two percent of the selected ewes but no St. Croix ewes mated within the first 21 d of ram exposure. Matings by St. Croix ewes did not occur until at least 28 d after ram introduction, and by the end of mating, 71 percent of selected ewes but only 30 percent of St. Croix ewes had mated. Most studies have shown that fertility of lactating ewes is commonly near zero at this time. Our study thus demonstrates that selection to increase ewe fertility in May and June enhanced the ability of ewes to rapidly rebreed while lactating in spring, and animals from the line appear particularly well suited for use in accelerated lambing. If accelerated lambing is not practical, an alternative strategy for increasing efficiency of lamb production involves identification of so-called easy-care breeds with
reduced shepherding requirements. Hair sheep composite breeds such as the Katahdin and Dorper are reported to possess a range of easy-care characteristics. We therefore compared productivity over 4 yr for straightbred Katahdin ewes and two alternative ewe types produced by mating Dorper or Dorset rams to ewes from the out-of-season breeding line described above. Differences among breeds in numbers of lambs born or weaned were modest at 1 and 2 yr of age with average numbers of lambs born per ewe lambing of 1.25 and 1.67, respectively. However, in adult ewes the number of lambs born plateaued at 1.77 in Dorper crossbred ewes but continued to increase to 1.93 in Dorset crossbred and 2.12 in Katahdin ewes. As a result, adult Katahdin ewes weaned 26 percent more lambs than Dorset and Dorper crossbred ewes. Lambs out of Katahdin ewes were lighter at 60 d than lambs out of Dorset or Dorper crossbred ewes but adult Katahdin ewes were still predicted to wean over 20 percent more kilograms of
lamb than ewes of the other two types. These results confirm the productive potential of Katahdin ewes compared to other wool and hair sheep types.
Impacts
Relevance: efficiency of the ewe flock is mainly determined by the weight and number of lambs produced and by labor and shepherding costs. In intensive production, efficiency can be increased by increasing lambing frequency. Ewes can ideally lamb every 7 to 9 months, but often only lamb annually because of their seasonal breeding pattern, with low fertility in spring and summer. Identification of less-seasonal ewe types would enhance efficiency. Under extensive conditions, minimization of production costs is critical, and there is interest in development of so-called easy-care ewe types. Response: a line of sheep with an extended breeding season was developed at Virginia Tech and their suitability in accelerated lambing was assessed by evaluating their ability to rebreed while lactating in March and April. For most ewe types, fertility at this time is near zero, but over 70 percent of our selected ewes conceived while lactating in spring. Two hair sheep types, the
Katahdin and Dorper, were also assessed in annual lambing, low-input production. Productivity of Katahdin and crossbred Dorper ewes was compared to that of crossbred Dorset ewes over 4 years. Katahdin ewes weaned 26 percent more lambs that ewes of the other two types and produced over 20 percent more kilograms of lamb. Results: these experiments identified genetic resources that can be used to optimize ewe performance in both intensive accelerated and extensive low-input production. Different genetic types are clearly required for different systems, and unique resources were identified.
Publications
- Vanimisetti, H. B. and D. R. Notter. 2006. Predicting genetic merit for total weight of lamb weaned. Proc. 8th World Congr. Genet. Appl. Livest. Prod. CD-ROM communication no. 04-09.
- MacKinnon, K. M., J. L. Burton, and D. R. Notter. 2006. Microarray analysis of abomasal tissues of hair and wool sheep to identify genes associated with resistance to the gastrointestinal parasite, Haemonchus contortus. Proc. 2nd Intern. Symp. Anim. Functional Genomics, East Lansing, MI, CD-ROM communication no. P28.
- Notter, D. R. 2005. Potential of hair sheep in the U.S. Proc. North American Hair Sheep Symposium, San Angelo, TX.
- Kuehn, L. A. 2005. Implications of connectedness in the genetic evaluation of livestock. Ph.D. Dissertation.
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Progress 10/01/04 to 09/30/05
Outputs
Improvement of ewe productivity, defined as total weight of lamb weaned per ewe lambing, is an important goal in most sheep breeding programs. Methods to improve ewe productivity have been assessed under experimental conditions, but extension of those methods to industry flocks participating in national genetic evaluation programs has been difficult, mainly because of incomplete or inconsistent reporting of lamb survival and weaning weights. An alternative to direct evaluation of ewe productivity is the use of selection index methods to derive expected progeny differences (EPD) for ewe productivity from EPD for component or associated traits. This approach does not require reporting of all component traits. Data from 26 Katahdin flocks participating in the U.S. National Sheep Improvement Program were therefore used in a genetic analysis of the following ewe productivity traits: (1) number of lambs born per ewe lambing; (2) number of lambs weaned per ewe lambing; (3)
average weaning weight of lambs weaned; and (4) total litter weight weaned. After editing, 3,214 ewe records were available for numbers born and weaned, and 2,657 records were available for average and total weight weaned. Records were from 1,626 breeding ewes that were offspring of 236 sires and were mated to 131 sires. Percentages of single, twin, triplet, and quadruplet litters at birth were 34.1, 52.0, 13.1, and 0.7 percent, respectively, and 4.2, 41.4, 48.8, and 5.5 percent of ewes weaned 0, 1, 2, or 3 lambs, respectively. Means were 1.80 for number of lambs born, 1.56 for number of lambs weaned, 21.8 kg for average lamb weaning weight, and 27.8 kg for total weight of lamb weaned. The four traits were initially analyzed together to estimate heritabilities and genetic correlations. Heritability estimates were 0.13 for number born, 0.10 for number weaned, and 0.14 for both average and total lamb weaning weights. All traits were positively intercorrelated, indicating no genetic
antagonisms among components of ewe productivity. Total weight of lamb weaned had a very high genetic correlation (0.94) with number of lambs weaned and a substantial correlation (0.77) with average lamb weaning weight, but only a modest genetic correlation (0.42) with number of lambs born. The genetic correlation between number born and number weaned was surprisingly low (0.16) but consistent with observed frequencies for numbers of lambs born and raised; only 40 percent of the triplet litters were weaned as triplets and no quadruplet litters were weaned as quadruplets. These results suggest that EPD for ewe productivity can be accurately predicted from information on number born, number weaned, and average lamb weaning weight. To test this assertion, EPD and ewe producing abilities (PA) were predicted for 1,068 breeding ewes and 301 sires using a multiple-trait model including only these three traits. EPD for ewe productivity ranged from -3.5 to 4.5 kg per ewe and PA ranged from
-7.5 to 8.7 kg. Even though heritabilities for these traits were relatively low, variation in ewe productivity traits was large, leading to large differences in resulting EPD.
Impacts
These results provide a strategy for genetic improvement of ewe productivity in commercial flocks participating in industry performance recording and genetic evaluation programs. Ewe productivity, defined here as the total weight of lamb weaned per ewe lambing, is an economically important trait but can be difficult to assess in industry recording programs due to inconsistent or incomplete recording and reporting of lamb survival and weaning weights. The approach taken here allows prediction of genetic merit for ewe productivity from three component traits: number of lambs born, number of lambs weaned, and average lamb weaning weights. The contribution of this work is to allow for missing information on one or two of these traits, with prediction of genetic merit for ewe productivity based on remaining measures. This strategy allows for breed-wide prediction of genetic merit for ewe productivity without concerns regarding missing data and associated potential for bias
in resulting predictions of breeding values.
Publications
- Notter, D. R. and N. E. Cockett. 2005. Opportunities for detection and use of QTL influencing seasonal reproduction in sheep: a review. Genet. Sel. Evol. 37(Suppl. 1):39-53.
- Notter, D. R., R. C. Borg, and L. A. Kuehn. 2005. Adjustment of lamb birth and weaning weights for continuous effects of ewe age. Anim. Sci. 80:241-248.
- Duckett, S. K., S. P. Greiner, and D. R. Notter. 2005. Cholesterol, CLA and fat content of lamb loin chops by breed type. J. Anim. Sci. 83(Suppl. 1):64 (Abstr.).
- Vanimisetti, H. B., D. R. Notter, and L. A. Kuehn. 2005. Genetic (co)variance components for ewe productivity traits in Katahdin sheep. J. Anim. Sci. 83(Suppl. 1):102 (Abstr.).
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Progress 10/01/03 to 09/30/04
Outputs
Selection to improve fertility in fall lambing began in 1988 in a flock of 50 percent Dorset, 25 percent Rambouillet, and 25 percent Finnsheep breeding and ended after the fall, 1998 lambing. After 10 years of selection, mean fertility of adult ewes had increased from about 60 percent in 1989 to around 87 percent in 1998. Current experiments focus on characterization of seasonal reproductive performance in selected animals. Over 2 yr (2003 and 2004), 71 ewes (61 from the selection line and 10 purebred Dorsets) that lambed in January were evaluated for ability to cycle in March while lactating. Dorset ewes were represented only in 2004. Ewes were randomly assigned to a control group or a group that would be separated from their lambs for 72 h at the time of ram introduction. In early March, two blood samples were collected 7 to 10 days apart from each ewe and assayed for progesterone to identify ewes that had an active CL. In mid-March, all ewes began a 5-d regimen of
twice-daily subcutaneous injections of 25 mg of progesterone to attempt to stimulate expression of estrus in ewes that ovulated. Ewes in the lamb-separation group were removed from their lambs the next morning; two fertile rams were placed with each treatment group on that afternoon. Ewes and lambs from the two treatment groups, and all four breeding rams, were combined 3 d after lamb separation. Blood samples were collected thrice weekly for 5 wk beginning the day after lamb separation. Rams were equipped with marking harnesses, and marks were recorded at each bleeding date. Rams were removed in late April. Ewes were an average of 64 d postpartum at the start of breeding and 104 d postpartum at the end of breeding. Results were consistent across years. No differences in mating behavior were observed between ewes that were separated from their lambs and control ewes. Over 70 percent of the ewes had elevated progesterone levels suggestive of ovulation prior to the start of the
progesterone treatments. An average of 85 percent of the ewes in each treatment group mated within 2 weeks of ram introduction; an additional 6 percent mated over the next 3 weeks. Ultrasonic pregnancy diagnosis in early June indicated that 85 percent of the ewes were pregnant. Actual lambing rates in August were less encouraging than results of June pregnancy tests in year 1 when only 65 percent of the ewes from the selection line lambed, despite an ultrasonically determined June pregnancy rate of 77 percent. However, in year 2, 95 percent of the ewes lambed, yielding no indication of lamb losses during gestation. Dorset ewes in this flock are normally mated in April to lamb in September and generally have lower fertility rates than select-line ewes mated in May. However, these Dorset ewes gave no indication of poorer fertility while lactating compared to crossbred ewes. The performance of these Dorset ewes raises several questions about whether the somewhat mediocre fall-lambing
performance of Dorset ewes in our flock is related to lamb losses in late pregnancy, or could perhaps be improved by running Dorset ewes with either crossbred teaser rams or cycling crossbred ewes.
Impacts
This research documents that selection can be used to develop ewes that are capable of rebreeding while lactating in spring. Access to such animals is required to make systems of accelerated lambing (e.g., the production of three lambs crops every 2 years) practical. The current results also further documents that ewes in this selection line appear to have a strongly abbreviated seasonal anestrus and may serve as a model for further study of genetic factors controlling seasonality in sheep and other specied. Within-flock selection is thus a useful stragegy for both extending the breeding season and for development of animals adapted to accelerated lambing systems.
Publications
- Notter, D. R. and N. E. Cockett. 2003. Opportunities for detection and use of QTL influencing seasonal reproduction. Proc. Intern. Workshop on Major Genes and QTL in Sheep and Goats, Toulouse, France, December 8-11, CD-ROM communication no. 2-08 (8 pp.).
- Vanimisetti, H. B., S. P. Greiner, A. M. Zajac, and D. R. Notter. 2004. Performance of hair sheep composite breeds: resistance of lambs to Haemonchus contortus. J. Anim. Sci. 82:595-604.
- Notter, D. R., S. P. Greiner, and M. L. Wahlberg. 2004. Growth and carcass characteristics of lambs sired by Dorper and Dorset sires. J. Anim. Sci. 82:1323-1328.
- Vanimisetti, H. B., S. L. Andrew, D. R. Notter, and A. M. Zajac. 2004. Inheritance of fecal egg count and packed cell volume and their relationship with production traits in sheep infected with Haemonchus contortus. J. Anim. Sci. 82:1602-1611.
- Notter, D. R. 2004. Genetic evaluation opportunities for small ruminants. J. Anim. Sci. 82(Suppl. 1):27 (Abstr.).
- Borg, R. C., D. R. Notter, R. W. Kott, and L. A. Kuehn. 2004. Developing breeding objectives for Targhee sheep. J. Anim. Sci. 82(Suppl. 1):391 (Abstr.).
- Greiner, S. P., S. K. Duckett, and D. R. Notter. 2004. Shear force and sensory attributes of lamb from hair sheep composite breeds. J. Anim. Sci. 82(Suppl. 1):311-312 (Abstr.).
- Borg, R. C. 2004. Developing breeding objectives for Targhee sheep. M. S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg.
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Progress 10/01/02 to 09/30/03
Outputs
Selection for fertility in spring matings is continuing in a flock of 125 crossbred (50 percent Dorset, 25 percent Rambouillet, 25 percent Finnsheep) ewes. Additional experiments are being carried out to determine the changes in the timing and duration of the seasonal anestrus in selected animals. For 2003, 59 ewes were evaluated for ability to cycle in March while lactating. Ewes lambed in early January. Jugular blood samples from all ewes were collected on February 27 and March 10 and assayed for progesterone to identify ewes that appeared to have an active CL at this time. Ewes were stratified by lambing date and age and assigned to a control group or a group that would be separated from their lambs for 72 h at the time of ram introduction. On March 12, all ewes began a 5-day regimen of twice-daily subcutaneous injections of 25 mg of progesterone dissolved in corn oil. Ewes in the lamb separation group were removed from their lambs at the end of the 5 days, and two
fertile rams were introduced to ewes in each treatment group on that afternoon. Ewes and lambs from the two treatment groups, as well as the four breeding rams, were recombined on the morning of March 20. Jugular blood samples were collected thrice weekly (Tuesday, Thursday, and Saturday) from all ewes for 5 wk beginning March 18. Rams were equipped with marking harnesses and marks were recorded at each bleeding date. Rams were removed on April 22. Ewes were thus an average of 70 d postpartum at the start of breeding and an average of 106 d postpartum at the end of breeding. Over 70 percent of the ewes exhibited elevated progesterone levels suggestive of ovulation in late February or early March at an average of 52 to 63 d postpartum. No differences in mating behavior were observed between treatment groups: 80 percent of ewes in the lamb separation group and 84 percent of ewes in the control group mated between March 17 and April 1. Only one additional ewe mated between April 1 and
April 22. These results suggest that most ewes exposed to rams at this time either mate promptly or do not mate at all. It may be that many ewes from this flock are not anestrus prior to April 1, even when lactating. Alternatively, ewes may be responding to ram introduction but do not continue to cycle after the initial ovulations. Pregnancy rates, based on observed progesterone profiles and incidence of mating, were nearly identical for the two treatment, averaging 77 percent. In other experiments, 124 Suffolk, hair sheep, and Suffolk x hair sheep crosses were mated in autumn 2003 to investigate the inheritance of fiber type in hair x wool crossbred sheep and to assess the potential to develop productive, wool-free sheep populations. Finally, a flock of approximately 150 Dorset crossbred, Dorper crossbred, and Katahdin ewes were mated to Suffolk sires to evaluate the maternal performance of hairsheep crossbred ewes.
Impacts
This research documents that selection can be used to develop ewes that are capable of rebreeding while lactating in spring. Access to such animals is required to make systems of accelerated lambing (e.g., the production of three lambs crops every 2 years) practical. The current results also further documents that ewes in this selection line appear to have a strongly abbreviated seasonal anestrus and may serve as a model for further study of genetic factors controlling seasonality in sheep and other specied. Testing of various hair sheep crossbred types will provide information on strategies to develop "easy-care" sheep germplasm in order to reduce costs of lamb production.
Publications
- Notter, D. R., Andrew, S. A., and Zajac, A. M. 2003. Responses of hair and wool sheep to a single fixed dose of infective larvae of Haemonchus contortis. Small Ruminant Res. 47:221-225.
- Notter, D. R., Cockett, N. E., and Hadfield, T. S. 2003. Evaluation of melatonin receptor 1a as a candidate gene influencing reproduction in a fall-lambing sheep flock. J. Anim. Sci. 81:912-917.
- MacKinnon, K. M., Kuehn, L. A., and Notter, D. R. 2003. Pedigree analysis of a closed population of crossbred sheep. J. Anim. Sci. 81(Suppl. 1):67 (Abstr.).
- Greiner, S. P., Duckett, S. K., and Notter, D. R. 2003. Effect of breed type on sensory evaluation and fatty acid content of lamb. J. Anim. Sci. 81(Suppl. 1):124 (Abstr.).
- Notter, D. R. 2003. Contributions of hair sheep to meat production in temperate climate countries. In: dos Santos, E. S. and W. H. de Sousa (Eds.), Anais do II Simposio Internacional sobre Caprinos e Ovinos de Corte, EMEPA-PB, Joao Pessoa, Paraiba, Brazil, September 29-October 5, pp. 457-467.
- Vanimisetti, H. B. 2003. Genetics of resistance to Haemonchus contortus infections in sheep. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg.
- MacKinnon, K. M. 2003. Analysis of inbreeding in a closed population of crossbred sheep. M. S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg.
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