Factors affecting differences between birth weight of littermates (BWTD) and the effects of BWTD on lamb performance.

The objectives of this study were to determine factors affecting lamb birth weight (BWT) and differences between BWT of the largest and smallest littermates (BWTD) and to assess the relative importance of BWT and BWTD on lamb survival and growth to weaning. Records from twin (n = 5369) or triplet (n = 1664) litters born on pasture were utilised. Breeds included Coopworth, Romney, Perendale, composite and Texel. Ewe and sire breed, age and weight of the dam, sex, and number of lambs born (NLB) affected BWT. Neither embryo loss nor ovulation pattern (i.e. all ova released from one ovary or some from each) were associated with BWT. Dam weight and NLB affected BWTD but dam age, sire or dam breed did not. Loss of an embryo reduced BWTD, and BWTD was lower when the ewe ovulated from both ovaries versus only one ovary for twins, but not triplets. Whereas BWT was moderately heritable (h2 = 0.20), BWTD was not heritable (h2 = 0.003). Lamb survival was positively associated with BWT. Lambs with BWTD > 1.3 kg were less likely to survive (73.3%) than those from litters of smaller BWTD (range 82.8-85.7% survival). Growth rate of twin and triplet lambs was positively associated with BWT. Surprisingly, lambs from litters with moderately high BWTD had slightly greater (approximately 3%) growth rate than those from lambs of low to intermediate BWTD. Thus, while large BWTD were negatively associated with survival, BWT itself, which was moderately heritable, appeared to be a stronger driver of lamb survival and average daily gain.

Identification of a line of sheep carrying a putative autosomal gene increasing ovulation rate in sheep that does not appear to interact with mutations in the transforming growth factor beta superfamily.

Sheep lines with mutations in single genes that have major effects on ovulation rate have been very useful in gaining a better understanding of pathways important in controlling follicular development and ovulation rate. To date however, all known mutations are in the transforming growth factor beta (TGFB) superfamily. Ovulation rates were measured in 720 progeny of 20 rams that were descendants of a single prolific ewe. Evaluation of ovulation rates of daughters of closely related sires suggests the presence of a segregating major gene Fecundity Davisdale (FECD) that increases ovulation rate between 0.4 and 0.8 in heterozygous daughters. Key features of mutations in genes of the TGFB superfamily pathway, such as synergistic interactions with other family members, infertility in homozygous carriers, and increased responsiveness to exogenous gonadotropins, were not observed in this line; thus, the mutation does not appear to be acting in the TGFB pathway. Hence, there is likely a novel mutation being carried in this line of sheep that alters ovulation rate. Future identification of the causative mutation may provide new insights into regulation of follicular development and ovulation rate.

Discovery of quantitative trait loci for resistance to parasitic nematode infection in sheep: I. Analysis of outcross pedigrees.

BACKGROUND: Currently most pastoral farmers rely on anthelmintic drenches to control gastrointestinal parasitic nematodes in sheep. Resistance to anthelmintics is rapidly increasing in nematode populations such that on some farms none of the drench families are now completely effective. It is well established that host resistance to nematode infection is a moderately heritable trait. This study was undertaken to identify regions of the genome, quantitative trait loci (QTL) that contain genes affecting resistance to parasitic nematodes. RESULTS: Rams obtained from crossing nematode parasite resistant and susceptible selection lines were used to derive five large half-sib families comprising between 348 and 101 offspring per sire. Total offspring comprised 940 lambs. Extensive measurements for a range of parasite burden and immune function traits in all offspring allowed each lamb in each pedigree to be ranked for relative resistance to nematode parasites. Initially the 22 most resistant and 22 most susceptible progeny from each pedigree were used in a genome scan that used 203 microsatellite markers spread across all sheep autosomes. This study identified 9 chromosomes with regions showing sufficient linkage to warrant the genotyping of all offspring. After genotyping all offspring with markers covering Chromosomes 1, 3, 4, 5, 8, 12, 13, 22 and 23, the telomeric end of chromosome 8 was identified as having a significant QTL for parasite resistance as measured by the number of Trichostrongylus spp. adults in the abomasum and small intestine at the end of the second parasite challenge. Two further QTL for associated immune function traits of total serum IgE and T. colubiformis specific serum IgG, at the end of the second parasite challenge, were identified on chromosome 23. CONCLUSION: Despite parasite resistance being a moderately heritable trait, this large study was able to identify only a single significant QTL associated with it. The QTL concerned adult parasite burdens at the end of the second parasite challenge when the lambs were approximately 6 months old. Our failure to discover more QTL suggests that most of the genes controlling this trait are of relatively small effect. The large number of suggestive QTL discovered (more than one per family per trait than would be expected by chance) also supports this conclusion.