Genome-wide association studies of female reproduction in tropically adapted beef cattle.

The genetics of reproduction is poorly understood because the heritabilities of traits currently recorded are low. To elucidate the genetics underlying reproduction in beef cattle, we performed a genome-wide association study using the bovine SNP50 chip in 2 tropically adapted beef cattle breeds, Brahman and Tropical Composite. Here we present the results for 3 female reproduction traits: 1) age at puberty, defined as age in days at first observed corpus luteum (CL) after frequent ovarian ultrasound scans (AGECL); 2) the postpartum anestrous interval, measured as the number of days from calving to first ovulation postpartum (first rebreeding interval, PPAI); and 3) the occurrence of the first postpartum ovulation before weaning in the first rebreeding period (PW), defined from PPAI. In addition, correlated traits such as BW, height, serum IGF1 concentration, condition score, and fatness were also examined. In the Brahman and Tropical Composite cattle, 169 [false positive rate (FPR) = 0.262] and 84 (FPR = 0.581) SNP, respectively, were significant (P < 0.001) for AGECL. In Brahman, 41% of these significant markers mapped to a single chromosomal region on BTA14. In Tropical Composites, 16% of these significant markers were located on BTA5. For PPAI, 66 (FPR = 0.67) and 113 (FPR = 0.432) SNP were significant (P < 0.001) in Brahman and Tropical Composite, respectively, whereas for PW, 68 (FPR = 0.64) and 113 (FPR = 0.432) SNP were significant (P < 0.01). In Tropical Composites, the largest concentration of PPAI markers were located on BTA5 [19% (PPAI) and 23% (PW)], and BTA16 [17% (PPAI) and 18% (PW)]. In Brahman cattle, the largest concentration of markers for postpartum anestrus was located on BTA3 (14% for PPAI and PW) and BTA14 (17% PPAI). Very few of the significant markers for female reproduction traits for the Brahman and Tropical Composite breeds were located in the same chromosomal regions. However, fatness and BW traits as well as serum IGF1 concentration were found to be associated with similar genome regions within and between breeds. Clusters of SNP associated with multiple traits were located on BTA14 in Brahman and BTA5 in Tropical Composites.

Effect of nutrition during pregnancy on fetal growth and survival in Fec(B) Booroola x South Australian Merino ewes.

The effect of nutritional manipulation during mid and late pregnancy on fetal growth and survival in high fecundity (Fec(B) Fec(+)) Merino ewes was examined. A total of 4 treatment groups was utilized in each of 3 years (1987, 1988 and 1989); and 2 groups were given low nutrition during mid pregnancy while 2 groups received high nutrition during this period. One group from each of these treatments subsequently received a high protein diet 1 to 2 weeks prior to term. Differential feeding during mid pregnancy resulted in the high and low treatment groups varying by 7.5 and 5.0 kg liveweight at Days 100 and 135 of pregnancy, respectively. Fetal survival was not improved by variation in nutritional intake during mid pregnancy (P>0.05) and neither were placental and fetal growth (P>0.05), as indicated by observations on cotyledon diameter and fetal head width, respectively. However, fetal survival during late pregnancy was reduced by a high level of nutrition during both mid and late pregnancy (P<0.05).

Factors influencing lamb survival in a high fecundity Booroola Merino x South Australian Merino flock.

The effects of year of lambing, age of ewe and litter size on lamb survival and birthweight, and the effects of ewe mating weight and pregnancy wastage (ovulation rate minus litter size) on birthweight were examined in Booroola Merino x South Australian Merino ewes. Year of lambing, litter size and their interaction were significant (P<0.001) sources of variation for lamb survival. When birthweight was included as a linear and quadratic covariate for lamb survival, year of lambing and litter size and their interaction remained as statistically significant sources of variation. Year of lambing, litter size and pregnancy wastage contributed significantly (P<0.05) to variation in birthweight. Ewe liveweight at mating was not an important source of variation (P>0.05). Birthweight was significantly (P<0.05) reduced with an increase in pregnancy wastage. Improvement of birthweight of multiple birth lambs has some potential for increasing lamb survival. Other factors influencing lamb survival (year of lambing, litter size, pregnancy wastage) require further study so that strategies for reducing lamb loss in high fecundity Merino flocks can be developed.

Fertilization and embryo loss in Booroola Merino x South Australian Merino ewes: Effect of the F gene.

The effects of Booroola genotype (F+, ++); the number of ovulations per ewe (one, two or three); and the age of a ewe (2.5 yr vs 3.5 to 6.5 yr) on the percentage of ova fertilized, embryo loss and fetal loss were examined in Booroola x South Australian Merino ewes slaughtered on Days 4, 21 and 90 after insemination. Ewes slaughtered on Day 90 were examined by real-time ultrasound imaging (RUI) on Day 45. Fertilization failure was independent of ewe genotype, ovulation rate and age of ewe, and it was not an important source of wastage (F+, 9.4%; ++, 6.7%). Most embryo loss occurred during the first 21 d (F+, 54.7%; ++, 40.3%). Interpretation of the effects of genotype and ovulation rate on embryo wastage measured on Days 21, 45 and 90 was obscured by significant (P < 0.05) genotype and ovulation rate interactions with the day of slaughter/RUI. The effect of age on embryo loss was not significant (P > 0.05). Reasons for the high rate of wastage observed in this experiment require further study.

Uterine presensitization and embryo survival and growth in Booroola Merino x South Australian Merino ewes.

The fertility enhancing effects of semen were examined following the intra-uterine insemination of killed spermatozoa plus seminal plasma 17 d prior to insemination with viable spermatozoa. Three experiments were conducted: two on 1.5-yr old and 2.5 to 5.5 yr-old Booroola Merino x South Australian Merino ewes in 1986 and one on 1.5 yr-old ewes in 1987. Differences between treatment and control groups for the percentage of ewes exhibiting estrus by Days 21 and 35 following fertile insemination, the percentage of ewes with viable embryos at Day 35, the number and weight of viable embryos per ewe, the nubmer of caruncular implantation sites and the progesterone level were not statistically significant (P>0.05). There were no statistically significant treatment by experiment interactions for any of the variables examined. Inflammation and edema of the endometrial tissue was not observed following the presensitization treatment.

The use of physiological traits in genetic selection for litter size in sheep.

The objective was to quantify and compare the genetic responses by direct selection on litter size, by indirect selection through a physiological trait and by combined selection, combining litter size and the physiological trait in a selection index. Three kinds of physiological trait were considered, male sex-limited (e.g. testes size), female sex-limited (ovulation rate) and traits measurable in both sexes (gonadotrophin levels). The results are presented graphically and cover a wide range of possible situations and show the size of the responses for different parameters of the physiological trait. There is usually scope for improvement in the rate of response with combined selection, and also in special cases (high heritability and genetic correlation) with indirect selection. The increases in predicted response may range from zero to two or three times the direct response, depending on the genetic parameters. However, the need for reliable estimates of the genetic parameters is stressed, because the predicted responses might otherwise be overestimated and the selection effort misplaced.