Five generations of divergent selection for plasma concentration of insulin-like growth factor-1 (IGF-1) and for 12-week body weight were carried out in mice, including randomly selected control lines for each trait. All lines were replicated once (12 lines in total). Each replicate line consisted of eight male and eight female parents per generation. Litter size was standardized to eight pups at birth. Mass selection was applied in the selected lines and within-family random selection in the control lines. Blood was taken from the orbital sinus of individual mice at 12 weeks of age for IGF-1 assay. Realized heritabilities were 0.10±0.01 for IGF-1 and 0.41 ± 0.02 for 12-week weight. The realized genetic correlation between IGF-1 and 12-week weight was 0.58 ± 0.01, with a phenotypic correlation of 0.38. Although the genetic correlation between IGF-1 and body weight in mice is moderately positive, 12-week weight responded 3.5 times as fast to weight selection as to selection for IGF-1.

A flock of mainly Romney × Coopworth ewes was established to test sires for genetic differences in resistance to ryegrass staggers (RGS). This disease is a neurotoxic condition caused by ingestion of endophyte-infected perennial ryegrass containing the mycotoxin, lolitrem B. Lambs, 18-month males and females, and ewes of all ages were scored from January to March each year for RGS (0 or 1; 0 = no staggers), while grazing toxic pastures during natural outbreaks of RGS. Over 6 years (the autumns of 1988-93) and over all stock classes scored, age-group means for RGS ranged from 0 to 0.16. A breeding value for RGS was calculated for each animal, incorporating data from all years and all age groups, provided that the mean year x age group score was at least 0.04. RGS data were also recorded on another Romney flock managed at the same site over the same time period. The heritability of RGS score over both flocks was 0.07 ± 0.02 and the repeatability over years was 0.24 ± 0.04. In March 1993, 170 ewes were allocated to a resistant (R) or susceptible (S) line according to their breeding values, for mating respectively to three elite R or three elite S rams (also selected on breeding values for RGS). The difference in breeding value for RGS score (S flock minus R flock means) for the 133 lambs born in the 1993 crop was 0.081 ± 0.005 (P < 0.001), A94097Received 30 November 1994; accepted 24 March 1995 corresponding to an expected difference between lines of 8.1 % in the percentage of lambs staggering; the observed difference, however, was not significant, but selection is continuing. A positive correlation (0.31) was obtained between breeding values for RGS and breeding values for another mycotoxic disease, facial eczema. These results suggest that it should be possible to change the degree of RGS susceptibility in sheep by selection using progeny-testing under field conditions.

No abstract

Romney sheep have been evaluated for resistance to facial eczema (FE) using a process which involves challenge with the FE toxin, sporidesmin, and they have been bred in selection lines for increased resistance (R) or susceptibility (S) to FE. There is evidence that sporidesmin exerts its toxic effects by generating reactive oxygen species, and protection can be afforded by a number of antioxidants and antioxidant enzymes. Our objective was to summarise three separate experiments to determine whether the R and S lines differed in antioxidant mechanisms, in search of a non-invasive genetic marker. Lines were compared for the activities of four enzymes in blood in Experiment 1, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), and glutathione reductase (GR), and for the concentration of the tri-peptide thiol, glutathione (GSH). SOD, CAT, and GSH were also recorded in Experiment 2, and GPX alone in Experiment 3. Heritabilities were estimated for SOD, A03050 Online publication date 27 May 2004 Received 11 September 2003; accepted 18 February 2004CAT, and GSH. SOD activity was lower and CAT activity was higher in the R than in the S line (P < 0.01 for both enzymes). GPX activity was higher in the R than in the S line in Experiment 1 (P = 0.002) and Experiment 3 (P = 0.06), whilst neither GR activity nor GSH concentration differed significantly between lines. Heritability estimates for SOD, CAT, and GSH were 0.19 ± 0.11, 0.63 ± 0.14, and 0.34 ± 0.14, respectively. It is concluded that selection for divergence in sensitivity to sporidesmin in sheep may have been successful partly because of correlated responses in activities of three antioxidant enzymes (SOD, CAT, and GPX), capable of scavenging reactive oxygen species. However, the divergence between resistant and susceptible sheep in blood antioxidant activity was not large enough to be a reliable indicator of individual FE resistance.

Angus (A) and Hereford (H) cows at one location and A cows at a second location were used to generate purebred controls and six first-cross (FI) types of calf. Foundation sires were A, H, Friesian (F), Jersey (J), and South Devon (Sd) at Location 1, generating FH, FA. JA, SdA, and HA (plus AH) FI calves, and at Location 2 sires were A and Blonde d' Aquitaine (Ba), generating BaA Fis. Sire breeds were selected as potentially contributing to productive crossbred cows, based on previous FI cow comparisons. In subsequent years F2, F3, and F4 calves were produced from each of the two-breed crosses, with overlapping generations and contemporary controls providing balanced comparisons of breed types and generations. The experiment consisted of 395 unique sires and 5243 calves born in 1973-88 at Location 1 and 105 different sires and 2404 calves born in 1976-86 at Location 2. Growth, reproductioQ, maternal, and carcass traits were studied. Heterosis was estimated from HA (plus AH) and purebred A and H animals, but only breed x A 93009 generation effects were estimated from the other two-breed crosses because of the absence of reciprocal FI matings. Heterosis from HA crosses was 3.5 and 6.3% for direct effects on weaning and yearling weights respectively, and 9.0% for the maternal (i.e. Fl cow) effect on weaning weight. For reproduction of FI HA cows, heterosis was 15.2% for calf crop weaned per cow joined, 25.4% for productivity (calf weaning weight per cow joined), and 18.1 % for an efficiency ratio (i.e. productivity/ average cow weight). Over all six two-breed crosses, relative to purebred controls, advantages of Fis averaged 4.4 and 8.1 % for direct effects on weaning and yearling weights respectively, and 13.3% for the . maternal (FI cow) effect on weaning weight. For reproduction, the FI cow advantage over controls averaged 9.7% for calf crop weaned per cow joined (range -7.3 to +15.2%), 24.7% for productivity (range -8.9 to 47.3%), and 16.8% for the efficiency ratio (range -15.7 to +36.3%). For the efficiency ratio, breed types were ranked FH (greatest), JA, FA, HA, SdA, A, H, and BaA. On average, the F3 and F4 calves out-perfonned the Fls by 6.3% in weaning weight but were similar to Fls in yearling weight, whereas for maternal weaning weight the means for F3 cows' calves were 3.4% below those for FI cows. For reproduction, averages of F3s were below those of Fls but superior to the purebreds by 7.2, 17.7, and 16.3% for calves weaned per cow joined, productivity, and the efficiency ratio respectively. There were also significant breed-group differences in carcass composition.