SELECTION IN HEREFORD CATTLE I. SELECTION INTENSITY, GENERATION INTERVAL AND INDEXES IN RETROSPECT

Selection intensity and generation interval were evaluated in a Hereford cattle herd of 14 inbred lines and 14 linecross groups corresponding to the lines of inbred sire at the San Juan Basin Research Centre, Hesperus, Colorado. Selection indexes practised were calculated in retrospect. The records analysed were weaning weight and postweaning traits in males and females collected from 1946 through 1973. Analyses were performed by line for the inbreds and pooled analyses for the inbred and linecross populations. From records of 1,239 calves weaned, age of sire averaged 3.75 years compared with 4.52 years for age of dam, showing faster generation turnover for sires than for dams. Generation interval determined 98 actual age of midparent was 4.13 years.Selection applied, evaluated as annual selection differentials within inbred lines and then pooled over all lines, averaged .55 standard deviations per generation for sires for weaning weight. Selection of females was much less. Midparent selection differential amounted to .33 standard deviations per generation. Pooled standardized selection differentials per generation over all lines for sires were .49, .46, 40, -.20, -.10, and .69, respectively, for initial weight, final weight, feed consumption, unadjusted feed efficiency, adjusted feed efficiency, and average daily gain. Selection of females for postweaning traits was not intense. Selection Indexes actually practiced in retrospect were: for sires, IS = .4661(WW) -.0092(FE) + .6126(ADG); for inbred dams, ID = .1824(WW) -.0284 (12W) + .0736 (18W) - 1097 (SPW) -.1097 (FAW); for linecross dams, ID= .2693 (WW) - 2960 (12W) + .0147 (18W) + 1185 (SPW) -.0354 (FAW). The corresponding index selection differentials were .818, 203, and .209. Sire index selection differentials represent about 79 percent of the total selection differentials.

Comparing the efficiency of base and multiplicative selection indices for yield and quality traits in Cassava

The efficiency of two selection indices; base index and the multiplicative index was compared to determine the effectiveness of each in identifying superior genotypes in cassava (Manihot esculent Crantz) population. Genotypic data on various yield and quality traits among 570 cassava genotypes were used to construct these selection indices. The efficiency of these indices was compared by determining the performance means of each genotype for studied traits and computing selection differentials of each index. Best linear unbiased predictions (BLUPs) for dry matter content and harvest index were also used for comparison of the two selection indices. The base index produced high-performance means of all five studied traits and proved to be more efficient compared to the multiplicative index in the improvement of cassava genotypes. In addition, the base index produced high selection differentials for three out of five studied traits. It was concluded that the base index is useful in cassava germplasm selection

Fluctuating selection across years and phenotypic variation in food-deceptive orchids

Nectarless flowers that deceive pollinators offer an opportunity to study asymmetric plant-insect interactions. Orchids are a widely used model for studying these interactions because they encompass several thousand species adopting deceptive pollination systems. High levels of intra-specific phenotypic variation have been reported in deceptive orchids, suggesting a reduced consistency of pollinator-mediated selection on their floral traits. Nevertheless, several studies report on widespread directional selection mediated by pollinators even in these deceptive orchids. In this study we test the hypothesis that the observed selection can fluctuate across years in strength and direction thus likely contributing to the phenotypic variability of this orchid group. We performed a three-year study estimating selection differentials and selection gradients for nine phenotypic traits involved in insect attraction in two Mediterranean orchid species, namely Orchis mascula and O. pauciflora, both relying on a well-described food-deceptive pollination strategy. We found weak directional selection and marginally significant selection gradients in the two investigated species with significant intra-specific differences in selection differentials across years. Our data do not link this variation with a specific environmental cause, but our results suggest that pollinator-mediated selection in food-deceptive orchids can change in strength and in direction over time. In perennial plants, such as orchids, different selection differentials in the same populations in different flowering seasons can contribute to the maintenance of phenotypic variation often reported in deceptive orchids.

Changes in nutritive characteristics associated with plant height, and nutrient selection by dairy cows grazing four perennial pasture grasses

Previous research has documented nutritive characteristics of perennial ryegrass-based pastures and subsequent nutrient-selection differentials when dairy cows graze such pastures, but there has been little comparable research on alternative pasture grasses. The aim of the present study was to compare the pre-grazing nutritive characteristics of four perennial grasses, how nutrients vary with plant height, and selection differentials achieved by dairy cows grazing these grasses in late winter and late spring. The study utilised an established field experiment, with four replicates of monoculture swards of perennial ryegrass (Lolium perenne L.), tall fescue (Festuca arundinacea Schreb.), cocksfoot (Dactylis glomerata L.) and prairie grass (Bromus willdenowii Kunth.), in western Gippsland, Victoria. Eighty individual tillers per replicate were sampled to ground level immediately pre- and post-grazing in late winter (July–August, vegetative tillers only) and late spring (November–December, vegetative and reproductive tillers sampled separately), dissected into three height categories (0–5 cm, 5–10 cm and 10+ cm) and analysed for nutritive characteristics. For vegetative tillers in both seasons, perennial ryegrass had the highest estimated metabolisable energy concentration and lowest neutral detergent fibre concentration of all species. In spring, reproductive tillers had consistently lower nutritive characteristics than did vegetative tillers. Selection differentials, calculated as the ratio of nutritive characteristics selected by the herd to that available pre-grazing, showed that cows selected herbage with higher crude protein concentration but there was little evidence for selection of higher metabolisable energy concentration. The selection differentials reflected the vertical distribution of nutrients in the tillers. The present results have provided new information to assist in developing grazing guidelines for alternative perennial grasses.

Nutrient selection by dairy cows grazing chicory or perennial ryegrass during spring

Traditional measures of the nutrient concentration of herbage before grazing may not accurately depict the nutrient intake of grazing animals. An experiment was conducted to investigate differences in DM and nutrient intake, and milk production of lactating cows offered chicory- or perennial ryegrass-based diets during spring on a commercial dairy farm in south-west Victoria. This experiment was conducted in mid-lactation (spring), with chicory (Cichorium intybus L.), perennial ryegrass (Lolium perenne L.) and a mixed sward of chicory and perennial ryegrass offered at two daily allowances (nominally 20 and 30 kg DM/cow to ground level). Selection differentials for estimated metabolisable energy (MJ/kg DM), neutral detergent fibre (NDF; g/kg DM) and acid detergent fibre (g/kg DM) varied (P < 0.05) with forage type, while allowance affected selection for estimated metabolisable energy, crude protein, acid detergent fibre and NDF (P < 0.01). However, the differences in selection differentials observed between the chicory, mixed sward and perennial ryegrass under the same conditions indicated that preferential grazing for leaf rather than stem in chicory may affect nutrient selection in a way that differs from perennial grass species, resulting in much lower intakes of NDF than indicated by traditional measurements of feed characteristics.