1998). Effect of duration of fasting period on short-term intake rates of lactating dairy cows. AbstractThe effects of duration of fasting on the short-term feeding behaviour of 12 grazing and 12 silage-fed lactating Holstein-Friesian dairy cows were examined. Four groups of three cows were rotated around fasting treatments of 1, 3, 6 or 13 h following a balanced Latin-square design. Herbage intakes for each treatment group were assessed over al-h period. As intended there were no significant differences in sward characteristics between the experimental plots grazed by cows from different treatment groups. However, total dry-matter (DM) intake, biting rate and DM intake per bite, measured over the 1-h grazing period, increased significantly when the duration of fasting was extended from 1 to 6 or 13 h (P < 0-05). There were only minor differences in grazing behaviour following fasting durations ofl and 3, or 6 and 13 h. In a parallel study, undertaken to assess the influence of fasting duration on appetite independently of grazing, four groups of three cows were housed indoors and offered 30 kg of a high quality grass silage. Silage DM intakes, measured over a 1-h period, increased significantly with extended fasting periods (P < 0-01), though silage DM intake was considerably lower than that of grazing cows for each fasting treatment. These results suggest that dairy cows grazing on good quality swards may be able to compensate for increased degree of hunger by increasing both biting rate and DM intake per bite to increase DM intake rate. Although the DM intakes of silage and grazed grass followed similar patterns of increasing intake with extended fasting duration, DM intake rates were considerably higher in grazing cows for each fasting treatment.

Perennial ryegrass (Lolium perenne L.) is by far the most widely sown grass species in Ireland. Genotype · environment (G · E) interactions are a frequent occurrence in herbage yield evaluations. The objectives were to determine (i) the nature and relative magnitudes of the pertinent G · E interaction variance components for dry matter yield of perennial ryegrass sward plots in Ireland and (ii) the optimal allocation of replicates, locations and years in a testing programme. Sixteen perennial ryegrass cultivars were sown at six locations throughout Ireland between 2000 and 2004. Plots from each sowing were harvested for 2 consecutive years under a simulated mixed grazing and conservation management programme. The largest component of the G · E variance was generally genotype · location · year emphasizing the need for evaluation of genotypes across locations and years to adequately characterize genotypes for differences in yield. Relative differences among genotypes from year to year and location to location were due mainly to changes in genotype rankings. Weather was estimated to have a greater effect on annual variation in herbage yield than age of stand. The optimum allocation of resources for a testing programme was estimated at four replicates per location, and either two locations and 3 sowing years or three locations and 2 sowing years with 2 harvest years for each sowing year. The most appropriate option depends on the relative importance of time vs. financial resources.

In some European countries, the majority of annual enteric methane (CH 4 ) emissions by ruminants occur at pasture -a direct result of the predominance of grazing within ruminant production systems. However, there are only limited data available as to the effect of perennial ryegrass cultivar and season of harvest on CH 4 production. Using the in vitro gas production technique, the effect of perennial ryegrass cultivar on fermentation characteristics and CH 4 production was determined (Experiment 1) and the persistence of these traits throughout the growing season for two cultivars, identified from Experiment 1 as having either a high or low methanogenic potential, was examined (Experiment 2). In Experiment 1, organic matter (OM) digestibility and cumulative total gas production profiles were unaffected by cultivar but, with regard to the kinetics of CH 4 production, the asymptote value (A), cumulative CH 4 yield at 72 h, and the fractional rate (l) of CH 4 production at both time of 0AE5A(T 1 2 )(lCH 4 T 1 2 ) and at 48 h (lCH 4 48h) were significantly (P < 0AE05) different. The amount of digested OM, as a proportion of cumulative CH 4 production (DigOM/CH 4 ) at 24 and 72 h after commencement of inoculation, revealed that the amount of substrate required to produce 1 ml of CH 4 also differed significantly between cultivars (P < 0AE01). In Experiment 2, regrowth number significantly modified the majority of measured samples (P < 0AE01); cultivar effects were limited to the lag phase of the cumulative CH 4 production curve and DigOM/ CH 4 at 8 h only (P < 0AE05). These results suggest that differences exist between cultivars in how OM is partitioned following microbial fermentation and that these differences demonstrate persistency throughout the growing season. In the course of time it may be possible to exploit these differences through cultivar selection and plant breeding programmes, and thereby reduce enteric CH 4 emissions within pastoral production systems.

Position in and contribution of leaf laminae to the canopy of forage grasses are important both in determining herbage growth rates and intake rate by grazing animals. These canopy characteristics are controlled by the way dry matter is apportioned between sheath and lamina in growing leaves. The objective of this work was to determine how the development of individual leaves is affected by altering the effective length of the psuedostem tube, on the assumption that the light environment within the tube varied. The development of a leaf from initiation at the apex to maturity was followed by successive destructive dissections of tillers. Vertical incisions were made in the pseudostem of each tiller to three different depths. The three treatments imposed wereno incision (control), moderate and severe incision of the sheath length. Destructive harvests of tillers followed 3, 6, 12 and 24 days after imposition of treatments. Incision resulted in the length of the monitored leaf being reduced significantly at all harvests, and differentiation of the sheath beginning earlier. The length reduction reflected a reduction in both cell size and cell number and the effects were evident at the earliest harvest. The data support the theory that leaf size and timing of onset of sheath development are influenced by the environment of the developing leaf. The present results indicate that sheath tube length affects leaf development and suggests that the effects are substantially explained by a direct light effect on the location and depth of the elongation zone.

The effects of genotypic variation in ryegrasses on sward structure, bite dimensions and intake rate by dairy cows were investigated. Two experiments were conducted. In Experiment 1, swards were in a vegetative state whereas, in Experiment 2, they were partly reproductive and were taller with higher herbage mass but lower leaf proportion than in Experiment 1. Applicability of relationships between sward structure and bite characteristics, previously established from artificial or hand‐constructed swards, to field conditions were tested. Additional short‐term intake rates and/or sward structural characteristics were considered as indicators of potential intake for use in protocols for the evaluation of grass varieties. Four cultivars were studied: AberElan, Twins (diploid and tetraploid perennial ryegrasses respectively), Polly, a hybrid ryegrass (perennial × Italian ryegrass) and Multimo (Italian ryegrass), each established in 200‐m2 plots in four replicated blocks. Herbage intake rate was determined by short‐term liveweight change (taking account of insensible weight loss) using 16 dairy cows allocated to four balanced groups with each plot grazed by one group for a 1‐h assessment period. One block was grazed per day, over a 4‐d experimental period, with each group grazing each variety in a complete crossover design. Sward characteristics and bite rate were also measured in both experiments. Bite dimensions were subsequently estimated, with bite depth being determined as a function of extended tiller height (ETH) in both experiments. Within both experiments, bite mass and intake rate did not differ significantly between swards of different cultivars despite swards containing Multimo generally having a higher ETH and water‐soluble carbohydrate concentration and lower green leaf mass, sward bulk density and neutral‐detergent fibre concentration than the other swards. However, bite depth was significantly higher (P < 0·01) in swards containing Multimo swards than in the others and, in Experiment 1, bite depth, as a proportion of ETH, was higher in swards containing Multimo and lower in those containing Twins than in the other two cultivars, whereas there was no difference in Experiment 2. Taking both experiments together, the mean bite depth was 0·5 of ETH with sward bulk density accounting for almost half the variance in the relationship between bite depth and ETH. The bulk density of the bite (bite mass per unit bite volume), measured in Experiment 2, followed a similar pattern to sward bulk density, increasing in the order Multimo, Polly, AberElan and Twins. It is concluded that the relationships between sward characteristics and bite dimensions, derived from artificial swards, are applicable to field swards, although the range in natural ryegrass sward characteristics is usually not as wide as in experiments using artificial swards. Lack of precision in the measurement of short‐term intake and in sward‐based measurements is likely to preclude their use in the evaluation of grass varieties.