The relationship between level of sward height reduction in a rotationally grazed sward and short-term intake rates of dairy cows

The objective of this study was to investigate the relationship between level of sward height reduction (SHR) and short-term intake of herbage by lactating dairy cows offered swards differing in initial bulk density (BD). Three experiments were carried out in which cows were presented with swards representing different levels of SHR (nominally described as ungrazed, low, moderate and high). Experiments 1 and 2 differed with respect to initial sward BD [ungrazed sward 1·7 vs. 2·5 kg dry matter (DM) m^?3 respectively]. Experiment 3 investigated the interaction between BD and SHR. In each experiment, sixteen Holstein/Friesian cows (fitted with excreta collection bags) grazed for a 1-h period in 200-m² plots that had been grazed on the previous day to predetermined sward surface heights (SSH) (= levels of SHR). Herbage intake rates were assessed from changes in live weight, with a correction for insensible weight loss (IWL). Biting rates were recorded from visual observation. High levels of SHR were associated with a significant reduction in SSH, herbage mass and leaf fraction, and a significant increase in sward BD, stem and dead fraction, and DM concentration. Herbage intake, expressed either as DM intake per bite or DM intake per hour, declined as level of SHR increased from low to high. The level of SHR generally had no effect on biting rate. Intake rates varied from 1·9 to 4·4 kg DM h^?1, whereas DM intake bite^?1 ranged from 0·5 to 1·3 g. Pooled regression analysis identified SSH (P < 0·001; r² = 0·94) as the principal determinant of DM intake bite^?1. The regression equation was not significantly improved by the addition of terms for leaf fraction, BD, or herbage mass. In Experiment 3, a significant interaction between level of SHR and sward BD was observed. It is concluded that the principal factor controlling intake (g DM bite^?1 or kg DM h^?1), as swards are progressively grazed down, is SSH, but at a high level of SHR, sward BD also influences intake bite^?1.     

The effect of sward structure as influenced by ryegrass genotype on bite dimensions and short-term intake rate by dairy cows

Abstract 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.