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 1á7 vs. 2á5 kg dry matter (DM) m A3 respectively]. Experiment 3 investigated the interaction between BD and SHR.In each experiment, sixteen Holstein/Friesian cows (®tted with excreta collection bags) grazed for a 1-h period in 200-m 2 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 signi®cant reduction in SSH, herbage mass and leaf fraction, and a signi®cant 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 1á9 to 4á4 kg DM h A1 , whereas DM intake bite A1 ranged from 0á5 to 1á3 g. Pooled regression analysis identi®ed SSH (P < 0á001; r 2 0á94) as the principal determinant of DM intake bite A1 . The regression equation was not signi®cantly improved by the addition of terms for leaf fraction, BD, or herbage mass. In Experiment 3, a signi®cant interaction between level of SHR and sward BD was observed. It is concluded that the principal factor controlling intake (g DM bite A1 or kg DM h A1 ), as swards are progressively grazed down, is SSH, but at a high level of SHR, sward BD also in¯uences intake bite A1 .

A full lactation study compared the performance of autumn‐calving dairy cows of high genetic merit under two contrasting systems of milk production: high forage (HF) and high concentrate (HC). During the winter, animals on system HF were offered a silage with a high feeding value characteristics, supplemented with 5·5 kg of concentrate [crude protein content of 280 g kg−1 dry matter (DM)] through an out‐of‐parlour feeding system. From 14 March, these animals were given increasing access to grazing, achieving 24‐h turnout on 15 April. Thereafter, until day 305 of lactation, these animals were offered a large daily herbage allowance (23·0 kg grass DM cow−1, measured above a height of 4·0 cm), supplemented with 0·5 kg d−1 of a ‘high‐magnesium’ concentrate. During the winter, animals on system HC were offered a silage of medium feeding value, mixed with ≈14·0 kg of concentrate d−1 (crude protein content of 202 g kg−1 DM) in the form of a complete diet. These animals commenced grazing on 9 April, achieving 24‐h turnout on 18 April. From 18 April until 9 June, daily herbage allowances and concentrate feed levels were 17·0 kg DM and 5·0 kg respectively; thereafter, and until day 305 of lactation, these daily allowances were reduced to 15·0 kg of herbage DM and 4·0 kg of concentrate. Animal performance during the first 305 days of lactation for systems HF and HC, respectively, were as follows: total concentrate DM inputs, 842 and 2456 kg; total silage DM intakes, 2205 and 1527 kg; total grass DM intakes, 3019 and 2044 kg; total feed DM intake, 6061 and 6032 kg and total milk output, 7854 and 8640 kg. Total milk output per cow with system HF was 786 kg lower than for system HC, despite similar total DM intakes, suggesting a greater total nutrient requirement with the former to support a given milk production. However, the study confirms that relatively similar levels of animal performance can be achieved from systems based on very different sources of nutrient supply.

Three experiments were carried out on perennial ryegrass‐dominant swards to provide a basis for recommendations for the limits to (a) building up and timing of utilization of a herbage ‘bank’ for out‐of‐season grazing and (b) duration and intensity of early spring grazing in the United Kingdom and Ireland. In experiment 1, the effect of regrowth interval (from 7 September, 20 October, 17 November or 15 December) in autumn on herbage accumulation, leaf turnover and on subsequent spring growth was investigated. Swards regrown from early September reached maximum herbage mass (about 3 t ha–1 DM) and leaf lamina content in mid‐November, by which time senescence rate exceeded rate of production of new leaves. New leaf production and senescence rates were greater in swards remaining uncut until December than in those cut in October or November. Time of defoliation up to December had no effect on spring herbage mass in the subsequent spring. Defoliating in March reduced herbage mass in late May by less than 20%. Experiment 2 investigated the progress in herbage growth and senescence in swards regrowing from different times in late summer and autumn to produce herbage for utilization beyond the normal grazing season. Treatments in a randomized block design with three replicates were regrowths from 19 July, 8 August, 30 August and 20 September. Based on a lower ceiling of leaf and total herbage mass being reached with progressively later regrowths, beyond which leaf senescence generally exceeded leaf production and herbage mass declined, it was concluded that currently recommended rotation lengths for this period should extend from 3 weeks in late July to 8 weeks for swards previously grazed in mid‐September. In both experiments, leaf senescence commenced earlier (by one leaf‐age category) than previously published estimates and so brought forward the time at which senescence rates balanced leaf growth rates. In experiment 3, designed to evaluate the effect of daily grazing period and intensity in early spring on herbage regrowth, dairy cows grazed successive plots (replicates) for 2 or 4 h each day at two intensities (target residual heights of 5 or 7 cm) in March to mid‐April. Regrowth rate was similar in all treatments including the ungrazed control, despite soil moisture content being relatively high on occasions. Tiller density was significantly reduced in May by grazing plots in early or mid‐April. It is concluded that in autumn there are limits to which rotation lengths should be extended to produce herbage for out‐of‐season grazing owing to attainment of ceiling yields. Although utilization in early spring may reduce herbage availability in spring, out‐of‐season utilization need not reduce herbage growth rates in early spring.

The response of swards which have been previously grazed to N fertilizer applied in early February was studied in two experiments in Northern Ireland. The effect of N fertilizer applied at a range of dates in autumn and spring on swards for out‐of‐season utilization was studied in a further experiment. Deep soil coring was also undertaken, subsequent to grazing with dairy cows, in grazed and protected areas in November and March to investigate the effect of out‐of‐season grazing on soil mineral N levels. Dry‐matter (DM) yield response to early spring N application in previously grazed swards was low, with no effect on DM yield in February or March. Progressively delaying N application (and commencement of herbage accumulation) in autumn from 8 September until 18 October reduced herbage availability in late autumn and early spring but increased leaf lamina content. The greater the amount of herbage accumulated to 1 December, the lower the tiller density in the following April. N fertilizer had a greater impact on soil mineral N in spring than in late autumn/early winter, suggesting that fertilizer N was more prone to loss in the latter. Soil mineral N was not significantly affected by out‐of‐season grazing. It is concluded that in well‐fertilized, previously grazed swards response to N for out‐of‐season herbage is low and the probability for N loss is increased. Herbage quality will decline and the sward may be damaged if about 2 t DM ha−1 or more of harvestable herbage accumulates for use in winter or in early spring.