Pattern of herbage intake rate and bite dimensions of rotationally grazed dairy cows as sward height declines

Transcription

1 Pattern of herbage intake rate and bite dimensions of rotationally grazed dairy cows as sward height declines P. D. Barrett*, A. S. Laidlaw*², C. S. Mayne*³ and H. Christie³ *Department of Applied Plant Science, Queen's University Belfast, UK, ²Applied Plant Science Division, Plant Testing Station, Crossnacreevy, Belfast, UK, and ³Agricultural Research Institute of Northern Ireland, Hillsborough, UK Abstract To allow improved prediction of daily herbage intake of dairy cows in rotational grazing systems, intake behaviour was assessed throughout the day in 24-h paddocks. Herbage intake in 16 lactating Holstein±Friesian cows was assessed using the short-term (1-h) weight gain method at four predetermined natural meal times throughout the day (early morning, T1; late morning, T2; mid-afternoon, T3; and early evening, T4). The study comprised two 4-day experiments, each with a cross-over design of four blocks. In both experiments, cows grazed a 24-h paddock daily, and the effect of the immediately previous grazing experience on intake behaviour was investigated throughout the day, taking account of daily uctuations in the short-term physiological condition of the cows. Experiment 1 was carried out to investigate overall grazing behaviour during meals as a sward is progressively depleted during the day, with intake being assessed within the paddock and, hence, on a depleted sward. Experiment 2 similarly investigated the effect of sward depletion and physiological condition throughout the day on intake, but cows were removed to fresh, undefoliated swards during intake measurement periods; thus, intake rate was not in uenced by differences in sward condition. Intake behaviour from both experiments was compared to establish the effect on herbage intake of changes in sward state and non-sward factors. In Experiment 1, sward surface height, available herbage mass, proportion of leaf and green leaf mass declined as the day Correspondence to: Dr P. D. Barrett, Queen's University Belfast, Department of Applied Plant Science, The Plant Testing Station, 50 Houston Road, Crossnacreevy, Belfast BT6 9SH, UK. p.barrett@qub.ac.uk Received 24 February 2000; revised 28 March 2001 progressed. Bite mass declined with sward depletion, and mean intake rate was 1á64 kg dry matter (DM) h ±1, which was signi cantly lower at T3 (P <0á01) than during other meals. In Experiment 2, plot sward conditions did not change throughout the day, and intake behaviour also remained constant, with a mean intake rate of 2á11 kg DM h ±1. Mean bite depth as a proportion of pregrazing extended tiller height was constant throughout the day (mean 0á32). The results show that, although cows grazed throughout the day on progressively depleted swards, indicative of rotationally grazed paddocks (Experiment 1), bite mass declined linearly and intake behaviour was variable. However, where intake was assessed on high-quality, undefoliated swards (Experiment 2), intake behaviour was similar regardless of the time of day and the immediately previous experience. There was some indication of an interaction between the effects of the sward and the physiological condition of the animal on herbage intake. Keywords: Lolium perenne L., sward canopy structure, short-term herbage intake, sward depletion, animal physiological condition Introduction The main factor limiting milk production from pasture is low herbage intake. Herbage intake can be considered as the result of the interaction between plant, animal and environmental components. Daily dry matter (DM) intake is the product of grazing time, biting rate and bite mass (DM intake per bite) (Spedding et al., 1966). Bite mass, the most critical factor determining intake rate, makes an important contribution to daily herbage intake (Stobbs, 1973; Black and Kenny, 1984; Phillips and Leaver, 1986) and has been shown to be in uenced by sward structure (Hodgson, 1985; Laca et al., 1992; Brereton and McGilloway, 1998). It is important that 362 Ó 2001 Blackwell Science Ltd. Grass and Forage Science, 56, 362±373

2 Herbage intake of dairy cows as sward height declines 363 bite mass is maintained throughout the day in order to achieve a high daily DM intake but, in rotationally grazed paddocks, sward structure changes continually as grazing proceeds. Changes in quantity and quality associated with the depletion of the sward have a detrimental effect on bite mass and intake rate (McGilloway et al., 1999). The physiological state of the animal also has a signi cant in uence on instantaneous and daily intake (Mangel and Clark, 1986; Penning et al., 1998; Gibb et al., 1999; Christie et al., 2000). Overall, the broad physiological condition of the animal is largely xed and is the result of factors such as body weight, parity, stage of lactation and milk yield. However, as the day proceeds, the cow will experience short-term oscillations in appetite as a result of hunger-stimulating and satiety-inducing factors (Baile and McLaughlin, 1987). These have a regulating effect on grazing behaviour, with cattle being motivated to feed by hunger, which is alleviated by satiation (Phillips, 1993). However, the grazing pattern of the cow is also in uenced by other factors such as type of grazing system (Arnold, 1987), including milking times (Rook et al., 1994), herbage availability and quality (Minson, 1987) and extent and timing of supplementary feeding (Sayers, 1999). Therefore, throughout the day, a lactating dairy cow engages in periods of grazing, ruminating and idling with two or more interruptions for milking, resulting in the cow ingesting herbage during a number of grazing bouts or meals. Often, herbage intake studies do not consider or make allowance for such diurnal uctuation in grazing behaviour and, for various reasons, studies are carried out on arti cial or manipulated swards, which may not be representative of grazing conditions throughout the day. Differences in intake rate at different times throughout the day have implications regarding the prediction of daily herbage intake based on short-term intake rate and daily grazing time. Gibb et al. (1998) found that, with continuous stocking, where herbage growth and intake are more or less in steady state, over the course of the day intake rate was variable, despite only slight changes in the sward. Intake was lowest after morning milking and highest after milking in the afternoon. To improve the predictability of daily herbage intake and, hence, develop more ef cient grazing management systems, it is necessary to understand how the cow grazes or adapts its grazing behaviour throughout the day and to changes in sward condition. The objective of this study was to investigate the daily pattern of grazing behaviour of cows grazing paddocks typical of those used in a rotational system, in which fresh paddocks are allocated once daily (i.e. 24-h paddocks), by assessing intake characteristics at each of four meals throughout the day. The study comprised two, 4-day experiments. In Experiment 1, grazing behaviour was investigated in response to time of day and depletion of herbage during grazing of a 24-h paddock, with intake being estimated within the paddock as the sward was depleted. Experiment 2 was similar in design to Experiment 1, except that, for the intake measurement periods, the cows were moved from the paddock, and the depleted sward, to undefoliated swards of fresh regrowth similar to those in the paddock before grazing. Therefore, the intake behaviour of cows in uenced by their immediately previous experience was investigated throughout the day on swards of near constant structure. Grazing behaviour was compared between experiments to determine the effects of both changing sward conditions and shortterm changes in animal physiological condition throughout the day. Materials and methods Evaluation of meal patterns The study was conducted at the Agricultural Research Institute of Northern Ireland (ARINI), Hillsborough (54 27 N and 6 04 W). The four times of measurement of intake were chosen to coincide with the early stages of grazing bouts (or meals) for cows under similar conditions to those during the experimental periods. These were determined on 22 July 1998 by monitoring the grazing behaviour of seven cows using automatic bite meters similar to those described by Rutter et al. (1997). Cows entered a new paddock after morning milking, and their jaw movements were recorded for 24 h. Grazing and non-grazing (including ruminating) jaw activity was differentiated in the recordings produced by the bite meters, with cows considered to be grazing actively when their bite rate was >12 bites min ±1. Meal times were determined as periods when most or all of the cows were grazing actively. Four daily meals were identi ed, one after each morning and afternoon milking (milking beginning»05.30 and h respectively) with two additional meals being discernible, one in late morning and another mid-afternoon. From this, it was decided to assess intake at the target times of approximately (T1), (T2), (T3) and h (T4). Experiment 1 The objective of Experiment 1 was to quantify intake behaviour of cows throughout the day as sward height within a 24-h paddock was reduced and, hence, the sward conditions changed as a result of its progressive depletion. The experiment took place from 28 September

3 364 P. D. Barrett et al. to 1 October Sixteen cows were assigned to four groups. The appropriate group was drawn from the herd at each of the four assessment times per day so that intake could be determined. Plots and treatments The experimental area was fenced off on an established, predominantly perennial ryegrass (PRG) (Lolium perenne L. cv. Talbot) sward previously used for grazing dairy cows in a rotational grazing system. After grazing, the area was topped to»6 cm on 22 August and fertilized with 60 kg N, 15 kg P 2 O 5,30kgK 2 Oha ±1 on 24 August Four 24-h paddocks (each 2400 m 2 ) were established and grazed sequentially over 4 days. Each paddock was equivalent to a block. A fence post was placed at the centre of each paddock so that onequarter could be temporarily fenced off, creating a plot for intake assessment when required. At each of the grazing time treatments, one plot was fenced off and grazed with the nominated group of cows for 1 h over the 4 days in a complete cross-over experimental design (Latin square), i.e. each of the cow groups was assessed in one of the four periods per day. Sward characteristics were measured before and after grazing. Determination of herbage intake Herbage intake throughout the study was determined using the short-term weight gain method, adapted from Penning and Hooper (1985), over 1-h assessment periods, whereby intake was determined as the difference between pre- and post-grazing live weight, to which insensible weight loss (IWL) was added. Live weight was measured on a weighbridge (Mettler Balances, Greifensee, Switzerland), with a measurement range of 0±3000 kg 0á01 kg. A cattle crush was mounted on the weighbridge, and the unit was enclosed with a windproof shelter. All liveweight measurements were recorded as the mean of three consecutive readings, and cows were tted with excreta collection bags throughout the intake assessment periods so that no excreta was lost between the two weighings. Insensible weight loss is the rate of reduction in live weight observed over a period of time, not associated with the loss of excreta but mainly caused by body moisture loss from respiration (Dumont et al., 1994), and is affected by meteorological conditions (Gibb et al., 1995). The relationships between IWL and a range of meteorological factors (solar radiation, temperature and relative humidity) were determined by multivariate regression analysis. Insensible weight loss was only signi cantly related to solar radiation (P < 0á001) and so, in both experiments, mean solar radiation for each treatment time was used to calculate IWL. Figure 1 shows mean solar radiation over the day in both experiments. The data set was generated from 21 determinations of IWL, each being the mean of generally 15±20 cows, from a core group of about 30. These determinations of IWL were made at ARINI under differing climatic conditions over 4 years. They were calculated from the rate of reduction in live weight of cows, tted with excreta collection bags and con ned in a holding area with no access to food or water for 1 h. The cows used in the present study were drawn from this core group of animals. The relationship between IWL (y; g min ±1 ) and solar radiation (x; Wm ±2 ) measured at the time of intake assessment is described by the equation: y ˆ 00324x 881 r 2 ˆ Animals Sixteen mid-lactation, medium genetic merit Holstein± Friesian cows were selected and randomized into four groups, balanced on the basis of milk yield, live weight and parity. Average daily milk yield was»20 kg in the week before the experiment. After morning and afternoon milking (beginning»06.00 and h Figure 1 Mean solar radiation of the 4-day experimental periods in Experiment 1 (r, 28 September to 1 October 1998) and Experiment 2 (, 27±30 July 1998).

4 Herbage intake of dairy cows as sward height declines 365 respectively), the group of cows due to graze the T1 or T4 plots, respectively, were separated from the herd, and the remainder of the cows were turned into the paddock. The four cows from the group were tted with excreta collection bags, weighed and turned into the appropriate plot for intake measurement. After the 1-h grazing period, cows were removed from the plot, reweighed and the bags removed. Cows were returned to the main herd, and the temporary fence was removed to allow cows access to the whole paddock once again. For intake estimation at T2 and T3, the appropriate cows were drawn from the herd, tted with the excreta collection unit, weighed and brought back to the relevant plot. As they were being removed from the paddock, the temporary fence was erected around the appropriate plot, and all other animals were excluded from it, although they continued to have access to the remaining three-quarters of the paddock. No fasting was imposed at any time in either experiment other than that associated with the time taken for milking, i.e.»1á5 h. All cows were tted with bite meters after morning milking, which were removed after T4 measurements. Intake rate was determined as the liveweight gain, adjusted for IWL, divided by the time spent by the group of cows in the plot. DM intake rate was calculated as the product of fresh weight intake rate and DM content of the herbage. Herbage DM content was determined from hand-plucked samples, representative of the herbage consumed by the cows. This was achieved by closely observing one cow in the group for the 1-h period and removing herbage apparently similar to the herbage grazed. Bite number and total grazing jaw movement (GJM) during the 1-h measurement periods were determined from the bite meters, and bite rate was calculated as the total number of bites divided by the total time spent in the plot. Bite number was also expressed as a proportion of GJM. Sward measurements As plots were created within the paddock, sward condition was constantly changing and, hence, sward measurements were made as close to the time when cows entered the plots as possible. Sward surface height (SSH) was measured before and after grazing using the HFRO sward stick (Barthram, 1986) at 30 positions in a `W' shape in each plot. Available herbage mass (HM >4 ) was measured by cutting herbage within 25 0á5 0á5-m quadrats taken throughout the plot at 4 cm above ground and drying triplicate subsamples at 80 C for»24 h. The proportions of leaf (lamina) and stem (mainly pseudostem) were subsequently determined from a subsample stored at ±18 C. Stubble mass (herbage below 4 cm) was estimated from herbage removed to ground level in 0á3 0á3-m quadrats within 10 of the above quadrats. This material was washed to remove all soil contamination, and DM mass was determined. Total herbage mass (HM) was calculated as the sum of HM >4 and stubble mass. Sward bulk density (BD) was calculated for each plot as total HM divided by pregrazing SSH, and green leaf mass (GLM) from the proportion of leaf and HM >4. Herbage from the hand-plucked samples was dried at 60 C for at least 48 h, milled and analysed for nitrogen (N), ash, acid-detergent bre (ADF), neutral-detergent bre (NDF) and water-soluble carbohydrate (WSC) concentration using methods described by Cushnahan and Gordon (1995). Experiment 2 The objective of Experiment 2 was to investigate the intake characteristics and bite dimensions of dairy cows grazing 24-h paddocks at intervals throughout the day in the absence of the effect of sward depletion during intake measurement periods. The experiment took place over a 4-day period from 27 to 30 July Its design was similar to that of Experiment 1 but, instead of intake being estimated within the paddock, cows were moved from the paddocks to plots with undefoliated swards for the 1-h duration of intake measurement, although they grazed in a 24-h paddock at all other times. Herbage allowance within the paddock was»23 kg DM per cow. Plots Sixteen 200-m 2 plots of undefoliated swards were set up on an established, predominantly PRG (cv. Tyrone) sward. The area was previously used for rotational grazing and was topped to 6 cm and fertilized with 60 kg N, 15 kg P 2 O 5 and 30 kg K 2 Oha ±1 before the experiment on 3 July Animals Sixteen mid-lactation medium genetic merit Holstein± Friesian cows were selected and allocated to four groups, balanced on the same criteria as in Experiment 1 with some cows common to both experiments. Each morning, cows were milked (beginning»05.30 h), and the group due to graze during the morning period (i.e. T1) was separated and walked to the weighbridge handling area, tted with bite meters and excreta collection bags, weighed and led to the plot while the remainder was walked to the paddock. This process was repeated after the evening milking (beginning»16.00 h) for cows being measured during period T4. For intake measurement at both T2 and T3, the four cows from the

5 366 P. D. Barrett et al. appropriate group were removed from the herd within the paddock, with minimum disturbance to the others, tted with bite meters and excreta collection bags, weighed and led to the plot with the fresh, ungrazed sward. After intake measurements at each of the treatment times, the four cows were returned to the paddock and the rest of the herd. Insensible weight loss, bite mass, bite rate and intake were determined as for Experiment 1. Sward measurements Sward surface height was measured as for Experiment 1. On the day before a plot being grazed, the extended tiller height (ETH) of 112 marked tillers per plot was measured from ground level to the uppermost extended point and, immediately after grazing, each tiller was recorded as being either grazed or ungrazed, and ETH was measured again. Tillers were marked using PVC rings placed at the base of the tiller, with seven tillers (each 20 cm apart) marked on 16 1á5-m transects per plot placed in a notional 4 4 matrix over the plot. Bite depth was calculated as the difference between pre- and post-grazing ETH, and proportional bite depth was determined relative to the pregrazing ETH. Available herbage mass and stubble mass were estimated as in Experiment 1, but nine 0á5 0á5-m quadrats cut to 4 cm above ground and 0á3 0á3-m quadrats were cut to ground level from within all nine quadrats for stubble mass. Proportions of leaf and stem and GLM were determined as for Experiment 1. DM content of the herbage grazed was estimated from a sample of herbage collected by cutting to a height of approximately half of SSH (based on the depth of bite from previous short-term intake studies, e.g. McGilloway et al., 1997) at random positions in the plot, using hand clippers (Gardena, Ulm, Germany). DM intake was calculated from DM content and fresh weight intake. The dried sample was milled and analysed chemically as for Experiment 1. The proportion of the plot area grazed was estimated by recording whether the plant part (leaf, stem or pseudostem), at each of 100 points in a notional matrix over the plot, had been grazed. Mean bite area was calculated from the area of the plot grazed divided by the total number of bites taken per plot. Bite volume was subsequently calculated as the product of bite area and bite depth. Statistical analysis For both experiments, plots were considered as experimental units for all sward and intake measurements. Differences between treatments were tested for signi cance by analysis of variance, and linear and quadratic trends in intake characteristics with time were tested using GENSTAT 5 (Genstat 5 Committee, 1993). Multivariate regression analysis was conducted to determine the relationship between IWL and solar radiation using GENSTAT 5. Results Meal times Figure 2 shows the distribution of grazing periods throughout the day of the seven cows monitored before the start of the experiments, differentiating between active grazing and non-grazing activities. Meal times were considered to be periods when all or most of the cows were actively grazing at any one time. Mean daily grazing time was 8 h 28 min, with 0á27 and 0á34 of this corresponding to meal times after morning and evening milking respectively. Two additional meals were identi ed but were less well de ned than the post-milking meals because of lower synchronicity of active grazing between cows and shorter meal duration, together comprising 0á39 of the total grazing time. One of these meals was in late morning and the other in early afternoon. This provided the basis for choosing measurement times immediately after both morning and afternoon milking and additionally at approximately and h. Mean entry times to plots for T1, T2, T3 and T4, respectively, were 7á44, 11á12, 14á07 and 18á13 h in Experiment 1 and 6á56, 11á20, 14á50 and 19á13 h in Experiment 2. Experiment 1 In Experiment 1, sward state in the plots in which intake was estimated changed markedly as the day progressed (Table 1). Sward surface height decreased from 23á8 cm at T1 to 13á0 cm at T4 (P <0á001), whereas HM >4 declined (P <0á05) from 1882 to 1412 kg DM ha ±1. Proportion of leaf declined (P <0á05), while proportion of stem increased (P <0á05) as the day progressed. Green leaf mass differed signi cantly (P <0á001) over the course of the day, declining from 1503 kg DM ha ±1 at T1 to 948 kg DM ha ±1 at T4. Bulk density increased signi cantly (P < 0á001) as the day progressed, the highest mean being recorded at T3, but was not signi cantly different from that of T4. Herbage DM did not vary between treatments, but WSC concentration increased (P <0á001) as the day progressed, whereas N concentration decreased (P <0á001). There was no difference in NDF concentration throughout the day, and ADF concentration

6 Herbage intake of dairy cows as sward height declines 367 Figure 2 Daily grazing pattern of seven cows used to determine meal times. Active grazing shown by black shading. tended to increase, although differences were not signi cant (P ˆ 0á07). Intake characteristics are shown in Table 2. Bite mass differed signi cantly (P < 0á05) among treatment times and had a signi cant linear effect, declining as the day progressed. There was no difference in the rate of total GJM throughout the day, mean rate being 61á2 GJM min ±1. Bite rate tended to be lower (P ˆ 0á06) at T3 as a result of the proportion of bites to GJM being lower (P ˆ 0á07) at that time but, for both, values were similar at the other meals. Intake rate varied signi cantly (P <0á01) between treatments, being lower at T3, with signi cant linear and quadratic trends. Experiment 2 In Experiment 2, as intended, sward measurements did not differ signi cantly between treatments. Mean SSH, HM >4 and BD were 17á9 cm, 1707 kg DM ha ±1 and 1á76 kg DM m ±3 respectively. Herbage DM content increased (P <0á05), as did WSC (P <0á05) concentration, throughout the day, but there was no difference in N concentration or ash content (Table 3).

7 368 P. D. Barrett et al. Table 1 Sward characteristics and chemical composition of herbage representing that consumed by cows at different treatment times in Experiment 1. Target time of intake estimation (h) Mean s.e.m. Signi cance SSH (cm) 23á8 16á9 13á6 13á0 16á8 0á69 *** Stubble mass (kg DM ha )1 ) á6 NS HM >4 (kg DM ha )1 ) á7 * HM (kg DM ha )1 ) á6 NS BD (kg DM m ±3 ) 1á30 1á66 2á23 2á16 1á84 0á103 *** Proportion of leaf 0á80 0á76 0á67 0á67 0á72 0á027 * GLM (kg DM ha )1 ) á3 *** Proportion of stem 0á10 0á10 0á17 0á16 0á13 0á017 * DM content (g kg )1 ) 151á7 171á3 167á6 161á8 163á1 9á04 NS WSC concentration (g kg )1 DM) 89á3 128á5 150á5 154á2 130á6 8á94 *** N concentration (g kg )1 DM) 38á0 34á4 30á6 30á1 33á3 0á86 *** ADF concentration (g kg )1 DM) 237á2 235á1 245á4 248á5 241á6 3á69 NS NDF concentration (g kg )1 DM) 569á6 559á2 559á7 548á6 559á3 6á40 NS Ash concentration (g kg )1 DM) 87á1 85á9 86á7 95á3 88á8 3á04 NS SSH, sward surface height; HM >4, available herbage mass; HM, total herbage mass; BD, bulk density; GLM, green leaf mass; DM, dry matter; WSC, water-soluble carbohydrates; N, nitrogen; ADF, acid-detergent bre; NDF, neutral-detergent bre. NS P >0á05; *P <0á05; ***P <0á001; P ˆ 0á07. Table 2 Intake characteristics of cows during 1-h measurement periods in Experiment 1. Target time of intake estimation (h) Signi cance Mean s.e.m. Time Linear Quadratic Bite mass (g DM) 0á74 0á70 0á55 0á62 0á65 0á041 * * NS Total GJM (GJM min )1 ) 62á1 59á4 60á0 63á4 61á2 1á90 NS NS NS Bite rate (bites min )1 ) 45á0 42á3 32á9 45á2 41á4 3á05 NS NS * Bites GJM )1 0á72 0á71 0á55 0á71 0á68 0á046 NSà NS NS Intake rate (kg DM h )1 ) 2á01 b 1á78 b 1á09 a 1á66 b 1á64 0á138 ** * * GJM, grazing jaw movements. NS P >0á05; *P <0á05; **P <0á01; P ˆ 0á06; àp ˆ 0á07. Values in row with different superscript letters signi cantly different at P < 0á05. Intake characteristics for Experiment 2 are shown in Table 4. There were no signi cant differences in bite dimensions between treatments. Mean bite depth, area and volume were 64 mm, 124 cm 2 and 0á80 l respectively. Bite depth relative to ETH was constant throughout the day, the mean being 0á32 of ETH. There was no signi cant difference in bite mass over the course of the day, although it ranged from 0á71 g DM in the morning meal to 0á86 g DM at T3. Although failing to achieve signi cance (P ˆ 0á07), biting rate was more variable throughout the day, tending to be higher at the meal after each milking, i.e. T1 and T4. Total GJM and the number of bites per GJM were constant at different meals, means being 60á8 GJM min ±1 and 0á71 bites GJM ±1 respectively. Similarly, DM intake rate did not differ signi cantly over the course of the day, with a mean rate of 2á11 kg DM h ±1. Discussion Treatment times and insensible weight loss Meal times were identi ed and intake measurement times chosen to coincide with the estimated early stages of a meal to avoid confounding effects on grazing behaviour of stage of progress through a grazing meal with time of day. Meal times were easily identi ed immediately after milking, both morning and evening, when all cows began grazing at the same time for prolonged periods. However, the other meals in the day

8 Herbage intake of dairy cows as sward height declines 369 Table 3 Sward characteristics and chemical composition of herbage representing that consumed by cows at different times in Experiment 2. Target time of intake estimation (h) Mean s.e.m. Signi cance SSH (cm) 18á5 17á4 18á0 17á9 17á9 0á45 NS Pre-ETH (mm) 201á2 201á7 200á8 203á6 201á8 5á96 NS Stubble mass (kg DM ha )1 ) á5 NS HM >4 (kg DM ha )1 ) á3 NS HM (kg DM ha )1 ) á6 NS BD (kg DM m ±3 ) 1á65 1á75 1á85 1á80 1á76 0á110 NS Proportion of leaf 0á80 0á80 0á74 0á78 0á78 0á020 NS GLM (kg DM ha )1 ) á6 NS Proportion of stem 0á13 0á13 0á17 0á14 0á14 0á010 NS DM content (g kg )1 ) 150á0 177á4 188á4 198á3 178á5 10á37 * WSC concentration (g kg )1 DM) 185á4 222á4 246á3 277á2 232á8 19á16 * N concentration (g kg )1 DM) 33á8 31á2 31á3 29á1 31á3 1á71 NS ADF concentration (g kg )1 DM) 226á6 227á7 210á8 201á5 216á7 3á75 ** NDF concentration (g kg )1 DM) 473á3 473á9 443á0 425á6 453á9 7á07 ** Ash concentration (g kg )1 DM) 71á5 66á2 67á3 63á7 67á2 3á43 NS SSH, sward surface height; HM >4, available herbage mass; HM, total herbage mass; BD, bulk density; GLM, green leaf mass; DM, dry matter; WSC, water-soluble carbohydrates; N, nitrogen; ADF, acid-detergent bre; NDF, neutral-detergent bre. NS P >0á05; *P <0á05; **P <0á01. Table 4 Bite dimensions and intake characteristics of cows during one-hour measurement periods in Experiment 2. Target time of intake estimation (h) Signi cance Mean s.e.m. Time Linear Quadratic Bite depth (mm) 67á7 63á3 64á1 60á3 63á8 5á72 NS NS NS Bite depth (proportion 0á34 0á31 0á32 0á29 0á32 0á024 NS NS NS of ETH) Bite area (cm 2 ) 119á4 132á2 136á2 109á6 124á3 10á96 NS NS NS Bite volume (l) 0á81 0á83 0á88 0á66 0á80 0á114 NS NS NS Bite mass (g DM) 0á71 0á82 0á86 0á82 0á80 0á046 NS NS NS Total GJM (GJM min )1 ) 61á8 59á6 57á7 64á2 60á8 2á06 NS NS NS Bite rate (bites min )1 ) 44á8 40á4 41á1 46á2 43á1 1á54 NS NS * Bites GJM )1 0á73 0á68 0á71 0á72 0á71 0á022 NS NS NS Intake rate (kg DM h )1 ) 1á92 1á96 1á93 2á23 2á11 0á135 NS NS NS ETH, extended tiller height; GJM, grazing jaw movements. NS P >0á05; *P <0á05; P ˆ 0á07. were less well de ned. At these times, periods of active grazing were less synchronized between animals, and their duration was also more variable than the postmilking meals. The times chosen for T2 and T3, however, coincided with simultaneous grazing activity for the majority of the cows. Insensible weight loss was estimated from solar radiation in this study in order to take account of the effect of meteorological conditions and their variation over the course of the day. Gibb et al. (1995) stated that temperature and relative humidity in uence IWL. However, from the data set used in the present study, the relationships of IWL regressed on temperature or relative humidity were not as strong as for solar radiation and did not signi cantly improve (P > 0á05) the prediction of IWL when included in the multivariate regression with solar radiation. Similarly, tting a quadratic relationship made no signi cant improvement over a linear relationship. Additionally, as Gibb et al. (1997) described, IWL and intake rate can be adversely affected by measuring IWL immediately before or after measuring intake rate.

9 370 P. D. Barrett et al. As cows were required to be in a paddock at all other times when they were not being used for intake estimation in this study, IWL could not be measured regularly. The mean daily variation in solar radiation over both experiments is shown in Figure 1, and mean daily meteorological conditions over both experimental periods are shown in Table 5. Sward depletion and grazing behaviour (Experiment 1) In Experiment 1, the state and condition of the sward in the plots, and hence the paddock, changed markedly as the day progressed, as expected. Other things being equal, SSH is considered to be the most important sward factor in uencing herbage intake (Hodgson, 1981; Black and Kenny, 1984; Sporndly and Burstedt, 1996; Mayne et al., 1997). However, in a sward subject to rotational grazing, the decline in SSH is known to correspond to a substantial reduction in the proportion of leaf and hence GLM (McGilloway et al., 1999). In the present study, SSH, HM >4, proportion of leaf and GLM decreased signi cantly over the course of the day, whereas BD increased. Decline in leaf material as a result of grazing is rapid because of the animal's ability to select high proportions of leaf relative to the overall herbage content (Arnold and Dudzinski, 1978) and its concentration in the upper layers of the sward (Casey, 2000). Penning et al. (1994) showed that, compared with continually stocked swards, rotationally grazed swards of similar SSH had substantially less GLM and gave rise to signi cantly lower intake rates with sheep. In Experiment 1 of the present study, the relationship between GLM (kg DM ha ±1 )(x) and bite mass (mg) (y) is described by the equation: y ˆ 00003x r 2 ˆ The differences in chemical composition of the herbage also re ect the changes in the structure of the sward as grazing proceeded through the day. No differences in intake behaviour were found between the rst two meals of the day but, by midafternoon (T3), bite mass was signi cantly reduced and resulted in a lower intake rate than earlier in the day. Similarly, on swards subject to levels of depletion similar to those in the present study, McGilloway et al. (1999) found that, despite increased bulk density as swards were grazed down, decreasing bite depth markedly reduced bite mass. Bite rate in that study was generally constant at different stages of sward depletion and tended to decrease signi cantly when the reduction in sward height was severe, resulting in a decline in intake rate. Likewise, in Experiment 1 of the present study, bite rate tended to be lower at T3 than at other times because of a reduction in the proportion of GJM being bites, even though the rate of total GJM remained constant. Therefore, bite rate was not adapted to compensate for a low bite mass and, as a result, intake rate was signi cantly lower than at any other meal. Bite mass declined linearly with the extent of sward depletion. However, at T4, irrespective of low bite mass, intake rate was signi cantly higher than at T3 because of an increase in bite rate by 0á37 (P ˆ 0á06). Although bite rate was substantially lower at T3 than at other meals, this was not the result of reduced jaw activity, because total GJM was similar at each mealtime, but was the result of a reduction in the proportion of GJM being bites. This may suggest that, if the mouth is not being used so extensively for prehension, then the cow will take advantage of the opportunity to increase Rainfall (mm) Relative humidity (%) Mean temperature ( C) Solar radiation (W m )2 ) Table 5 Meteorological data recorded during experimental periods in 1998 for Experiments 1 and 2. Experiment 1 28 September 0á2 91á5 12á1 49á4 29 September 0á0 84á6 12á4 60á6 30 September 6á0 98á1 12á8 15á6 1 October 2á4 92á9 13á6 46á9 Mean 2á2 91á8 12á7 43á9 Experiment 2 27 July 0á8 89á2 14á8 99á4 28 July 2á4 90á5 14á5 87á9 29 July 4á4 89á0 13á8 122á2 30 July 2á8 89á4 13á8 90á7 Mean 2á6 89á5 14á2 100á0

10 Herbage intake of dairy cows as sward height declines 371 mastication and perhaps reduce the subsequent rumination demand. Intake behaviour of cows grazing undefoliated swards immediately after grazing in a paddock (Experiment 2) The purpose of Experiment 2 was to determine the grazing behaviour of dairy cows at different meals in the day in response to the immediately previous grazing experience, unaffected by sward depletion. It is evident that, as the day progressed, there were no large uctuations in grazing behaviour as a consequence of animal physiological condition. Increased hunger in animals induced by imposed fasting is an extreme case of a change in short-term physiological condition. Fasted animals usually adapt their intake behaviour from that under normal conditions and have been shown to increase their bite rate, bite depth and bite mass as a result of fasting (Greenwood and Demment, 1988; Illius et al., 1995; Patterson et al., 1998). The aim of the present study precluded the use of fasted animals and, in Experiment 2, on no occasion within the day did cows increase bite rate, bite depth, bite mass or intake rate. This suggests that physiological condition, at least as far as it affects grazing behaviour, remained relatively constant throughout the day, and any variation was not to the extent of that experienced by cows when fasted. However, bite rate tended to be more variable between treatments (P ˆ 0á07) than bite mass or intake rate, although rate of total GJM was constant. Proportionally, bite rate at both meals in the middle of the day (T2 and T3) was substantially lower than at T1 or T4. It may be that a small reduction in appetite is experienced in the middle of the day resulting in this minor reduction in bite rate. In the present study, bite rate was more variable than bite mass. It may be that bite rate is more easily adjusted by the cow than bite mass and, hence, used as a means of regulating intake in preference to adjusting bite mass in response to small physiological changes. Similarly, Patterson et al. (1998) found that, after a limited period of fasting (3 h), cows increased their bite rate but not their bite mass; however, as the period of fasting was extended (to 6 h), bite mass was also increased. It is therefore likely that, when a small increase in degree of hunger is experienced, increasing intake rate by raising the rate of biting has greater priority than increasing bite mass. The interaction between the effect of sward depletion and short-term physiological condition In Experiment 1, where intake was measured on swards declining in height, herbage mass and leaf content throughout the day, differences in grazing behaviour were found. Sward state was shown to in uence intake, but its restrictive effect, as grazing down proceeded, was compensated for at T4, and intake rate was not lower at this time, as found at T3. However, this differentiation in grazing behaviour between T3 and T4 was not apparent in Experiment 2, where intake rate was measured on similar sward types throughout the day, as there were no signi cant differences in the grazing behaviour of the cows, including the dimensions of bites. Therefore, although the immediately previous grazing history was similar for cows at similar times in both experiments, the state of the sward being grazed had an effect on their grazing behaviour. The effect of the progressive depletion of the sward and the uctuation in animal physiological condition therefore appear to interact. In practice, under normal grazing circumstances in a 24-h paddock, in mid-afternoon (i.e. T3), although bite mass may be low, bite rate would also appear to be low, and a low intake rate would be the result. Later in the day, however, there appeared to be a greater need to achieve a high intake rate, dictated by the physiological condition of the cow and, in this study, cows compensated for the low bite mass by increasing bite rate. This apparent need to achieve a relatively high intake in the evening may be brought about because the cow has not satis ed her intake requirements as the day proceeds, thus cumulating in either a nutrient de cit or simply failure to attain satiation from an adequately lled rumen. Additionally, treatment T4 represents the last meal of the day before nightfall. It is possible that increased grazing activity at dusk is the result of the forthcoming onset of nightfall when grazing usually stops during the hours of darkness. When meal pro les were determined, only one meal was observed between evening and morning milking, and this was substantially longer than the one after morning milking. From the bite meter data, where meal times were determined, the evening meal contributed 0á34 of the total daily grazing time. Lengthening grazing time by increasing the duration of individual meals is one way that cows increase daily herbage intake (Gibb et al., 1999). It is known that animals adapt feeding behaviour in anticipation of future events and energy requirements and can be hyperphagic under certain conditions (Baile and McLaughlin, 1987), most evident in the cases of hibernation and migration by some animals. The cow endeavours to achieve a high intake rate before nightfall as grazing usually stops during the hours of darkness, which is the period where most of the daily rumination is concentrated (Phillips and Leaver, 1986; Phillips, 1993).

11 372 P. D. Barrett et al. Implications These ndings have implications for management in rotational grazing systems and also for the further development of daily intake prediction models. Within a paddock, because intake rate is not maintained at each meal in the day, an option available to livestock managers is to increase herbage allowance, thus decreasing the extent of grazing down and relieving the restriction imposed by the changes in sward state. However, topping or the use of a leader/follower system is necessary to maintain sward quality. The effect of sward depletion on instantaneous intake rate has an overall detrimental effect on daily herbage intake. Where the sward was not limiting intake (i.e. Experiment 2), daily herbage intake would be nearly 0á20 more than that found in Experiment 1 (i.e. 17á9 vs. 13á9 kg DM day ±1 ). This was determined from the mean intake rate in each experiment and assuming a daily grazing time of 8 h 28 min, as determined from the evaluation of meal patterns (Figure 2), to remain unchanged in both experiments. There is evidence to suggest that offering dairy cows fresh herbage more frequently than once per day will result in an increase in daily intake and, if 12-h paddocks or strips were used, the timing of the change of paddock may be important. For example, the results suggest that, by providing fresh herbage in mid-afternoon, as opposed to the conventional and convenient timing of after evening milking, intake rate would be increased by providing suf cient impetus to promote high intake otherwise not achieved when the sward is grazed down. Similarly, if feeding concentrates, substitution effects might be reduced if they were offered at this time in the paddock, resulting in a DM intake otherwise not possible under normal grazing systems. At this latter time, cows have a high impetus to graze irrespective of herbage state. If cows are moved to new paddocks after the morning milking and in early afternoon, the area for the paddock grazed after the morning milking should be about 0á65 of that for the paddock grazed after the afternoon milking, based on grazing times from the study to evaluate meal times (Figure 2). This work has additional implications for the development of predictive models for daily herbage intake in rotational systems. As found for continuously stocked swards (Gibb et al., 1998), in rotationally grazed swards, intake characteristics determined on one occasion in the day are not necessarily applicable to another time of the day. Therefore, simple multiplication of known intake rates by grazing duration is not appropriate, and further study is required to quantify changes better and to investigate reasons for such changes in grazing throughout the day. 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