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Influence of sward height, daily timing of concentrate supplementation, and restricted time for grazing on forage utilization by lactating beef cows^1,2

O. J. Gekara^*, E. C. Prigge^*, W. B. Bryan^*,,3, E. L. Nestor^* and G. Seidel⁴

^* Division of Animal and Veterinary Sciences and and Division of Plant and Soil Science, West Virginia University, Morgantown 26506-6108

	Abstract

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To establish the effect of sward height, concentrate feeding time, and restricted time for grazing on forage utilization by grazing cattle, 32 crossbred beef (24 Angus and eight Hereford) cows (632 kg BW) and calves (104 kg BW) were grouped by weight and calving date. They were assigned randomly to two sward height treatments (4 to 8 or 8 to 12 cm), replicated four times. The herbage comprised mainly Kentucky bluegrass, orchardgrass, some forbs, and white clover. The cows were restricted to 12 h/d grazing (0700 to 1900) or unrestricted to 24 h/d grazing and fed a concentrate supplement (4.1 kg DM·cow^–1·d^–1, approximately 0.65% of BW or 33% of total DMI) either at 0700 or 1800. The experiment was repeated over three 15-d periods in May, June/July, and August 2000. The herbage on high sward height pasture was higher (P = 0.06) in NDF and ADF and lower (P < 0.01) in CP than low sward height herbage. For cows restricted to 12 h/d grazing, supplementing at 0700 as opposed 1800 resulted in greater (P = 0.04) forage DMI (8.6 vs. 8.1 kg/d), whereas cows that were unrestricted showed little change (8.2 kg/d at 0700 vs. 8.4 kg/d at 1800). Supplementing at 1800 as opposed to 0700 resulted in greater (P = 0.03) herbage DM digestibility (67.7 vs. 64.5%) for cows on high sward height, whereas cows on low sward height exhibited minimal differences (65.4% at 1800 vs. 66.3% at 0700). Cows restricted to 12 h/d grazing and supplemented at 0700 as opposed to 1800 resulted in greater (P = 0.06) digestible DMI (5.0 vs. 4.7 kg/d), whereas unrestricted cows exhibited the opposite response (4.6 kg/d digestible DMI at 0700 vs. 4.9 kg/d at 1800). Supplementing at 1800 as opposed to 0700 increased the time spent grazing to a greater (P = 0.09) extent for restricted than for unrestricted cows. When forage availability or grazing time was limiting (due to a low forage allowance and restricted access to forage, respectively) supplementing concentrates at 0700 resulted in greater forage utilization and intake rate because of increased forage DMI, DM digestibility, and digestible DMI. However, when forage or grazing time was not limiting, supplementing concentrates at 1800 resulted in greater forage utilization because of increased forage DM digestibility.

Key Words: Cows • Digestibility • Intake • Restricted Time for Grazing • Sward Height • Timing of Supplementation

	Introduction

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When cattle consume forages as their only energy source, intake of available energy may be inadequate to meet desired production demands for lactating cattle. To increase milk production of lactating dairy cows under grazing situations, supplementation with concentrates is a viable option (Kolver et al., 1998). Gekara et al. (2001) reported that concentrate supplementation of cows grazing lower sward height (SH; 6.6 cm) pasture (mainly Kentucky bluegrass, orchardgrass, some forbs, and white clover) resulted in greater forage DMI compared with cows on higher SH (8.8 cm) pasture. This finding suggests that forage management could be an important variable, influencing the effectiveness of supplementation. The decreased forage intake and DM digestibility often observed with levels of concentrate supplementation greater than 0.7% of BW (Horn and McCollum, 1987; Gekara et al., 2001) could be attributed to disruption of the rumen environment via fluctuations in pH, predominant microbial species and/or population (Mould and Ørskov, 1984; Hoover, 1986). The grazing activity of cattle varies throughout the day partly in response to environmental temperature, demand for other activities such as rumination and resting (Rook et al., 1994), or to diurnal changes in nutrient composition of pasture (Orr et al., 1997) and/or rumen metabolites (Van Vuuren et al., 1986). Because of diurnal changes in the ruminal environment brought about by the grazing pattern of cattle, the effectiveness of concentrate supplements on pasture could be influenced by forage and time of concentrate feeding via effects on fiber digestion and other variables associated with ruminal fermentation. Consequently, the objective of this study was to determine the effect of SH, daily timing of concentrate supplementation, and restricted time for grazing time on forage utilization by lactating beef cows.

	Materials and Methods

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Experimental Design

The experiment was conducted at the West Virginia University Animal Science Farm, Morgantown, during the 2000 grazing season on mixed cool-season forages. The mixed forages comprised grasses, predominantly Kentucky bluegrass (Poa pratensis L.) and some orchardgrass (Dactylis glomerata L.); legumes, predominantly white clover (Trifolium repens L.), and some red clover (Trifolium pratense L.); and other forbs (mainly broad leaved). The experimental area has a slope of 15 to 20% and soil types are Clarksburg (fine-loamy, mixed, mesic Typic Fragiudalfs); Dormont (fine-loamy, mixed, mesic ultic Hapludalfs); and Holly (fine-loamy, mixed, nonacid, mesic Typic Fluvaquents). Soil samples taken in 1999 (at a depth of 0 to 5 cm) from each pasture had an average pH of 6.0 and available P and K of 92 and 341 kg/ha, respectively. No lime or fertilizer was applied. Thirty-two multiparous crossbred (24 Angus and eight Hereford) beef cows (632 ± 14 kg BW) and their calves (104 ± 4 kg BW) were grouped according to calving date and BW. Cows calving between February 1 and April 1 were selected and assigned randomly to two SH treatments (4 to 8 or 8 to 12 cm), replicated four times. Each cow received a concentrate supplement (4.1 kg of DM/d; approximately 0.65% of BW or 33% of total DMI) fed at one of two times (either at 0700 or 1800) and either restricted to grazing 12 h/d (0700 to 1900) or unrestricted grazing 24 h/d. The restricted time for grazing was incorporated to simulate a pasture-based dairy system where cows would be allowed to graze for a length of time daily between milking for dairy cows. The experiment was conducted over three periods, each lasting 15 d: May 5 to 26 (Period 1), June 29 to July 13 (Period 2), and August 15 to 29 (Period 3). The experimental cow/calf units grazed on the same pastures during all three experimental periods, but were rerandomized within the time of supplement feeding or restricted time for grazing treatments during each period.

Animals and Diet

Before the start of Period 1, all animals were fitted with fly control ear tags (Cutter Blue, Bayer Corp., Shawnee, KS) and treated for internal parasites (SafeGuard, Hoechst Roussel Vet, Summersville, NJ). Protocol approved by the University Animal Care and Use Committee was applied in the management of animals and experimental procedures. Animals were weighed at the beginning and end of each experimental period. Cows were fed the supplements individually in 2.5-m x 0.8-m stalls located within each pasture. Stalls also were used for placement and removal of vibracorders, feeding of a fecal output marker, and collection of fecal samples. Animals usually were in the stalls for less than 15 min twice daily for these procedures.

The concentrate supplement was offered in one portion at the prescribed feeding time throughout each 15-d experimental period. To ensure uniform distribution of the rare earth marker, ytterbium, in the feces, Yb-labeled oats (Baker et al., 1988) were fed twice daily (50 g/feeding). The labeled oats were mixed with the supplement at the prescribed supplementation time or mixed with a portion of the supplement (100 g, as-fed basis) and fed to cows not receiving supplement at that particular time to encourage consumption. The supplement was comprised mainly of corn and soybean meal (Table 1). All animals were allowed free access to a trace mineral salt block (Morton Int., Inc., Chicago, IL) and water. Cattle restricted to 12 h/d grazing were removed from the pastures at 1900 and confined in nearby holding pens (supplied with drinking water only and no forage DM), where they remained overnight and were returned to their specific pastures for grazing at 0700. Nonexperimental yearling cattle of similar breeding as the cows were used on a put-and-take basis throughout the grazing season to maintain the swards at desired heights. Appropriate adjustments in yearling cattle numbers were made weekly. Between periods, the experimental cow-calf units were moved to nonexperimental pastures of similar type.

Forage conditions were monitored weekly throughout the grazing season by taking SH measurements (50 readings/ha of pasture) with an acrylic plastic meter stick as described by Rayburn and Rayburn (1998). Only measurements collected during the experimental period are reported in this experiment. Clip samples to determine botanical composition and growth rate of forage were clipped at ground level and collected once every 2 wk for the entire grazing season. Herbage growth rate was determined with exclusion cages (four per pasture) that were relocated after SH measurements and clipping as described by Gekara et al. (2001). Hand-plucked samples of forage representing the grazed horizon were taken every 2 d starting d 9 through 13. Fecal grab samples to determine fecal output were collected twice daily at the time the supplement was fed, from d 10 to 14 of each 15-d experimental period, and analyzed as described by Gekara et al. (2001). Samples were oven-dried to a constant weight (at 60°C for 6 to 7 d), and then ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass a 1-mm screen. The samples were subsequently analyzed for DM, CP, NDF, ADF, and alkaline peroxide lignin (APL). Dry matter and CP of the pluck samples were analyzed according to procedures described by AOAC (1990). Neutral detergent fiber and ADF were analyzed following procedures described by Van Soest et al. (1991) and Robertson and Van Soest (1981), respectively. In vitro DM digestibility of the supplement was determined as described by Goering and Van Soest (1970), using a ruminal fluid inoculum obtained from a ruminally cannulated dry dairy cow grazing a similar type of forage.

Measurements and Analytical Methods

To allow comparisons with other investigations, sward surface height (SSH) as determined by the Hill Farming Research Organization sward stick (Barthram, 1986) was calculated from the SH obtained using the acrylic plate disc meter. The following prediction equation, developed from paired samples (SH and SSH) taken on experimental pastures in 2001 using the method of Bryan et al. (1990), was used:

Furthermore, the relationship between SH and herbage mass was estimated from the equation developed from paired samples (plate height and herbage DM) taken on experimental pastures in 2001 using the method of Bryan et al. (1990) and is as follows:

Ytterbium was used as an external indicator to estimate fecal output. Ytterbium-labeled oats were prepared by spraying YbCl₃ solution on whole oats according to procedures described by Baker et al. (1988). As noted previously, each cow received 100 g of Yb-labeled oats/d (as-fed basis), from d 1 to 14 of each period (50 g at each feeding to coincide with supplement feeding time). Samples of the composited feces and labeled oats were analyzed for Yb concentration according to procedures of Baker et al. (1988). Fecal output (DM basis) was calculated from indicator concentrations using the following formula:

The contribution of forage to fecal output was estimated by subtracting the indigestible fraction of supplement (0.08) as determined by IVDMD from the total fecal output, as described by Gekara et al. (2001). The APL contents of forage and feces were used internal markers, which, together with fecal output, were used to estimate forage intake, as described by Sunvold and Cochran (1991). Forage digestibility was calculated using the estimated forage DMI and forage fecal output.

Grazing time was measured as described by Gekara et al. (2001), with vibracorder equipment (Kienzle Apparate GmbH, Villingen, Germany) fitted around the neck of each cow during each experimental period. Twenty-eight animals had prior experience wearing a vibracorder from previous studies. Animals with no prior experience showed some discomfort for the first 10 min (this period of adjustment was not considered when estimating grazing time). The vibracorder remained on the animal for 48 h. An animal was considered either grazing or ruminating/idle if it spent at least five continuous minutes doing a specific activity.

Statistical Analyses

Forage botanical and chemical composition variables for each pasture were averaged within period and analyzed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC), using a split-plot in time design and replicate x treatment as the error term for SH. Animal variables were analyzed using least squares procedures of SAS and ANOVA based on a split-split plot design, with SH as the main plot, supplement feeding time and restricted time for grazing in combination as subplot treatments, and period as sub-subplot. Pasture within SH and block was the error term used to test SH. Cow within height, plot, and block tested supplement feeding time and restricted time for grazing, whereas residual error tested all other independent variables. Fisher’s test (LSD) was used to make multiple comparisons among periods.

	Results

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Weather

The mean maximum and minimum temperatures (Figure 1A) and precipitation (Figure 1B) were within the 30-yr range for this area. The ambient temperatures were lower for Period 1 compared with Periods 2 and 3, and this was considered normal seasonal variation.

View larger version (18K): [in this window] [in a new window]   Figure 1. Mean 2000 and 30-yr maximum and minimum temperature (A) and precipitation (B) over the experimental period at the National Weather Station, Hart Field Municipal Airport, Morgantown, WV. Apr = April; Jun = June; Jul = July; Aug = August.

Mean SH and SSH for the low and high swards, as well as herbage growth rate and the corresponding herbage mass, are shown in Table 2. The SSH difference was 4.6 cm between the low and high SH pastures, which was what we expected to achieve by varying the grazing pressure. Sward height was found to influence (P = 0.04) herbage growth rate (Table 2), which was greater for the low (6.0 cm) compared with the high (9.9 cm) sward.

There was no interaction (P = 0.34) between SH and period for the proportion of legume and other forbs in the pasture. A SH x period interaction (P = 0.07) for the proportion of live grass and dead material was observed. For the high SH pasture, the proportion of grass decreased (Figure 2A), whereas dead material increased (Figure 2B) as the season progressed. For the low SH pasture, the proportion of grass was greater (Figure 2A) and the proportion of dead material was lower during Period 2 (Figure 2B) compared with other periods. Sward height influenced (P = 0.09) the proportion of other forbs in the sward but not legume (P = 0.16). Sward height influenced the concentrations of NDF (P = 0.03), ADF (P = 0.06), and CP (P < 0.01) in forage (Table 3). Period influenced the concentration of ADF (P = 0.02) and CP (P < 0.01), but not NDF (P = 0.92).

View larger version (13K): [in this window] [in a new window]   Figure 2. Influence of sward height and period on proportion of (A) live grass, and (B) dead material. High SH = high sward height; Low SH = low sward height; Period 1 = May; Period 2 = June/July; Period 3 = August; sward height x period interaction on live grass (P = 0.07) and dead material (P = 0.01).

There were no three or four-way interactions (P = 0.37) involving SH, supplementation time, restricted time for grazing, and period for forage DMI. An interaction between supplementation time and restricted time for grazing (P = 0.04) forage DMI was evident. For cows restricted to 12 h/d grazing, the trend in forage DMI was downward from 0700 (8.6 kg/d) to 1800 (8.1 kg/d) supplementation (Figure 3A). For cows that were allowed to graze for 24 h/d, there was little change in forage DMI (8.2 kg/d at 0700 vs. 8.4 kg/d at 1800).

View larger version (20K): [in this window] [in a new window]   Figure 3. Influence of supplement feeding time and restricted time for grazing on (A) forage DMI, and (B) forage digestible DMI of cows. R = restricted grazing (12 h/d); U = unrestricted grazing (24 h/d); supplement feeding time x restricted time for grazing interaction on forage DMI (P = 0.04) and on forage DDMI, P = 0.06; forage DDMI = forage digestible DMI.

There were no three- or four-way interactions (P = 0.26) involving SH, supplementation time, restricted time for grazing, and period for forage DM digestibility. An interaction between SH and supplementation time (P = 0.03) for herbage DM digestibility was detected. Supplementing at 1800 as opposed to 0700 resulted in greater herbage DM digestibility (67.7 vs. 64.5%) for cows on high SH (Figure 4). Cows on low SH exhibited little difference in response (65.4% at 1800 vs. 66.3% at 0700).

View larger version (15K): [in this window] [in a new window]   Figure 4. Influence of sward height and supplement feeding time on forage DM digestibility for cows. High SH = high sward height; Low SH = low sward height; sward height x supplement feeding time interaction, P = 0.03.

There were no three- or four-way interactions (P = 0.83) involving SH, supplementation time, restricted time for grazing, and period for forage digestible DMI (DDMI). An interaction between supplementation time and restricted time for grazing (P = 0.06) for forage DDMI was detected. For cows restricted to 12 h/d grazing, the trend in digestible DMI was downward from 0700 (5.0 kg/d) to 1800 (4.7 kg/d) supplementation (Figure 3B). Unrestricted cows exhibited the opposite trend (4.6 kg/d at 0700 vs. 4.9 kg/d at 1800).

Grazing Time Pattern

The grazing time pattern of cows was not disrupted (P = 0.98) following supplementation; however, restricted time for grazing influenced (P = 0.05) the grazing time pattern of cows.

Grazing Time and Rate of Forage Intake

Three- or four-way interactions involving SH, supplementation time, restricted time for grazing, and period for grazing time and rate of forage intake (kg of forage DMI/h of grazing time) were not evident (P = 0.54). However, an interaction between supplementation time and restricted time for grazing for grazing time was detected (P = 0.09). Supplementing at 1800, as opposed to 0700, increased the actual time spent grazing to a greater extent for restricted vs. unrestricted cows (Figure 5A). Supplementing at 0700, as opposed to 1800, increased the rate of forage intake (Figure 5B) for restricted cows (1.3 vs. 1.2 kg DM/h of grazing time), whereas there was no difference in response for unrestricted cows. An interaction between SH and period (P = 0.02) for grazing time was evident (Figure 6).

View larger version (18K): [in this window] [in a new window]   Figure 5. Influence of supplement feeding time (0700 vs. 1800) and restricted time for grazing on A) grazing time, and B) forage intake rate for cows. R = restricted grazing (12 h/d); U = unrestricted grazing (24 h/d); supplement feeding time x restricted time for grazing interaction on grazing time (P = 0.09) and intake rate (P = 0.01).

View larger version (8K): [in this window] [in a new window]   Figure 6. Influence of sward height and period on grazing time for cows. Period 1 = May; Period 2 = June/July; Period 3 = August; High SH = high sward height; Low SH = low sward height; sward height x period interaction, P = 0.02.

	Discussion

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The two measurements for forage height, SH and SSH, were included to allow for comparison with other literature on the subject. The SSH is measured without disturbing the canopy of a pasture, whereas to measure SH, pressure exerted by the plate disturbs the canopy of a pasture as the height is being taken. Herbage growth rate was greater for the low vs. the high sward (Table 2). In a previous study on the same pasture, carrying capacity per hectare or stocking density increased as SH decreased from 12 to 4 cm as measured by the Hill Farming Research Organization sward stick (Prigge et al., 1997). Prigge et al. (1997) suggested that a low SH (8 to 10 cm) was needed to optimize forage growth rate for these types of forages as well as animal performance; results of this study agree with that conclusion. In the present study, herbage growth rate decreased as the season progressed (Table 4), which was considered to be a normal response to temperature and moisture limitations for cool-season forages (Pearson and Ison, 1997).

Forage maturity and the resulting senescence (Hodgson, 1990) probably contributed to the differences in the proportion of grass and dead material observed in this study (Figures 2A and B). During Period 1, the high proportion of dead material (for both SH treatments) may have been due to carryover material from the fall (Prigge et al., 1999). During Period 3, increased maturity and decreased growth rate (Table 4) probably contributed to senescence, especially for the high sward. The proportion of other forbs was greater for low vs. high SH (12.7 vs. 9.1%), probably because SH influenced the competition between grass and legume and/or other forbs for resources such as light and soil nutrients (Bullock, 1996). Moreover, CP decreased, whereas NDF and ADF increased, as SH increased, as would be expected with more mature swards (Minson, 1990; Van Soest, 1994). The ADF content of forage increased (Table 5) as season progressed, as would be expected, again indicative of increasingly mature forage (Van Soest, 1994). Crude protein concentration was lower (P < 0.01) in Period 2 than in other periods, probably because of a proportional increase in NDF and ADF content over the same period.

Restricted time for grazing yielded different animal responses and depended on the time of supplementation. Cows restricted to 12 h/d grazing consumed more forage DM when supplemented at 0700 vs. 1800, whereas cows that were unrestricted exhibited the opposite trend. A possible explanation for these findings is that cows on restricted management, having been withheld from grazing during the night, perhaps exhibited a compensatory forage intake at mid-morning (0700 to 1000), when most grazing activity occurred. Satiety or rumen fill (Forbes, 1988) may have limited forage DMI for unrestricted cows fed at 0700 as opposed to 1800.

Adams (1985) reported that steers supplemented at 0730 (with corn at 0.3% of BW) had greater (P < 0.05) forage DMI than the group supplemented at 1330. The author suggested that steers fed a supplement at 1330 probably substituted corn for forage more than the group supplemented at 0730 did. In fact, steers supplemented at 1330 had greater (P < 0.10) DE intake (8.4 vs. 7.7 Mcal/100 kg of BW) and ADG (P < 0.01) than the 0730 group, probably because of increased intake of total DM (Adams, 1985). Differences in animal response to feeding time between this study and that of Adams (1985) probably lie in the nutritive value of forages available to animals, or perhaps to the density of the sward. The ADF and CP content of forage used in our study were 33% and >12%, respectively, whereas for Adams (1985), it was 48% and <7%, respectively. Adams (1985) did not report forage availability estimate.

Fiber utilization usually increases when ruminal availability of carbohydrate and N is synchronous (Hoover, 1986). Studies by Sinclair et al. (1993) concluded that a synchronized supply of N and energy substrates could achieve a stable ruminal environment (less pH fluctuation and optimal microbial protein synthesis). Supplying one or both of these nutrients (carbohydrate and/or N) at 1800 when the amount available from forage probably was diminishing or availability in the rumen was limited, may have elicited the increased forage DMI observed for the unrestricted grazed cows.

Forage Digestibility

Supplementation time exerted a greater influence on DM digestibility for cows on high SH (Figure 4), whereas it had minimal influence for cows on low SH. Because the CP content of high SH forage was lower and fiber content higher than that of low SH (Table 2), ruminal NH₃-N concentrations may have been limiting at certain times of the day for optimal fiber digestion. Supplementing cows grazing high SH forage at 1800 as opposed to 0700 may have provided additional ruminal NH₃-N at times when needed, resulting in greater microbial digestion of the forage fiber. The lower NDF and ADF levels of the low SH forage, in conjunction with a slower rate of NDF and ADF consumption as one would expect on a pasture of lower SH (Rook et al., 1994), may have resulted in the 0700 supplement having less of a pH-related inhibition of ruminal fiber digestion (Hoover, 1986). In addition, diurnal fluctuations of forage nutrients (Holt and Hilst, 1969; Youngberg et al., 1972) also could have contributed to the digestibility response observed in this study by influencing the ruminal environment.

Microbial growth and protein synthesis can be stimulated by stable ruminal fermentation with a constant supply of fermentation substrate and nutrients (Sniffen and Robinson, 1987; Khorasani et al., 1994). In our study, it is possible that N and energy synchronization may have been achieved at different times for each forage height: 0700 for low SH and 1800 for high SH.

Digestible Dry Matter Intake

Cows restricted to 12 h/d grazing consumed a greater amount of forage digestible DM (Figure 3B) when supplemented at 0700 as opposed to 1800 mainly due to increased forage DMI (Figure 3A). Unrestricted cows had greater forage DDMI when supplemented at 1800 as opposed to 0700, most likely because of enhanced or lesser inhibition of forage digestibility, whereas satiety or rumen fill (Forbes, 1988) probably limited forage DDMI for the cows supplemented at 0700. Restricted flow of digesta through the gastrointestinal tract can be attributed to consumption of slowly digestible feeds; consequently, forage DMI on these types of diets is thought to be limited mainly by gut fill (Allen, 1996). The characteristics that contribute most to forage intake include solubility or cell content of forage, insoluble but potentially fermentable fraction, degradation rate, ruminal outflow rate, and rate of particle size reduction (Ørskov and Fraser, 1975). These factors may have contributed to the animal responses in forage DDMI observed in this study.

Grazing Time Pattern

In this study, grazing intensity of both unrestricted and restricted cows was mainly concentrated in the mid-morning (0700 to 1000) and early evening between 1800 and 2000. Most grazing activities of cattle take place during the daylight hours (Stobbs, 1970; Rook et al., 1994). Penning et al. (1991) suggested that a large evening meal could be an optimal foraging response to increased levels of accumulated sugars from photosynthesis in plant leaves at this time. The large mid-morning meal (Figure 7) observed in this study could be attributed to animal response to decreased gut fill (Forbes, 1988) and low ambient temperature (Seath and Miller, 1946), especially during the summer months, generally agreeing with other reports (Stobbs, 1970; Rook et al., 1994).

View larger version (12K): [in this window] [in a new window]   Figure 7. Influence of restricted time for grazing and time of day on grazing time pattern for cows. R = restricted grazing (12 h/d); U = unrestricted (24 h/d); restricted time for grazing x time of day interaction, P = 0.06.

Grazing Time and Forage Intake Rate

Cows restricted to 12 h/d probably grazed with greater intensity (Figure 7) compared with unrestricted cows, in response to a stimulus initiated by low ruminal fill following a 12-h lapse of no grazing activity. However, the methodology of using pluck samples to represent nutritive value of herbage consumed in this study would not be sensitive enough to detect these differences. Cows grazing on low SH pasture allocated more time to grazing activity during Period 1 compared with Periods 2 and 3 (Figure 6). Cows grazing on high SH pasture spent more time grazing during Period 1 and 2 than in Period 3. The decreased grazing time observed for the later periods (for both groups) was possibly related to the high ambient temperatures at this time of the year and a decrease in nutrient requirements for the cows in the later stages of lactation or greater selectivity due to declining nutritive value of forage.

	Implications

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The results of this experiment show how a pasture-based livestock system using concentrate supplements can optimize production efficiency by timing supplementation and grazing activities. When SH is lower (less available forage), supplementation is more effective in the morning than in the evening; however, when SH is higher (more available forage), supplementation is more effective in the evening. In dry years when forage availability is decreased, supplementation in the morning may be the most effective way to maintain animal performance. For pasture-based dairy systems, where animals may have restricted grazing time, supplementation in the morning also may be beneficial.

	Footnotes

 
¹ Published with the approval of the Director of the West Virginia Agric. and Forestry Exp. Stn. as Scientific Article 2904, Div. of Anim. and Vet. Sci.

² Research was supported by the Northeast Regional Research Project NE-132: Environmental and Economic Impacts of Nutrient Management on Dairy Forage Systems.

⁴ Station Statistician.

³ Correspondence: P.O. Box 6108 (phone: 304-293-2631; fax: 304-293-2232; e-mail: wbryan{at}wvu.edu).

Received for publication July 16, 2003. Accepted for publication February 18, 2005.

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