| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
ANIMAL PRODUCTION |
*Southwest Research and Extensions Center, Division of Agriculture, University of Arkansas, Hope 71801
 |
Abstract |
|---|
 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
Key Words: cattle • fasting and herbage allocation • grazing behavior • nutrient flow • rumen metabolism • site and extent of digestion
|
INTRODUCTION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
). Herbage is characterized by an increase in its nutritive value during the day (Lechtenberg et al., 1971; Griggs et al., 2005; Mayland et al., 2005). Such increase is attributable to moisture loss, nonstructural carbohydrates accumulation, NDF and CP concentration dilution (Delagarde et al., 2000), and increases in IVDMD (Gregorini et al., 2006a) and palatability (Provenza et al., 1998). This fluctuation matches the daily grazing pattern of cattle (Gregorini et al., 2006b), which appears to maximize intake rate at dusk (Gibb et al., 1998), when herbage has the highest nutritive value. Afternoon herbage allocations have shown increases in herbage intake during dusk (Orr et al., 2001; Gregorini et al., 2006a). However, the positive relation between hunger and intake rate might indicate that dusk herbage intake is not yet maximized. Jung and Koong (1985) found that fasted sheep increased intake rate from 47 to 124 mg·min–1 kg–1 of BW–0.75. Greenwood and Demment (1988) found that fasting increased intake rate of steers by up to 62%, and Dougherty (1991) reported an increased intake rate by 50% in beef heifers fasted 16 h. Therefore, matching afternoon herbage allocation with fasting periods might maximize nutrient intake.
Intake pattern dictates the dynamics of fermentation and passage of particles through the rumen. Therefore, changes in the grazing pattern might generate differential patterns of nutrient supply and animal performance. The latter is supported by the results of Orr et al. (2001), Chilibroste et al. (2004), and Gregorini et al. (2006a), but the former statement is still an assumption, which signifies a lack of information. The experiment aimed to test the following hypothesis: combinations of plant and animal processes modify grazing pattern, thereby altering the nutrient supply of strip-grazed cattle.
|
MATERIALS AND METHODS |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
.
The experiment was conducted at the Southwest Research and Extension Center of the University of Arkansas (33°42'N, 93°31'W) from March to June of 2006. During this period, mean daily temperature and precipitation were 12.3°C and 153 mm, respectively. The pasture utilized was a monoculture of wheat (Tritucum aestibum L.) managed to keep a high level of horizontal and phenological homogeneity. Four sites with marked differences in phenological stage were selected to establish the set of strips for each of the 4 experimental periods. The first period began with the site that had the most advanced phenological stage in the scale of Stauss (1994). The animals utilized were 4 Angus heifers (279 ± 99 kg of BW, empty BW) fitted with silicon ruminal and duodenal (T type) cannulas. All surgeries were conducted by staff clinicians of the Oklahoma State University Veterinary Hospital.
Grazing Management, Experimental Design, and Treatments
Heifers strip-grazed individual pasture strips throughout the experiment. The strips to be grazed always offered an herbage allowance of 6% (DM basis) of BW to avoid any potential ingestive constraints to herbage intake (Combellas and Hodgson, 1979). The size of the strips was determined according to the herbage mass (kg of DM/m2) and allowance. Herbage mass was determined weekly; nine 30-cm x 30-cm squares per paddock were randomly selected and cut with manual mowers to ground level, and the clipped herbage was then collected, weighed, and sampled for DM determination. While grazing, the heifers had no access to water and were not provided shelter or salt. Because they had been previously fed a concentrate-based diet, all heifers were allowed to strip-graze wheat for 10 d before beginning the experiment. During these days, heifers were moved to a new strip at 1100 h. After this diet standardization period, the heifers were randomly assigned to treatments as a 4 x 4 Latin square design (Gill, 1978). Each period was 12 d (8-d adaptation and 4-d measurement).
Treatments consisted of: 1) morning herbage allocation (MHA; heifers were placed in an ungrazed strip at 0800 until 1900 daily), 2) afternoon herbage allocation (AHA; heifers were placed in an ungrazed strip at 1500 until 1900 h daily; the next day, the heifers were placed in the same strip at 0800 until 1500 h then moved to a new ungrazed strip at 1500 h), 3) morning herbage allocation plus fasting (MHAF; heifers were placed in an ungrazed strip at 0800 until 1200 h daily; from this time to 0800 h of the next day, the heifers were fasted), and 4) afternoon herbage allocation plus fasting (AHAF; heifers were placed in an ungrazed strip at 1500 until 1900 h daily; from this time to 1500 h of the next day, heifers were fasted). Fasting and overnight resting (from 1900 to 0800 h for all treatments) took place in a 6-m x 12-m pen beside the pasture, where they had access to water.
Measurements and Calculations
Grazing Behavior and Herbage Intake.
Behavioral and intake measurements took place on d 12 of each period. Four trained observers were randomly assigned to each treatment and period to visually determine (Hirata et al., 2002) eating, rumination, and idling behavior every 2 min while the heifers were in the pasture strips. From these data, eating, rumination, and idling times were calculated by multiplying each behavior frequency by a 2-min interval. While grazing, cattle search, acquire into the mouth, masticate, and swallow herbage (Gibb, 1998). For the purpose of this experiment, eating behavior was defined as heifers with heads down and completely engaged in acquiring herbage into the mouth, masticating, swallowing, and searching at the feeding station level. The observers also counted bites and eating steps during 1 continuous minute hourly while the heifers were eating. Each eating step was considered a feeding station because with each eating step the heifers define a potential eating area (Ruyle and Dwyer, 1985; Rook et al., 2004). Behavioral times (eating, rumination, and idling), bite rate, step rate, herbage intake rate, and bites per feeding station were then summarized into 3 periods: morning (0800 to 1200 h), afternoon (1200 to 1500 h), and evening (1500 to 1900 h), corresponding with the main grazing bouts (Gibb et al., 1998; Taweel et al., 2005; Gregorini et al., 2006b).
The daily herbage DMI was calculated as the sum of herbage DMI of each period (morning, afternoon, evening). The herbage DMI in each of these periods was estimated from changes in the DM ruminal pools as described by Taweel et al. (2005) based on Chilibroste et al. (2000). In brief:
 |
where RP = the DM ruminal pool size (kg), t2 = the ending time of each grazing bout (time of day), t1 = the starting time of each grazing bout (time of day), Kcl = the fractional rate of clearance of RP t1, and t = the time elapsed between the start and the end of each grazing bout.
The Kcl was estimated over the night fasting period (heifers were rumen evacuated at 1900 h and the next day at 0800 h) assuming a first-order kinetics (Robinson et al., 1986), with 1 pool being cleared at a constant fractional rate. The DM ruminal pool data were collected at 0800, 1200, 1500, and 1900 h by the ruminal evacuation technique described Taweel et al. (2005). To calculate herbage intake rate per time of day, each amount of herbage consumed was divided by the eating time. Bite mass was estimated by dividing the herbage consumed at each time of day by the product of bite rate (bites/min) and eating time (min).
Ruminal Environment.
To determine the diurnal pattern of ruminal fermentation, ruminal fluid was sampled during d 9 at 0800, 1200, 1500, 1900, and at 2300 h. To facilitate sampling at pasture, samples (approximately 200 mL) were collected from the ventral sac of the rumen using a solid PVC plastic tube (80-cm long and 2.5 cm in diameter). The pH of these samples was measured immediately, using an electronic pH meter (Check-Mite electrode, Nova Analytics, MA). Two subsamples were taken, acidified with 0.5 mL of 85% phosphoric acid, and stored frozen (–20°C) pending VFA and ammonia analyses.
VFA and Ammonia N Analysis.
Samples were thawed at room temperature and centrifuged for 10 min at 10,109 x g (TJ-6R, Beckman Coulter Inc., Fullerton, CA). Subsequently, 5 mL of supernatant were mixed with 1 mL of 25% metaphosphoric acid + 2-ethyl-butyric acid. The liquid was recentrifuged for 30 min at 10,109 x g. One milliliter of supernatant was pipetted into a vial and placed onto the GLC (5890 Series II, Hewlett Packard, Palo Alto, CA) autosampler. The GLC was equipped with a capillary column of 15 m, 0.53-mm ID, and 0.5-µm film thickness (Supelco Nukol, Bellefonte, PA). The hydrogen carrier flow rate was set at 15 mL/min, with the initial oven temperature at 110°C. Oven temperature was increased to 150°C at a rate of 8°C/min beginning at 0.10 min. Injector and detector temperatures were set at 250°C. Concentrations of acetic, propionic, isobutyric, butyric, isovaleric, and valeric acid were determined, integrating the peaks with a 3396A integrator (Hewlett Packard) and identified with commercially available VFA standards (Sigma Chemical, St. Louis, MO). The total concentration of VFA in the ruminal fluid was the sum of individual VFA. The calculation and expression of the glucogenic and nonglucogenic VFA, and the nonglucogenic:glucogenic VFA ratio (NGR, where NGR = [acetate + (2 x butyrate)]/propionate]), were similar to those used by Chilibroste et al. (2005).
Ruminal samples were also analyzed for ammonia N by the methodology of Broderick and Kang (1980). Briefly, 0.05 mL of sample was mixed with 2.5 mL of phenol reagent and then 2 mL of hypochlorite reagent was added and mixed. This mixture was incubated for 5 min in a 95°C water bath, cooled, and read on a spectrophotometer (Spectronic 20, Bausch & Lomb, Rochester, NY) at 630 nm.
Site and Extent of Digestion of Herbage OM.
To estimate total tract, site, and extent of digestion of herbage OM, the equations used by Galyean (1997), with the internal marker indigestible NDF (INDF), were used. True ruminal OM herbage digestibility was calculated from the apparent ruminal OM digestibility and the daily OM herbage intake, as affected by the microbial OM flow to the duodenum. It was assumed that microbial N contained 20% RNA (McCollum et al., 1987) and that bacterial OM contained 65% CP (Nocek and Tamminga, 1991; Van Vuuren et al., 1997). Lower tract digestibility was calculated by the difference of total tract digestibility minus ruminal digestibility.
Sampling.
Herbage was sampled 4 times daily (0800, 1200, 1500, and 1900 h) in correspondence with the main grazing bouts of grazing cattle (Gibb et al., 1998, Gregorini et al., 2006b). Three samples per treatment per period were taken on d 8, cutting three 30-cm x 30-cm squares with manual mowers to a stubble height of 3 cm. These samples were split for INDF (pooled by treatment and period) and chemical composition analysis (pooled by time of day, treatment, and period). Duodenal digesta (300 mL) and feces (200 g) sample collections lasted 2 d (d 10 and 11). A total of 12 samples were taken at intervals of 4 h during the first (starting time 0800 h) and second (starting time 1000 h) day. All samples were immediately placed in dry ice and were stored at –20°C.
Indigestible NDF was determined in filter bags (F-57, Ankom Co., Fairport, NY). These bags were filled with 1.5 g of dry (lyophilized) forage, duodenal, and fecal samples ground to pass a 1-mm screen in a Wiley Mill (Model 5, Thomas Scientific, Swedesboro, NJ). The bags were heat-sealed using an impulse sealer (Model CD-200, National Instrument Co., Baltimore, MD). All bags for each combination of heifer, treatment, and period were placed in a 36-cm x 50-cm mesh bag and inserted into the ventral sac of the rumen in a cannulated heifer. The bags were incubated for 120 h; during incubation, the heifer grazed a sward of similar condition and nutritive value to the experimental wheat. While grazing, the heifer had ad libitum access to fresh water and herbage only. All bags were removed simultaneously and placed immediately in water containing ice. The bags were subjected to 10 cold-water rinse cycles, with 1 min of agitation per rinse (Coblentz et al., 1997). The NDF analyses were conducted using the batch procedures outlined by Ankom Technology Corporation (Fairport, NY).
OM and Microbial Protein Flows.
Organic matter flow at the duodenum and fecal OM output were calculated as the ratio of INDF intake (g/d):indigestible NDF concentration (g/g of DM) in duodenal or fecal samples multiplied by their respective OM concentration (Gunter et al., 1997). The INDF intake (g/d) was estimated by multiplying the daily herbage DMI by the INDF concentration of the corresponding samples (animal·-treatment–1·period–1) of herbage. The flow of microbial N at the duodenum was determined from the ratio of purines:total N in the duodenal samples. Duodenal samples were analyzed for total N using FP-528 TruS-pec (St. Jones, MI) and 990.03 method (AOAC, 1995) and purines (Zinn and Owens, 1986).
Rates of Dilution and Ruminal Passage of Particles.
Liquid and particle passage rates were estimated by the double-marker technique (Faichney, 2005) using CoEDTA (Uden et al., 1980) and mordant Yb (Galyean, 1997) as markers for liquid and solid fractions, respectively. Cobalt EDTA crystals were dissolved in distilled water (65.2 g/L). Herbage potentially consumed (Gregorini et al., 2006a) was labeled with Yb (2.5 g of YbCl3). In brief, samples of herbage (approximately 200 g of fresh material) available to the heifers were taken by walking next to the heifers for 2 min, taking hand-plucked samples. Hand-plucked samples were obtained from ungrazed areas at the same feeding station. Samples were pooled based on and equal DM weight and oven-dried at 60°C for 48 h. Markers were dosed into the rumen as a pulse at a rate of 275 mL of CoEDTA solution and 40 g of Yb-labeled herbage. A portion of each duodenal sample (150 mL) was oven-dried at 60°C for 48 h, and then Co and Yb were extracted following the procedure of Hart and Polan (1984). The EDTA extract was analyzed on an atomic absorption spectrophotometer (Perkin-Elmer Co., Boston, MA). Cobalt was determined using an air and acetylene flame; Yb was determined using a nitrous oxide and acetylene flame. Rates of ruminal dilution and passage of particles were calculated by regression of the natural log of the Co or Yb sample concentrations against sampling time (Galyean, 1997). Turnover rates of liquid and solid ruminal phases were calculated as the inverse of the ruminal dilution and passage of particle rates.
Herbage Quality.
Herbage samples were oven-dried at 60°C for 48 h, ground to pass a 2-mm screen, and analyzed for OM, CP, NDF, NDS (NDS = 100 – NDF; NDS = neutral detergent solubles), and ADF. The latter 2 analyses were conducted sequentially using the batch procedures outlined by Ankom Technology Corporation. Concentrations of N in each sample were determined by rapid combustion (850°C), conversion of all N-combustion products to N2, and subsequent measurement by thermo-conductivity cell (Leco model FP-428, Leco Corp., St. Joseph, MI). Crude protein was calculated as the percentage of N in the sample multiplied by 6.25.
Statistical Analysis.
Data from behavioral activities during the day, ruminal environment, and ruminal fermentation pattern were analyzed by ANOVA as a 4 x 4 Latin square with repeated measurements in cells. These measurements were analyzed using the MIXED procedure (SAS Inst. Inc. Cary, NC) for repeated measurements (Littell et al., 1998). The model included treatment, period, time, and all interactions. The random effect of heifer within each treatment and period, specified in the RANDOM statement, accounted for the correlations among repeated observations on the same heifer. No treatment x time x period interactions (P > 0.10) were observed for the variables measured. However, if the treatment x time interaction was significant (P < 0.05), the means were evaluated at each time using the PDIFF function of SAS. Daily herbage intake and times of behavioral activities, as well as nutrient duodenal flows, OM fecal output, ruminal dilution rate, particulate passage rate, liquid and solid ruminal content turnovers, and site and extent of OM digestion were analyzed by ANOVA as a 4 x 4 Latin square using the MIXED procedure of SAS. The terms in the statistical model included treatment, heifer, and period least squares means were compared using the PDIFF function, when protected by a significant (P < 0.05) treatment effect. Data from chemical composition of herbage were analyzed by ANOVA as using GLM procedure of SAS. The terms in the statistical model included the fixed effects of time of day, period, and the period x time of day interaction. Least squares means were compared using the PDIFF function of SAS. A value of P < 0.05 was considered significant.
|
RESULTS AND DISCUSSION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
. This change was accelerated by unexpectedly warm temperatures and rain after period 1. These phenological stages may explain the low-quality values for the wheat available to heifers (Table 1) over the course of the experiment, and the effect (P < 0.05) of period for most of the variables analyzed. In spite of this, there was no period x time of day interaction (P > 0.05). Therefore, herbage quality and the remainder of the variables analyzed will be discussed only in terms of treatment and treatment x time of day effects.
|
with perennial ryegrass (Lolium perenne L.), and Gregorini et al. (2006a) in Italian ryegrass (Lolium multiflorum Lam.). These authors reported that seasonal effects (winter, spring) did not interact with the variation in chemical composition of herbage throughout the day light hours. In the present experiment, DM concentration increased by 19.6% (P < 0.05) from 0800 to 1900 h. The OM concentration also increased (6%; P < 0.05), reaching the greatest value at 1500 h. From 1500 to 1900 h, OM concentration decreased 4.8%. A similar pattern was followed by NDS concentration, and its inverse by NDF concentration. The former increased 9.9% by 1500 h, and decreased the same proportion by 1900 h (P < 0.05). This decrease may be related to an increase in respiration and metabolic activity of the plants during dusk. The CP concentration did not fluctuate (P > 0.05). These results are comparable with the diurnal variations of NDF concentration reported by Fisher et al. (1999) and Burns et al. (2005) for alfalfa hay cut at sunset after a sunny day compared with morning cutting. Delagarde et al. (2000), Mayland et al. (2005), and Gregorini et al. (2006a) state that the major change in diurnal nutritive value is attributable to an increase in photosynthates, which also leads to a passive dilution of NDF concentration and CP. Results found in this experiment (Table 1
) sustain this premise. Consequently, the herbage grazed by heifers in this experiment had greater nutritive value during the afternoon and early evening.
Herbage Intake and Behavioral Patterns
Daily Herbage DMI.
The daily herbage DMI did not differ among treatments (P > 0.05), averaging 1.65% of BW. The lack of difference between MHA and AHA (Table 2) agrees with results previously reported by Orr et al. (2001) and Gregorini et al. (2006a). These authors found no difference in daily herbage DMI when comparing morning and afternoon herbage allocations for dairy cows or beef heifers, respectively. The expected response of animals to fasting is an increase in short-term herbage intake rate (Jung and Koong, 1985; Greenwood and Demment, 1988; Patterson et al., 1998) that might increase daily herbage DMI. Results in this experiment (Table 2) indicate that the alteration of variables determining the short-term herbage intake rate (i.e., greater bite rate, bite mass) may be of short duration, and not enough to change daily DMI. This is supported by Gregorini et al. (2007b), who did not find changes (P > 0.05) in herbage DMI when beef heifers were fasted (8 h) before afternoon herbage allocation. However, the fasting effect was strong enough to alter the herbage DMI rate, and compensate the reduced grazing time. Independently of the timing of fasting, in less than 4 h (MHAF 187 min, AHAF 196 min), heifers consumed as much herbage as when they were not fasted. When fasted, heifers stopped eating before the end of the grazing session. This discards the possibility of a shortage of time to eat. Cessation of grazing was probably caused by the rumen being near maximum fill. This premise is supported by Taweel et al. (2005), who raised the importance of ruminal fill in controlling the level of intake and the ending of each grazing bout, and Gregorini et al. (2007a), who demonstrated the important role of ruminal fill in controlling grazing dynamics within bouts.
|
). In the present experiment, time dedicated to eating activity clearly differed (P < 0.05) among treatments (Table 2). Nonfasted heifers invested 48% of the time available for grazing to eating, compared with 80% for fasted heifers. Because of the lack of difference (P > 0.05) in herbage DMI, these results demonstrate that when herbage availability is high, daily fasting period may counteract reductions in available daily grazing times (Iason et al., 1999; Gregorini et al., 2007b). The time invested in idling activities during the time available to graze did not differ (P > 0.05) within fasting or nonfasting treatments (Table 2). Rumination time did not differ (P > 0.05) between MHAF and AHAF, but differed (P < 0.05) between MHA and AHA. The AHA not only increased rumination time (37%) over MHA, but also may have altered its temporal distribution during the day, leading heifers under AHA to ruminate longer during the morning hours. This premise is supported by results of Gregorini et al. (2006a), who showed how beef heifers under AHA allotted more rumination time during morning and afternoon, when compared with MHA.
Time of Day and Behavioral Activity.
The lack of difference in total eating time between MHA and AHA did not mean equal distribution of eating time (Table 3). Afternoon herbage allocation resulted in heifers concentrating eating time during the evening (Table 3), and rumination time during the morning and afternoon. Morning herbage allocation had the opposite pattern; heifers allocated a greater proportion of eating time during the morning. These results agree with the schematic representation of time allocation of daily behavioral activities shown by Orr et al. (2001) and Gregorini et al. (2006a). Therefore, these results support the premise that a simple change in timing of herbage allocation might alter duration and intensity of individual grazing bouts, and thereby modify the connection among them into the temporal distribution. In the present experiment, grazing intensity was assessed through feeding stations rate, bites/feeding station, bite rate, bite mass, herbage DMI rate, and herbage DMI per time of day (Table 3).
|
) varied (P < 0.05) according to treatments. In MHA it did not differ (P > 0.05) from morning to afternoon, but decreased (P < 0.05) 58% from the afternoon to the evening. In AHA, feeding station rate was also the slowest (P < 0.05) in the afternoon; however, it did not differ (P > 0.05) between morning and evening. In AHAF, heifers showed 3 feeding stations more (P < 0.05) per min than under MHAF. During the morning, feeding station rate did not differ (P > 0.05) among MHA, MHAF, and AHA. In the afternoon, this rate was the slowest (P < 0.05) in AHA and the slowest for MHA (P < 0.05) during the evening. As described by Ruyle and Dwyer (1985) and Rook et al. (2004), a feeding station defines a potential grazing area. Assuming a constant area per feeding station, the product of feeding station rate and eating time per time of day would show that potentially consumed area differs among treatments and time of day. It seems that allocating the new strip during the afternoon with or without fasting stimulated heifers to eat longer and potentially harvest more area during the evening when compared with MHA. Ruyle and Dwyer (1985), Hudson and Nietfield (1985), and Wade et al. (2006) observed that grazing sheep, wapiti, and beef heifers, respectively, tended to take fewer bites per feeding station when pastures had become depleted. In addition, Wade et al. (2006) detected changes in bites per feeding station throughout the day, and the effect of fasting. In contrast to these results, heifers of the present experiment did not show response to treatments in this variable, except with MHAF during the morning (P < 0.05). A possible explanation to the pattern of this variable may be related to the stimulant effect of the night resting period that all treatments went through (Jung and Koong, 1985; Iason et al., 1999), which was enhanced by the previous fasting for MHAF.
Bite Rate and Mass.
Bite rate decreased by 50% from morning to evening in MHA (Table 3), whereas in AHA, bite rate did not differ (P > 0.05) between morning and evening. When comparing all treatments, bite rate did not differ during the morning, but was different (P < 0.05) during the evening (greater for AHAF and AHA than MHA). Bite rate did not differ (P > 0.05) between AHA and AHAF, which agrees with results of Dougherty et al. (1989) and Chilibroste et al. (1998), who reported no impact of fasting on bite rate for beef heifers or dairy cows grazing tall fescue or perennial ryegrass, respectively. This premise also supports the lack of effect (P > 0.05) of fasting on bite rate during the morning for MHA vs. MHAF. The pattern of bite mass differed (P < 0.05) between AHA and MHA. For MHA, it was constant (P > 0.05) throughout the day, whereas bite mass increased for AHA (P < 0.05) 1.5 times from morning to evening. During the morning, bites were the heaviest (P < 0.05) for MHAF and heavier (P < 0.05) for MHA than AHA. Results found for MHA and MHAF show the effect of hunger on bite mass, which agree with previous works of Patterson et al. (1998) and Gregorini et al. (2007a). Despite MHA and AHA had the same time restriction to the pasture (resting evening), in AHA, heifers concentrated (P < 0.05) herbage intake during the evening. In the evening, heifers in AHA and AHAF took heavier (P < 0.05) bites than MHA. At this time of day, fasting did not affect (P > 0.05) bite mass. The depleted strip conditions faced during the morning by heifers in AHA may have lead to and explain the lowest bite mass (McGuilloway et al., 2001) at this time of day.
Pattern of Herbage Intake.
The herbage intake rate is the product of bite rate and bite mass (Hodgson, 1985). In the present experiment, the pattern of bite rate and bite mass approximately matched the pattern of herbage intake (Table 3) found in AHA and MHA. Herbage intake rate increased (P < 0.05) from the morning to the evening for AHA. During the evening this rate was the greatest (P < 0.05) for AHAF, and tended to be greater (P = 0.09) for AHA than MHA. Heifers in AHAF consumed 20 mg·min–1·kg–1 of BW of herbage DM more than MHA. The pattern of herbage intake per time of day differed per time of day (P < 0.05) and treatment (P < 0.05). These results indicate that strategic management of the grazing process can increase herbage intake during times of day in which herbage nutritive value is greater. This premise is supported by the inferences made by Orr et al. (2001) and Gregorini et al. (2006a), Gregorini et al. (2007b), and Chilibroste et al. (2007) with their results of milk yield, ADG, and herbage intake rate. In the current study, the highest nutritive value of herbage was during the afternoon declining from 1500 to 1900 h. Consequently, heifers did not face the greatest nutritive value during the entire evening period. However, the stimulus of opening the new daily strip and a combined fasting period may have led heifers in AHA and AHAF to concentrate and intensify their eating activity during the first bouts of the evening grazing event, still being able to harvest a greater amount of nutrients than in treatments MHA and MHAF. This idea is supported by results of Greenwood and Demment (1988) and Gregorini et al. (2007b), who observed that steers fasted for 36 h and heifers fasted for 8 h (previous to an afternoon allocation of the new daily strip) grazed longer than nonfasted cattle, in the initial grazing bouts.
Rumen Metabolism and Nutrient Supply as a Function of Grazing Management
Despite the existence of studies relating grazing behavior and ruminal environment (Chilibroste, 1999; Taweel et al., 2005; Bargo and Muller, 2005), and studies related to site and extent of digestion of herbage as well as digesta flow under grazing conditions (Dove and Milne, 1994; Gunter et al., 1997), there is a lack of information matching grazing management, eating behavior, and digestive processes. This lack may be related to the complexity of studying digestive factors in a dynamic feeding environment as grazed pastures, where researchers need to make certain mathematical assumptions in the calculations of digestive processes, which are not necessarily always met. The previous section showed the impact of specific grazing managements (MHA, AHA, MHAF, and AHAF) in altering certain parameters of ingestive behavior and herbage DMI intake patterns. This section discusses the effect of those alterations on digestive processes and its impact on nutrient supply.
Ruminal Environment and Fermentation Pattern.
Changes in ruminal fermentation can be achieved through dietary modifications (Van Soest, 1982), which shift the bacterial population dynamics of the rumen (France and Dijkstra, 2005; Theodorou and France, 2005), resulting in a modification of the nutrients supplied by rumen to the host animal.
Ruminal pH.
Diurnal variations in ruminal pH can be explained by the relationship between pH with meal frequency (Pitt and Pell, 1997). Under grazing environments, this frequency is linked to the diurnal grazing pattern (Gregorini et al., 2006b) and relates to the diurnal variation in pH found by Taweel et al. (2005) and Bargo and Muller (2005). In the current study, the diurnal pattern of ruminal pH (Figure 1) was affected (P < 0.05) by treatment following the altered patterns of eating behavior and herbage DMI intake. The comparison of these results with the diurnal pattern of pH under continuous stocking presented by Taweel et al. (2005) demonstrates that, grazing management can modify the pattern of ruminal pH. They also suggest that this pattern would not only depend on frequency, but also intensity and temporal arrangement of grazing bouts.
|
, 1998). This can be explained because heifers in MHAF had the summation of fasting and fresh herbage (ungrazed strip) stimuli. This premise is confirmed by the lack of differences (P > 0.05) between 0800 and 1200 pH with AHA. During the evening, the pH of AHA and AHAF did not differ (P > 0.05); however, it was lower (P < 0.05) than the ruminal pH of MHA and MHAF (Figure 1). The lack of difference between the ruminal pH of AHA and AHAF during the evening may be related to changes in chewing activity. The greater intake rate (P > 0.05) in AHAF may have lead heifers to reduce ingestive chewing investment (Laca et al., 1991; Shipley et al., 1994), releasing a lesser amount of rapidly degradable and hydrolysable cell contents (Boudon and Peyraud, 2001).
Ammonia N.
Data presented in Figure 2 show the impact of grazing management on the diurnal pattern of ammonia N in the rumen. These results highlight the relationship of this variable with the patterns of herbage intake and ingestive behavior. The true protein content of most pasture plants is about 70 to 90% of their CP content (Tamminga, 1986). In leaves, 75% of this protein is in chloroplasts, and from that percentage 50% is soluble (Nolan and Dobos, 2005). Moreover, the nonprotein N is almost immediately (200%/h) hydrolyzed when solublized in the ruminal medium (Mangan, 1982; Boudon and Peyraud, 2001). Consequently, damage to ingested herbage during ingestive mastication may have an important impact in releasing this N, and therefore, in the pattern of ammonia N in rumen. Then, in addition to the ingestion pattern, the chewing investment may have a significant impact. This idea supports the lack of difference (P > 0.05) between AHA and AHAF during the evening. It also may explain part of the difference (P < 0.05) between AHA and MHA from 0800 to 1500. At the period of time when herbage may have greater nonprotein N, heifers in AHA had smaller (P < 0.05) bite mass, and may have chewed each bite more intensively (greater ingestive chewing investment). The greater values of ruminal ammonia N of MHAF might be related to a more unbalanced degradable N:degradable carbohydrate ratio at that time of day (0800 to 1200 h), as well as a reduced (theoretically) ruminal microbial population to utilize the N released.
|
; France and Dijkstra, 2005). This is determined by the composition of microbial population, which in turn is largely determined by the basal diet (Dijkstra, 1994). Based on these concepts, the analysis of the patterns of VFA concentrations in this experiment (Figures 3 and 4) showed that at the same amount of herbage allocation, grazing management altered fermentation pattern. This underlies a dynamic change in microbial population.
|
|
). Acetate, propionate, and butyrate concentrations can be expressed as glucogenic (propionate) and nonglucogenic (acetate and butyrate) VFA when variations of degradation rate either are, or are expected to be, small (Groot et al., 1998; Chilibroste et al., 2005). In an in vitro fermentation experiment, Groot et al. (1998) evaluated the end product of fermentation of physically separated cell walls and cell contents from leaves of Italian rye-grass (Lolium multiflorum Lam.). These authors found different fermentation pattern between cell wall and cell contents, showing reductions in the nonglucogenic:glucogenic ratio between cell wall (3.61) and cell contents (1.60). In the present experiment, the difference in the diurnal pattern of this ratio (Figure 4) indicates the effect of grazing management on fermentation rates and source of fermentation, which seems to match the diurnal variation of chemical composition of herbage available and the potential differences in ingestive chewing as demonstrated by the delay in measurements in VFA concentrations in MHAF. It seems that VFA concentration in MHAF peaked after heifers were taken out of the strips and started ruminating. It is evident that grazing management can alter fermentation pattern, modifying the nutrients supplied from the rumen. This supports a strategic advantage to allocate herbage during the afternoon.
OM and Microbial Protein Flow to Duodenum.
In a given diet, digesta flow is considered a function of intake (Merchen et al., 1986; Clark et al., 1992; Faichney, 2005). However, this function does not account for variations in food quality, ingestive behaviors, or diurnal patterns of nutrient intake. Results of the present experiment show that at the same level but different pattern of herbage (and nutrient) intake, net amounts of OM, N, microbial protein, duodenal nonmicrobial OM, and microbial OM differed among treatments (Table 4). Results shown in Table 4 illustrate the superiority of AHA in nutrient supply to the duodenum. On average, AHA supplied 970, 40, and 290 g more (P < 0.05) total OM, N, and microbial protein per day, respectively. The difference in nonmicrobial OM flow to the duodenum was not that marked; however, AHA was still superior (P < 0.05) to the other treatments. Regarding microbial OM flow, AHA supplied 415 g/d more (P < 0.05) than MHA and AHF, and 704 g/d more than MHAF.
|
pointed out that the difficulty of establishing the relative effects of intake, ruminal fermentation pattern, and ruminal dilution on ruminal microbial yield is the high correlation among these variables (Dove and Milne, 1994). The different herbage ingestion patterns (P < 0.05) and their potential different pattern of nonstructural carbohydrates intake, certainly modified (P < 0.05) the ruminal environment, fermentation pattern, dilution, and particle passage rates. The dilution rate in AHA was 5.23 points faster (P < 0.05) than the average of the other treatments (Table 5). Particle passage rate was also faster (P < 0.05) in AHA. Eating and ruminating provide the major motor inputs for ruminal motility (Pittroff and Soca, 2006). Eating time was greater (P < 0.05) in MHA and AHA, the latter having a larger rumination time (P < 0.05). The early work of Freer and Campling (1965) showed that the number of ruminal contractions increased almost exponentially with eating rate. Consequently, ruminal motility would be greater in AHA, which may explain its greater dilution and particle passage rates. Faster dilution rates (Nolan and Dobos, 2005) and solid (Kennedy, 2005) turnover rates have been positively related to microbial protein synthesis and flows. Increased dilution rate may synergistically interact with particle passage rate, due to the feed particles being suspended in the fluid passing through the reticulo-omasal orifice (Poppi et al., 1981). Particles passing out also may act as bacterial vectors (Sniffen and Robinson, 1987). According to Cecava et al. (1990), forage particles in the rumen would have a greater colonization (more biofilm attached) when more fermentable forage is fed. These concepts, and the fact that AHA increased herbage intake when herbage has the greater concentration of nonstructural carbohydrates may explain the differential flows of microbial protein and OM to the duodenum.
|
) and ruminal packing capacity (Chilibroste et al., 2005). The greater intake rates by MHAF and AHAF may have led heifers to less ingestive chewing investments, making them swallow larger particles. This fact could have led to increased retention times (Kennedy, 2005). In addition, fasting may have generated an unstable ruminal environment as opposed to smooth environments found in frequently fed animals (Varga and Kolver, 1997). This enhancement of diurnal variation of fermentation end products, in combination with a greater imbalance of nutrient ingestion, may explain the lowest flow of microbial protein and OM from the rumen to the duodenum in MHAF. It is interesting to point out that flow of OM, N, microbial protein, and OM did not differ (P < 0.05) between MHA and AHAF. This may have been a result of better coupling (in a daily scale) of energy and protein for AHAF than MHA, which is supported by the lack of difference in dilution and particle passage rates between these treatments.
Site and Extent of Digestion.
The assessment of site and extent of digestion is vital for understanding nutrient utilization (Merchen et al., 1997) and supply by ruminants. Results of this experiment demonstrate that, at the same level of intake, grazing management alters the pattern of nutrient intake modifying site and extent of digestion of herbage (Table 6). The magnitude of total tract digestibility approximately matched the OM outflow shown in Table 4. The concentration of herbage intake in morning hours (MHAF) decreased (P < 0.05) total tract digestibility. This may be related to a proportional increase in NDF intake, reducing total tract digestibility (Van Soest, 1982). This premise is supported by the 10.7-point difference (P < 0.05) in total tract digestibility when compared with AHAF. In this case, NDF intake may have been diluted by a greater intake of nonstructural carbohydrates. Total tract digestibility did not differ (P > 0.05) between AHA and MHA. According to Cecava et al. (1990) and Merchen et al. (1997), increasing the proportion of nonstructural carbohydrates in a diet increases the total OM digestibility and shifts the site of OM digestion. This observation partially matches results found in AHA. Cecava et al. (1990) stated that an increase in nonstructural carbohydrates would increase intestinal and decrease ruminal digestion. In the current study, apparent ruminal digestion was 13.14 points greater (P < 0.05) for fasted treatments, without difference (P > 0.05) in the extent of true OM digested in the rumen among MHA, MHAF, and AHAF. A greater retention time in MHAF and AHAF may have equilibrated them with MHA (Table 5); however, this lack of difference remains unclear. In the case of AHA, the extent of true OM digested in the rumen was 19.25 percentage points greater (P < 0.05) than the remainder of the treatments. This result may not only be related to a greater ingestion of non-structural carbohydrates, but also to a qualitative and quantitative difference in the ruminal microflora dynamics. The population of ruminal microflora in AHAF and MHAF heifers may have not been enough to receive a higher intake rate of herbage. After a 20-h period of rumination, digestion, and rest, heifers would be expected to have cleared a major part of their ruminal fill.
|
, who evaluated the effect of plains bluestem and midgrass prairie grasses across summer months and years on site and extent of digestion. Lower tract OM digestibility differed (P < 0.05) among treatments (Table 6), the greatest being in AHA and MHA, decreasing to AHAF, and the lowest in MHAF. These differences could be the result of the fact that fasting treatments (MHAF, AHAF) had just 1 grazing bout, and there were at least 3 for AHA and MHA. These results agree with the results of Cecava et al. (1990), who found a greater hind-gut percentage of digestion for more frequently fed steers (2 vs. 12 times daily). In spite of these agreements, the differences in lower tract digestibilities found in this experiment remain unclear. The OM digested in the lower tract mainly consists of microbial cells and ruminal undigested fiber. Therefore, a tentative and partial explanation would relate to the microbial OM flow (g/d) and the percentage of NDF in digesta flowing to duodenum, which did not differ (average = 59.8%, SE = 0.004; P > 0.05) among treatments. All these results question the generally accepted idea that changes in site and extent of digestion within diet is a function of DMI.
Conclusions.
Temporal fluctuations of herbage quality imply that grazing behavior may not be constant in time. A simple change in the time of herbage allocation of strip-grazed cattle late in the afternoon instead of early in the morning modifies the connection among grazing bouts into the temporal distribution that makes cattle graze longer and more intensely during dusk, when herbage quality has peaked. This study not only supports those behavioral changes, but also demonstrates that grazing management changes the diurnal pattern of herbage DMI, modifies ruminal function, nutrient flow to the duodenum, and site and extent of digestion. Definitely, afternoon herbage allocations in strip-grazing management increases the nutrient supply. Although afternoon allocation and fasting did not enhance nutrient supply, it equaled the amount of nutrient supplied by morning herbage allocations. This fact may allow managers to graze cattle in short grazing sessions and reducing trampling of the sward. However, the question arises, as to whether this additional management is really worth the effort. Because the outcome of any grazing strategy results from the complex interaction among herbage dynamics, ingestion, digestion, nutrient absorption, and their feedback, it was believed that producers have partial control over herbage quality and nutrient availability. However, a simple match in plant and animal processes may help graziers "to start managing" nutrients supplied by a dynamic food, pasture.
|
Footnotes |
|---|
2 Present address: USDA-ARS, Pasture Systems and Watershed Management Research Unit. Bldg. 3702, Curtin Road, University Park, PA 16802.
3 Corresponding author: sgunter{at}uaex.edu
Received for publication July 17, 2007. Accepted for publication January 7, 2008.
|
LITERATURE CITED |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
This article has been cited by other articles:
|
|
 |
|
  S. A. Gunter, P. A. Beck, D. L. Kreider, P. Gregorini, and C. B. Stewart The Effects of a Modified Glucomannan on the Performance of Stocker Cattle Grazing Endophyte-Infected Tall Fescue Professional Animal Scientist, June 1, 2009; 25(3): 300 - 306. [Abstract] [PDF]
|
|
|
|
|
|
P. Gregorini, K. J. Soder, and M. A. Sanderson Case Study: A Snapshot in Time of Fatty Acids Composition of Grass Herbage as Affected by Time of Day Professional Animal Scientist, December 1, 2008; 24(6): 675 - 680. [Abstract] [PDF]
|
|
|
|
 |
|
 A. F. Brito, G. F. Tremblay, A. Bertrand, Y. Castonguay, G. Belanger, R. Michaud, H. Lapierre, C. Benchaar, H. V. Petit, D. R. Ouellet, et al. Alfalfa Cut at Sundown and Harvested as Baleage Improves Milk Yield of Late-Lactation Dairy Cows J Dairy Sci, October 1, 2008; 91(10): 3968 - 3982. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
