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ANIMAL PRODUCTION |
* Southwest Research and Extensions Center, Division of Agriculture, University of Arkansas, Hope 71801;
and
Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, La Plata 1900, Argentina
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: beef heifer • grazing pattern • herbage allocation • performance • strip grazing
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Orr et al., 1997; Mayland et al., 2003). The DM and nonstructural carbohydrate concentration of herbage increase over the day because of the loss of moisture and accumulation of photosynthates, which mainly occur in the upper layers of swards (Delagarde et al., 2000). This variation results in an increase in digestibility (Linnane et al., 2001), palatability (Provenza et al., 1998), and preference (Fisher et al., 1999; Burns et al., 2005) for herbage grazed at dusk, which may play a role in shaping the daily grazing pattern and nutrient intake of grazing ruminants.
Foraging decisions at broader scales, such as where to begin grazing, are probably irrelevant in small paddocks, because the entire area is readily accessible (Bailey et al., 1996). At smaller scales, when to begin, which frequency, and how to distribute the grazing bouts (GB) might be more important. Daily grazing time is a cluster of discrete meals, GB, and these decisions may determine how cattle allocate feeding time to meet their requirements. In temperate climates, ruminants show a daily frequency of 3 or 4 major GB (Gibb et al., 1998). Regardless of the frequency, the major GB occurs near sunrise and sunset; the GB at dusk is longer and more intense. However, frequency and distribution of GB are not inflexible, and interactions with external constraints (management or behavioral adaptations) may affect them.
The objectives of this work were to assess changes in the daily grazing pattern and performance by beef heifers strip grazing annual ryegrass when the new daily strip was allocated each morning or each afternoon (timing of herbage allocation).
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Research Site
These studies were conducted from June to October 2004 at the experimental farm El Amanecer of the Universidad Nacional de La Plata, (latitude 35°15'00''S, longitude 57°37'30''W), on a native temperate range of the Argentinean Flooding Pampa (central portion of Argentina). Average precipitation is 920 mm evenly distributed throughout the year. Monthly temperatures range from 7°C in July and August to 22°C in January (Jacobo et al., 2000). Due to the flat relief and the occurrence of a high water table, soils are classified in the halo-hydromorphic complexes and associations (INTA, 1977).
These environmental conditions support growth of both cool- and warm-season grasses on the Flooding Pampa range (Soriano et al., 1991). Dominant vegetation includes dallisgrass (Paspalum dilatatum Poir.), fox tail (Bothriochloa laguroides DC. Pilg), tears (Briza subaristata Lam.), pig hear (Distichlis scoparia Kunth), Esporobolo (Sporobolus pyramidatus Lam.), cloris (Chloris berroi Arech), centenillo (Hordeum stenostachys Gord.), paspalum (Paspalum vaginatum Sw.), and diplacne (Diplachne uninervia Presl.). Annual ryegrass (Lolium multiflorum Lam.) has become widespread in this range. During autumn, temperature and availability of resources, such as light and soil moisture, control competitive interactions between C3 and C4 grasses (Jacobo et al., 2000). Therefore, controlling the growth of warm-season grasses in late summer and early fall leads to swards mainly composed (>80% on a DM basis) of annual ryegrass in late fall, winter, and early spring.
In the experimental period, mean daily temperature, radiation, and precipitation were 10.7°C, 1,857.6 W/m2, and 49.1 mm for winter (June, July, and August), and 14.2°C, 3,869.2 W/m2, and 33.9 mm for spring (September and October).
Experimental Procedures
Behavioral and performance experiments were conducted simultaneously during the winter (from June 6 to July 26) and spring (from August 25 to October 20). The behavioral experiment analyzed grazing time, rumination time, idling time, their patterns, and bite rate of grazing heifers during the daylight, when herbage was allotted in the morning (animals were turned onto an ungrazed strip at 0700 each day; MHA) or afternoon (animals were turned onto an ungrazed strip at 1500 each day; AHA). The performance experiment evaluated ADG, changes in BCS, and daily herbage DMI with the same treatments.
Behavioral Experiments.
Eight Angus heifers (winter BW = 183 ± 2.1 kg, spring BW = 254 ± 7.5 kg) were grazed using a strip grazing method (Forage and Grazing Terminology Committee, 1992) throughout the experiments. Strips to be grazed were always in a vegetative stage (3 and 4 green leaves per tiller). Herbage allowance was 6% (DM basis) of BW (Combellas and Hodgson, 1979). The size of the daily strips was determined according to the herbage yield (kg/ha) and allowance. Herbage yield was determined weekly by cutting 9 squares of 30 x 30 cm with manual mowers at a stubble height of 3 cm. The clipped herbage was collected, weighed, and sampled for DM determination.
Each strip was delimited by a single-wire electric fence. In keeping with common practice in this area (Northeast Flooding Pampa), heifers were not provided shelter, salt, or free drinking water (because of the high water content of the herbage). Before beginning the experiments, all heifers were allowed to graze together for 15 d and were moved to a new strip at 1100 each day. After this period of standardization, heifers were randomly assigned to treatments: MHA or AHA, in a simple crossover (Gill, 1978) experimental design with 2 periods and 2 treatments. The experimental unit was the heifer. Each period was 10 d (9 of adaptation and 1 of measurement).
Grazing, rumination, and idling behavior were visually determined every 2 min (Hirata et al., 2002), from 0600 to 1800, by 2 trained observers who were randomly assigned to each treatment and period. From these data, grazing, rumination, and idling times were calculated by multiplying the frequency of each behavior by a 2-min interval. To graphically visualize GB distribution, breaks in grazing activity of more than 5 min in duration were considered as delimiting a GB, whereas breaks less than 5 min were intra-GB intervals (Rook et al., 1994). For the purpose of this study, a GB does not mean a period of eating but rather that the animal was directly engaged with eating (searching, acquisition into the mouth, mastication, and subsequent swallowing of herbage (Gibb, 1998). Daylight behavioral times were then summarized into 3 times of the day, morning (0700 to 1100), afternoon (1100 to 1500), and evening (1500 to 1900). To measure bite rate (bites per min), 8 trained observers were randomly assigned to each heifer every period. Bite rate was determined at the beginning, middle, and end of each day, in periods of at least 1 min of uninterrupted biting activity.
Performance Experiments.
Forty-eight Angus heifers (winter BW = 183 ± 2.6 kg and BCS = 5.6 ± 0.02; spring BW = 246 ± 0.3 kg and BCS = 6.12 ± 0.13) were selected by BW and BCS from a herd of 60. Grazing management, method, standardization, adaptation period, and treatments were the same as for the behavioral experiments. In these experiments, a completely randomized design (Gill, 1978) was used, with heifer serving as the experimental unit. The measurement period was 5 and 6 wk in length for winter and spring, respectively.
Heifers were weighed, and BCS (9-point scale) was assigned weekly. To minimize differences in ruminal fill, heifer BW were taken before they were moved to the new daily strip, according to treatment. Herbage DMI was estimated, by the sward cutting method, as herbage allowance minus herbage refused (Mejis, 1981). During the experiment, fresh herbage allowance and its percentage DM were determined weekly at noon. Herbage allowance was measured by hand-clipping herbage at a stubble height of 3 cm from nine 30 x 30-cm quadrants before grazing, and herbage refused was determined by clipping eighteen 30 x 30-cm quadrants after grazing (Mejis, 1981; Smit et al., 2005) weekly. Clipped herbage was collected, weighed, and sampled for DM determination (oven-dried at 60°C for 48 h).
Pastures and Herbage Quality.
To reduce competitiveness of warm season grasses and promote the growth of annual ryegrass, 22 ha of range (total experimental area) were sprayed with glyphosate (5 L/ha) in late February 2004 and fertilized a month later at 50 kg of N/ha in the form of ammonium nitrate. As a result, swards used for these experiments were 85% annual ryegrass (Lolium multiflorum Lam.), as determined by the dry-weigh-rank method (Gillen and Smith, 1986). Sward surface height was measured every week, using a drop plate meter. Herbage yield was also determined weekly using the same procedure for herbage allowance.
To estimate diurnal variation of chemical composition of herbage available to heifers, graminoid samples (150 g of fresh material) were taken 3 times daily (0700, 1300, and 1900), corresponding with the main GB (Gibb et al., 1998, Taweel, 2004), once each week. Samples were taken following a methodology similar to that of Smit et al. (2005) by walking next to 4 randomly chosen heifers per treatment for 2 min, taking hand-plucked samples (mimicking harvesting movements of the heifers), where and when heifers grazed previously un-grazed spots. Samples were pooled by time of day based on equal DM weight, oven-dried at 60°C for 48 h, ground to pass a 2-mm screen (Wiley Mill, Model 5, Thomas Scientific, Swedesboro, NJ), and analyzed for NDF and ADF according to Van Soest et al. (1991), nonstructural carbohydrates with the anthrone method (Yemm and Willis, 1954), CP (6.25 x N) using Micro Kjeldahl (Bremner and Mulvaney, 1982), and IVDMD according to Tilley and Terry (1963).
Statistical Analysis.
Daylight grazing, rumination, idling time, and herbage DMI were analyzed by ANOVA using GLM of SAS (SAS Inst., Inc., Cary, NC). The model included the fixed effects of pasture (strip), animal, and treatment. Chemical composition of herbage, grazing time, rumination time, idling time, bite rate per time of day, as well as ADG and changes in BCS, were analyzed using a statistical model that included the effect of time (time of day or week) as a repeated measurement using PROC MIXED of SAS. The error term used in this analysis to test for a treatment effect was heifer within treatment x week or time of day (according to experiment). The pooled residual error was used to test the effects of week and the interactions with week. An effect was considered significant at P < 0.05.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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). The latter may be related to differences in leaf:stem ratio (Laredo and Minson, 1975) and phenology (Van Soest, 1982). During winter, heifers always grazed previously ungrazed strips; therefore, they might have found a greater leaf:stem ratio, whereas in spring, stem proportion and senescent material might have increased over levels observed during the winter. Chemical composition of herbage varied throughout the daylight hours (P < 0.05) but not between seasons. Neutral detergent fiber decreased by 11%, and nonstructural carbohydrates increased by 35% from morning to evening across season (P < 0.05). Although CP concentration decreased numerically, it was not significant (Table 1). These results are similar to reports by Delagarde et al. (2000) who reported that in rotationally grazed perennial ryegrass (Lolium perenne), time of day for sampling had noticeable effects on NDF and soluble carbohydrate. Particularly in the layers 10 cm above ground, there was a 25-g decrease in NDF and a 40-g increase in soluble carbohydrates/kg of OM from dawn to dusk. Griggs et al. (2005) compared nonstructural carbohydrate concentrations during 24-h clipping sequences initiated at 1900 and 0700 in orchardgrass (Dactilis glomerata L). Despite the small differences in nonstructural carbohydrates over a 24-h period (October, June, and August average 7.6 ± 3.9 g of DM/kg), they suggest a potential increase in nonstructural carbohydrates concentrations at dusk. Delagarde et al. (2000) pointed out that such fluctuation in nonstructural carbohydrates primarily occurs in layers of the sward representing sites of photosynthesis and gas exchange. Mayland et al. (2003) demonstrated that this fluctuation mainly occurs in leaves. Moreover, they found that leaves exposed to 15 h of light had a 1.6 times greater concentration of sugars than in leaves exposed to 9 h of dark. In alfalfa hay (Medicago sativa L.) cut during the afternoon, Fisher et al. (2002) also found a 5% decrease in NDF and an 18% increase in nonstructural carbohydrates. Burns et al. (2005) report 2% less and 22% greater NDF and nonstructural carbohydrates, respectively, for alfalfa hay cut at sunset after a sunny day compared with morning cutting. These results and ours support the premise that the main change in diurnal chemical composition is due to an in increase in photosynthates, which leads to a passive dilution of NDF content or CP, or both (Delagarde et al., 2000).
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). Moreover, the greatest ruminal ammonia concentrations are generally found during the dawn GB (Chilibroste et al., 2005), which leads to an unbalanced supply of ruminally degraded nutrients (Chilibroste et al., 2005). This unbalanced condition naturally restrains the activity of the microbial population of the rumen, which is the smallest at that time of the day (M. J. Gibb, Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, UK, personal communication). This situation leads us to point out that under grazing situations, nutrient supply varies among GB not only in total supply, but also in nutrient balance. In addition, the nutritive and feeding value of herbage might be differentiated according to the time of day. Therefore, grazing management that would induce longer and more intense GB at dusk may improve animal performance.
Treatment did not affect ADG and changes in BCS in the 5 wk during the winter. The treatment x week effect was significant for ADG (P < 0.01) and BCS (P < 0.01). Analysis of the interaction showed that heifers in AHA gained more weight and points of BCS daily from wk 3 through 5 (P < 0.05) than heifers grazing with MHA (Figures 1 and 2). The mean ADG of 3 to 5 wk was 863 vs. 713 g for AHA and MHA, respectively. There was not a significant treatment x week interaction (P = 0.74) in the spring. Differences among ADG began to appear from wk 1. Heifers with AHA gained in average 549 g (P < 0.01) and 0.0145 points of BCS (P < 0.05) more daily than heifers with MHA. Even with a different animal model, our results of performance follow the same trend found by Orr et al. (2001), who strip grazed dairy cows, offering the new strip after morning or afternoon milking. They found no differences in milk yield throughout a 10-wk period. However, milk yield gradually began to differentiate (P = 0.07) over the last 4 wk (Orr et al., 2001). Dalley et al. (2001) allocated the new strip 1 or 6 times daily, reporting a decrease in milk production in the group with more frequent herbage allocations, (25.7 and 26.7 L/cow daily, respectively). The lower milk yield may have resulted from the consumption of herbage with lower nonstructural carbohydrates during the night grazing periods because 2 of the strips were allocated after 2100. Despite the different times of herbage allocation, beef heifers in the present work had the same herbage DMI in winter (AHA: 5.0 vs. MHA: 4.5 kg of DM daily, ± 1.48; P = 0.65) and spring (AHA: 5.6 vs. MHA: 5.1 kg of DM daily ± 0.80; P = 0.35), supporting the report by Orr et al. (2001). However, daily energy intake of heifers may have differed; IVDMD was 5.3 and 4.7% greater early in the evening (1900) than morning (0700), respectively (Table 1
). This assumption is also supported by the greater nutritive values found by Griggs et al. (2005) with simulated defoliation patterns of orchardgrass pastures; Burns et al. (2005) and Fisher et al. (1999, 2002) who fed alfalfa and tall fescue (Festuca arundinacea Schreb.) hays, respectively, harvested at sunrise or at sunset. Also, in the present work, heifers in AHA faced a depleted strip during morning and afternoon, which might have led to a reduction in herbage intake rate and a reduced ruminal fill at the time to enter to the new strip. This generates greater intake rate (Gregorini et al., 2006). Therefore, even at the same BR, a greater bite mass might be expected.
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and 3) showed that AHA decreased daylight GT (P < 0.01) compared with MHA, independently of season. In MHA, heifers grazed 63 and 54 min longer than under AHA during the daytime of winter and spring, respectively. This is opposed to the grazing time reported by Orr et al. (2001) with dairy cows, which grazed the same amount of time regardless of herbage allocation timing. Grazing bouts may be cumulative, leading to total daily grazing time (Gibb, 1998). As a result, daily grazing time may be seen as a cluster of discrete GB. As mentioned previously, under strip grazing, foraging decisions such as when to begin, which frequency, and how to spread the GB might determine how cattle allocate the time invested in feeding to meet their metabolic requirement for nutrients. Analysis of the GP demonstrates that frequency of GB (Figures 3 and 4) and distribution of grazing time are not completely inflexible, and a simple change in timing of herbage allocation may modify the time of day when grazing occurs, particularly under daily strip grazing managements. According to the summary of data shown in Tables 2 and 3, heifers with AHA concentrated 54 and 50% of their daylight grazing time in the 4 h of evening, in winter and spring, respectively, when they showed a greater bite rate (P < 0.01). Under MHA, heifers spent 39% of the daylight grazing time in winter and 36% during the spring in the morning, when herbage offers more constraints for a high DMI, such as lower DM concentrations (Van Soest, 1982), surface moisture (Gibb et al., 1998), and a low nonstructural carbohydrates:protein ratio (Chilibroste et al., 2005). Whereas in AHA, percentage of daylight grazing time spent in the morning was 25% in winter and 28% during the spring. Furthermore, the morning bite rate of heifers with MHA was 1.4 and 1.3 times greater in winter and spring, respectively, than the bite rate of heifers with AHA. In research reported by Orr et al. (2001), dairy cows receiving a new strip in the afternoon had longer dusk GB (4 vs. 2 to 3 h) and spent less time ruminating.
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reported that cows with AHA had lower intake rate in the next morning compared with those offered the new allocation the same morning (19 vs. 41 g of DM/min). This difference in intake rate was reversed when the measurements were made in the afternoon (55 vs. 31 g of DM/min). These results and ours, along with the herbage DMI estimations in the performance experiments, may suggest that timing of herbage allocation not only modifies frequency and distribution of GB, but also grazing intensity because herbage intake rate could have been greater at dusk. McGilloway et al. (2001) evaluated the relationship between the sward surface height reduction in a rotationally grazed sward and the short-term intake rate. They concluded that the main factor controlling the intake rate, as swards are progressively grazed down, is sward surface height. But with larger reductions in swards surface height, bulk density of the sward also influences bite mass (McGilloway et al., 2001). Barret et al. (2001) evaluated the patterns of herbage intake rate and bite mass of rotationally grazed dairy cows as sward surface height declined, and found that cows grazing throughout the day on a progressively depleted sward had a linearly declining bite mass even though intake rate was similar compared with cows grazing previously undefoliated paddocks regardless of the time of the day. In our case AHA heifers found a depleted strip in the morning. This may have led to a reduced intake rate at that time, which seemed to be compensated during the afternoon leading to equal daily herbage DMI between treatments. |
IMPLICATIONS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Footnotes |
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2 Corresponding author: sgunter{at}uaex.edu
Received for publication September 21, 2005. Accepted for publication January 30, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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