Modification of ingestive behavior due to exercise in yearling horses grazing orchardgrass

Applied Animal Behaviour Science, 22 (1989) 335-345

335

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Modification of Ingestive Behavior due to Exercise in Yearling Horses Grazing Orchardgrass S.E. DUREN 1, C.T. DOUGHERTY2, S.G. JACKSON' and J.P. BAKER'

IDepartment of Animal Sciences, University of Kentucky, Lexington, KY 40546-0215 (U.S.A.) 2Department of Agronomy, University of Kentucky, Lexington, KY 40546-0091 (U.S.A.) (Accepted for publication 29 November 1988)

ABSTRACT

Duren, S.E., Dougherty, C.T., Jackson, S.G. and Baker, J.P., 1989. Modification of ingestive behavior due to exercise in yearling horses grazing orchardgrass. Appl. Anita. Behav. Sci., 22: 335345. Eight yearling horses were used in a balanced change-over design to determine: ( 1) voluntary herbage dry matter (DM) intake during a 3-h grazing session; (2) whether ingestive behavior is modified by exercise. The exercise treatments were either no forced exercise (NEX) or 30-min forced exercise (EX) at a rate of 3.6 m s -1 on a mechanical horse-walker. Following exercise treatments, the yearlings were tethered and allowed to graze a plot of orchardgrass (Dactylis glomerata L.) during a closely monitored 3-h morning grazing session. Intake (DM) was estimated by subtracting the harvested residual herbage from the calculated herbage mass allowance. Voluntary herbage DM intake was not significantly different between NEX (1.81 kg) and EX {1.75 kg) horses during the 3-h grazing session. The biting rate during the first 20 rain of grazing was higher (P<0.05) for NEX {14.8 bites min-') than for EX (12.6 bites min -1) yearlings. Mean biting rate, calculated for the 3-h session, was higher (P<0.05) for NEX than for EX animals ( 11.9 and 10.9 bites min- ', respectively). Estimated herbage intake per bite tended to be smaller for NEX (861 rag/bite) than for EX (865 mg/bite ) yearlings. Since forage intake during the grazing session was not significantly different between treatment groups, these data indicate that the 30 rain of forced exercise given in this experiment did not alter intake. Apparently the EX horses modified their grazing behavior by taking fewer, but larger, bites.

INTRODUCTION I n m a n y h o r s e p r o d u c t i o n s y s t e m s , grazed forage is t h e p r i n c i p a l c o m p o n e n t of t h e diet. O r c h a r d g r a s s (Dactylis glomerata L.) is a cool-season, perennial, b u n c h - t y p e grass, w h i c h h a s c o n s i d e r a b l e c a p a c i t y for a n i m a l p r o d u c t i o n a n d is o f t e n a m a j o r c o m p o n e n t of h o r s e pastures. I n f o r m a t i o n c o n c e r n i n g v o l u n t a r y c o n s u m p t i o n of herbage, o f a n y p l a n t species type, b y horses is limited. I n a r e c e n t review of t h e l i t e r a t u r e on equine b e h a v i o r ( C a r s o n a n d W o o d Gush, 1983), it was c o n c l u d e d t h a t grazing b e h a v i o r is i n f l u e n c e d b y m a n y

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variables and is more complex than the feeding behavior of a stabled horse. Mayes and Duncan (1986) reported that the daily intake of a grazing animal depends on many factors, including the amount of time spent foraging, the bite rate and the bite size. They further concluded that in free-ranging horses, feeding was patterned into meals separated by intervals of non-random length. Detailed studies of the time/activity budget of adult and weaned sub-adult horses (Duncan, 1980) as well as of foals (Boy and Duncan, 1979), and of the time spent grazing by yearling horses (Hansen et al., 1987), have been reported. Research has also been reported on the palatability of both grasses and legumes (Archer, 1971) and preferences of plant species for grazing horses (Archer, 1973). In addition, limited equine data relative to chemical composition of grazed forage, digestibility and animal performance is available for young horses (Moffitt et al., 1987; Webb et al., 1987; Conrad et al., 1988). Cantillon (1986) and Moffitt et al. (1987) have reported estimates of forage intake by grazing horses, but an accurate means of determining the nutrients consumed by young grazing horses has not been established. The objectives of this study were to determine: (1) voluntary herbage intake during a closely monitored grazing session; (2) whether ingestive behavior is modified by exercise. This information would be valuable in terms of optimizing both horse and forage management. MATERIALS AND METHODS

Experimental design and analysis A study using a series of 4 × 4 change-over designs with complete balance for first residual effects (Berenblut, 1964 ) was conducted to measure forage intake of tethered horses during grazing sessions. The 4 × 4 design of Berenblut (1964) uses 4 animals alternated between 2 treatments during 4 feeding days. In this study, 8 horses were accommodated by simultaneously repeating the design using a second square. The 8 horses were assigned randomly to squares (treatments ) and columns (days) which established the treatment schedule for each TABLE1 Analysis of variance and appropriate F-tests Source of variation

df

ms

F-test

Animals (adjusted) Days (unadjusted) Treatment (adjusted) Residual effects (adjusted) Error

7 3 1 1 19

M, M2 M:~ M4 M~

M1/M.~ M2/M5 MffM5 M4/M,~

337 animal. Horses (with their scheduled treatments) were assigned randomly to 8 grazing plots each day. Data from both squares were combined for statistical analysis using the linear model represented by the analysis of variance shown in Table 1. In the design, the direct effect and first residual effects are orthogonal (Berenblut, 1964), and the permanent effect can be approximated by summation of the direct and residual effects (Gill, 1978). The treatments used in this study were the amounts of exercise imposed prior to a 3-h morning grazing session. The treatments were either no forced exercise ( N E X ) or 30-min forced exercise ( E X ) at a rate of 3.6 m s -1.

Animal management Eight 1-year-old horses (6 colts and 2 fillies), of both Thoroughbred and Quarter Horse breeds with an average weight of 374 kg, were trained to graze while tethered. Tether ropes were inserted through rigid rubber hoses to reduce the problem of tangling and secured to iron stakes driven to soil level. Horses were exposed to several grass species, to the tether technique and to exercise on a mechanical horse-walker during a 4-week period prior to the start of data collection. Horses were maintained in 5 × 21-m individual herbage-free paddocks at all times except during grazing and exercise sessions. During the experimental phase horses were exercised according to treatments at 06.00 h EDT, and were then transferred by horse trailer at 06.30 h E D T to fresh ungrazed plots where they were permitted to graze 3 h. In addition to ingested herbage, each yearling received a textured concentrate mix (Table 2 ) at a rate of 5.4 k g / h e a d / d a y and free choice timothy (Phleumpratense L. ) hay, salt and water.

Forage management A field of orchardgrass (Dactylis glomerata L. cultivar Boone) was used in this study. The field was mowed on 7 June 1987 and the herbage harvested as TABLE2 Composition of textured concentrate mixture Ingredient

% of mixture

Rolled oats Cracked corn Soybean meal Molasses Ground limestone Dicalcium phosphate Trace mineral salt

39 35 15 8 1 1 1

338 TABLE 3 Pre-grazing forage conditions Condition

Units

Experiment

Measurement phase Plot area Herbage mass Canopy height Herbage allowance Grazing time

Date m~ kg m - 2 cm kg/horse/3 h h

21-24 July 30 0.32 24 10.3 3

hay. On 9 March and again on 14 May 1987, N as ammonium nitrate was broadcast at 50 kg h a - 1 Management of harvest, soil fertility, weed and pest control was in accordance with current recommendations for orchardgrass in Kentucky. Blocks of 8 plots (diameter 6 m) were laid out for each of the 4 experimental days. Plots were delineated by mowing a 0.55-m strip around the circumference with a rotary mower tethered to a stake located at the center. Plots distributed on either side of each grazing plot served as ungrazed controls. Herbage mass allowance, on a dry matter (DM) basis, was estimated each day of the experiment by harvesting 17.5 X 1.5-m strips (control plots ) at a height of 5 cm with a Haldrup forage harvester (Haldrup 1500 plot combine; Alberten's EFTF v/ 5. Haldrup, Logstor, Denmark) (Sheldrick et al., 1985). Pre-grazing forage conditions such as plot area, herbage DM allowance (above 5 cm) and canopy height are presented in Table 3. Measurements

Residual herbage DM (RDM) above 5 cm was harvested, and moisture content (MC) and chemical composition were determined after drying at 75 °C to constant weight (Dougherty et al., 1987). Concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF) and crude protein (CP, N X 6.25) were determined by near infrared reflectance spectroscopy (Martin et al., 1985). Grazed plots were harvested 3 h after the cessation of grazing, after post-grazing canopy height measurements were completed and the altitude of the trampled forage was partially restored. Dry matter intake was estimated from the difference between herbage mass allowance and RDM (Meijs et al., 1982). Cantillon ( 1986 ) established biomass collection as a viable means of estimating forage intake in horses by simultaneously comparing biomass collection to an internal marker technique for determining intake. The rate for DM intake was estimated by dividing total intake by grazing time. Rates of biting were established by observers who counted and recorded the

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number of bites taken during 1 min for each animal at 20-min intervals (8 observations/session). Mean rates of biting were estimated for all horses for all treatment groups and grazing sessions. RESULTS

Since residual effects were not statistically significant, only direct effects of exercise on ingestive behavior are presented. In this study, the permanent effects are equal to the direct effects. Forage conditions

The chemical composition of the orchardgrass grazed in this study is shown in Table 4. There were no significant differences between treatments in chemical composition of the forage. Forage canopy height averaged 24 and 17 cm for the pre-grazing and post-grazing measurements, respectively. There were no significant differences in pre- or post-grazing canopy heights between treatment groups, indicating that both groups decreased canopy height an average of 7 cm during the grazing sessions. Grazing plot area and herbage DM allowance were also not significantly different between treatments (Table 3). Bite rate

Forage biting rates, measured at 20-min intervals (8 observations/session) during grazing, are shown in Fig. 1. Biting rate was observed to decrease in both NEX and EX yearlings as a grazing session progressed. Mean bite rate during the first 20 min of grazing (Observation 1 ) and after 120 min of grazing (Observation 6) was higher (P < 0.05) in NEX than in EX horses. Mean bite rate (MBR), calculated as the average of the 8 observations, was higher (P<0.05) for NEX than for EX animals (11.9 and 10.8 bites min -1, respectively) (Table 5). TABLE 4 Chemical composition of orchardgrass a Component

Crude proteins Neutral detergent fiber Acid detergent fiber aDry matter basis.

Treatment ( % ) NEX

EX

12.8 69.3 35.4

12.8 70.7 35.1

340

15

14

10-

3

2

4

5

£

7

OBSERVATIONS EXERCISE

NON-EXERCISE mmmmmmmm

Fig. 1. Horse biting rate. TABLE 5 Least square means of grazing behavior variables Variable

Intake DM Intake DM MBR BS DM

Units

kg/3 h kg h- 1 bites min- 1 g/bite

Treatment

SE

NEX

EX

1.81 0.63 11.9° 0.861

1.75 0.60 10.8b 0.865

0.26 0.09 0.25 0.11

a bMeans within the same row that do not have a common superscript differ P< 0.05.

Forage intake Forage i n t a k e during t h e 3-h grazing session was n o t significantly different b e t w e e n N E X a n d E X yearlings (1.81 a n d 1.75 kg DM, respectively) ( T a b l e 5). I n t a k e expressed on a n h o u r l y basis was also n o t significantly d i f f e r e n t b e t w e e n N E X (0.63 kg h -1) a n d E X (0.60 kg h -1) horses.

Intake per bite T h e total n u m b e r of bites d u r i n g the grazing session was calculated by multiplying the M B R by the grazing time. M e a n bite sizes ( T a b l e 5) were estim a t e d by dividing the forage i n t a k e b y t h e n u m b e r of bites t a k e n during the

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grazing session. In this study, the estimated forage bite size tended to be smaller for NEX (0.861 g/bite) than for EX (0.865 g/bite) yearlings. DISCUSSION

Forage conditions The plot area and herbage allowance provided in this study were greater than those routinely used in beef cattle experiments (Dougherty et al., 1987) to allow the horses unrestricted forage access. It was observed that limiting allowance by shortening tether ropes to lengths less than 3 m caused the horses to quit grazing and perceive themselves as being tied, rather than free to graze. Since there were no significant differences between treatments in canopy height measurements, it was concluded that both treatment groups were grazing very similar forage plots. The orchardgrass was in a vegetative stage of growth, but was beginning to turn brown in color as a result of drought conditions, evidenced by the low moisture (74%) content of the herbage. Despite this characteristic, all yearlings readily consumed the herbage. The time when animals graze during the day is known to be influenced by weather conditions (Arnold and Dudzinski, 1978). Mayes and Duncan (1986) reported that feeding declined to exceptionally low levels at mid-day during summer months in free-ranging horses. In this study, grazing sessions were conducted for 3 h in the early morning to avoid high mid-day temperatures, which have also been shown to decrease rate of biting in ruminants (Rodriquez Capriles, 1973). The daily relative humidity and temperature during grazing, for the present study, are shown in Table 6. It was observed that, even with the hot and humid weather conditions, the horses grazed continuously.

Forage biting Forage biting rates (Fig. 1 ) decreased in both NEX and EX yearlings as a grazing session progressed. Possible explanations for the decreased rates of TABLE6 Summary of daily weather variables a Date

21 July 22 July 23 July 24 July

Temperature ( ° C )

Relative humidity (%)

High

Low

High

Low

33 33 35 35

18 19 18 20

98 96 94 93

46 42 38 43

'~No measurable precipitation was recorded.

342 biting may be a sense of satiety caused by increased gut fill, fatigue due to either active grazing or the exercise treatments imposed or, finally, heat stress due to environmental conditions. Similar changes have been reported in beef cattle grazing alfalfa (Dougherty et al., 1987). In contrast, Mayes and Duncan (1986) reported no effect of stage in a meal on bite rate. Mean biting rates during the entire 3-h grazing session (Table 4) for NEX (11.9 bites min -1 ) and EX (10.8 bites min -1) horses were significantly different. The EX yearlings were easily identified during the grazing sessions due to their more contented grazing behavior. The NEX yearlings seemed to be more restless and easily distracted than the EX yearlings, although no quantitative measurements were made. The bite rates recorded in this study are lower than those reported for non-exercised mature horses grazing endophytefree tall fescue (18 bites min -1), but higher than for mature horses grazing alfalfa (8 bites m i n - 1) (Cantilton, 1986). The low frequency of biting for horses grazing alfalfa can be explained partly by the anatomical design of the horse's mouth and partly by the forage canopy structure. Since horses have both upper and lower incisors, a rapid rate of biting is favored on short herbage rather than on the generally taller alfalfa plants, due to the ease with which the forage can be clipped at ground level. In another study, Duren et al. (1987) reported that non-exercised yearling horses grazing endophyte-free tall fescue had a MBR of 15.7 bites m i n - 1, which is slightly higher than either NEX or EX yearlings grazing orchardgrass. The greater biting rates were possibly due to the fescue being in an early vegetative state, greener, and having a higher nutritive value than the orchardgrass in the present study. Arnold and Dudzinski (1978) stated that grazing animals will generally prefer green (young) herbage instead of brown (old) herbage. Mayes and Duncan (1986) also reported bite rates which exceeded those reported in this study. They concluded that bite rates for 6 free-ranging mares averaged between 30 and 50 bites m i n - 1. Research conducted using both beef cattle and sheep grazing ryegrass (Forbes and Hodgson, 1985) indicated that bite rates exceeded 30 bites min-1 in both species. These rates are considerably higher than those reported in this study.

Forage intake Forage intake rates were not significantly different between treatment groups (Table 5 ). It was concluded that forage intake was not affected by the exercise treatments imposed in this study. In a similar tethered grazing study, Duren et al. {1987) reported that nonexercised yearling horses grazing endophyte-free tall fescue had a forage intake of 3.2 kg DM during a 3-h grazing session. As with the biting rates, these forage intake values are larger than either the NEX or EX yearlings grazing orchardgrass in the present study. In another study using the tethered grazing tech-

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nique, Cantillon (1986) reported mature horses to have forage DM intakes of 1.5 and 1.65 kg h-1 while grazing endophyte-free fescue and alfalfa, respectively. Moffitt et al. (1987) estimated mean grazing forage DM intakes for ten 2-year-old horses to be 10.3 and 12.7 kg/day for orchardgrass and fescue, respectively. Intake was determined using a non-digestible internal marker. Hansen et al. (1987) reported that yearling horses grazed an average of 13.8 h/day in a circadian pattern, with a depression in grazing just before and after sunset. If the average rate of intake for both NEX and EX horses in this study (0.615 kg DM h -1 ) is extrapolated to fit a circadian grazing pattern, the estimates are in agreement with published estimates of total DM intake for yearling horses (N.R.C., 1978). Aiken et al. (1987) reported voluntary intake of coastal bermudagrass hay by yearling horses to be 9.6 kg DM/day. Coastal bermudagrass hay was fed as the sole source of nutrients in that study and accounted for a daily DM consumption of 2.5% of body weight. In the present study, grazed forage and the supplemented textured concentrate and free-choice timothy hay accounted for approximately 11.75 kg DM/day or 3.1% of body weight. This higher daily DM intake may have been a result of the large body frame size and intake capacity of the rapidly growing yearlings used in the study. Compared to the herbage intake estimates reported in the present study, Dougherty et al. (1987) reported the herbage DM intake rate of beef cattle grazing alfalfa to be 3.0, 1.9 and 1.6 kg h - 1 during the first, second and third h, respectively, of a tethered grazing session. In that study, the cattle were fasted overnight prior to being grazed, which partially explains their higher rates of forage intake. Intake per bite

Mean bite size (Table 5) was not significantly different for NEX (0.861 g/ bite ) than for EX (0.865 g/bite) yearlings. Cantillon (1986) estimated forage bite size to be 0.90 g/bite for mature horses grazing three different forage swards. It is difficult to make comparisons of bite size between these studies since bite size is influenced by the structure of the sward (Carson and Wood-Gush, 1983 ). However, estimates between these studies are quite similar. Bite size estimates (Duren et al., 1987) for non-exercised horses grazing fescue were larger when compared to both NEX and EX yearlings grazing orchardgrass in the present study. Dougherty et al. (1987) reported larger estimates of intake per bite for beef cattle than were observed in this study. CONCLUSION

A few general conclusions regarding grazing behavior in young, exercised horses are possible from observations made on the tethered animals. The yearlings in this study readily adapted to the tethering technique and grazing pro-

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ceeded in a seemingly normal manner. The biomass collection scheme used in this study allowed calculation of forage intake, and these values are generally in agreement with previous published estimates. Mean forage intakes for the NEX and EX horses were 1.81 and 1.75 kg DM, respectively, during the 3-h grazing sessions. The forage biting rate decreased for both groups as grazing time increased, although the MBR was higher for the NEX than the EX horses. Bite size tended to be larger in EX compared to NEX yearlings. Since forage intake was not different between groups, these data indicate the 30 min of forced exercise given in this study did not alter forage intake. Although no quantitative measurements were made, it appeared that EX horses were more content to graze and not as easily disturbed as NEX horses. A probable explanation for this behavior difference is that the exercise regime was strenuous enough to decrease excitability, but not strenuous enough to decrease forage intake due to fatigue. Therefore, the EX horses seemed to modify their grazing behavior by taking fewer, but larger, bites. One of the most difficult aspects of formulating feeding programs for the horse is estimating the quantity of nutrients derived from pasture. The grazing behavior and forage intake information obtained from this study coupled with the knowledge of circadian grazing behavior will allow a more accurate means for determining the nutrient intake of rapidly growing horses. This would aid in determining the nutrients, if any, which may be deficient in the diets of young growing horses. Furthermore, an understanding of the nutrients obtained from grazed forage, coupled with accurate supplementation when necessary, would help correct nutritional imbalances which can cause a multitude of skeletal anomalies. Forage management is also optimized with a better understanding of both grazing behavior and forage intake, since pasture can be rotated and grazed according to the nutritional needs of the animals. ACKNOWLEDGEMENTS

This research could not have been conducted without the valuable technical assistance of M. Cochran and L. Lauriault. Journal paper (88-5-3-82) is published with the approval of the director of the Kentucky Agricultural Experiment Station.

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345 Arnold, G.W. and Dudzinski, M.L., 1978. Ethology of Free-Ranging Domestic Animals. Elsevier, Amsterdam. Berenblut, I.I., 1964. Change-over designs with complete balance of first residual effects. Biometrics, 20: 707-712. Boy, V. and Duncan, P., 1979. Time-budgets of Camargue horses. I. Developmental changes in the time-budgets of foals. Behaviour, 71: 187-202. Cantillon, J.D., 1986. Measurement of intake levels and examination of grazing parameters of Quarter Horse geldings. M.S. Thesis, University of Kentucky, Lexington. Carson, K. and Wood-Gush, D.G.M., 1983. Equine behaviour. II. A review of the literature on feeding, eliminative and resting behaviour. Appl. Anim. Ethol., 10: 179-190. Conrad, B.E., Webb, G.W., Hussey, M.A. and Potter, G.D., 1988. Response of yearling horses to various levels of forage-on-offer. J. Anim. Sci., 66:53-54 (abstract). Dougherty, C.T., Bradley, N.W., Cornelius, P.L. and Lauriault, L.M., 1987. Herbage intake rates of beef cattle grazing alfalfa. Agron. J., 79: 1003-1008. Duncan, P., 1980. Time-budgets of Camargue horses. II. Time-budgets of adult horses and weaned sub-adults. Behaviour, 72: 26-49. Duren, S.E., Dougherty, C.T., Jackson, S.G. and Baker, J.P., 1987. Forage intake of yearling horses grazing tall fescue. 100th Annu. Rep. Kentucky Agric. Exp. Stn., pp. 104-105. Forbes, T.D.A. and Hodgson, J., 1985. Comparative studies of the influence of sward conditions on ingestive behavior of cows and sheep. Grass Forage Sci., 40: 69-77. Gill, J.L., 1978. Design and Analysis of Experiments in the Animal and Medical Sciences. Vol. 2. Iowa State University Press, Ames, IA. Hansen, D.K., Rouquette, F.M., Jr., Florence, M.J., Walker, J. and Heitschmidt, R., 1987. Grazing behaviour of yearling horses. I. Time spent grazing different forages. Forage Research in Texas, Texas A&M University, College Station, TX, pp. 17-21. Martin, G.C., Shenk, J.S. and Barton, F.E., 1985. Near infra-red reflectance spectroscopy (NIRS). Analysis of Forage Quality Agric. Handbook No. 643, USDA, Washington, DC. Mayes, E. and Duncan, P., 1986. Temporal patterns of feeding behaviour in free-ranging horses. Behaviour, 96: 105-129. Meijs, J.A.C., Walters, J.K. and Keen, A., 1982. Sward methods. In: J.D. Leaver (Editor), Herbage Intake Handbook. British Grassland Society, Hurley, pp. 11-36. Moffitt, D.L., Meacham, T.N., Fontenot, J.P. and Allen, V.G., 1987. Seasonal differences in apparent digestibilities of fescue and orchardgrass/clover pastures in horses. Proc. 10th Equine Nutr. Physiol. Soc. Symp., pp. 79-86. N.R.C., 1978. Nutrient requirements of domestic animals. No. 6. Nutrient requirements of the horse. 4 rev. edn. National Academy of Sciences, Washington, DC. Rodriquez Capriles, J.M., 1973. The herbage intake of young grazing cattle. Ph.D. Dissertation, University of Reading. Sheldrick, R.D., Lavender, R.H., Teson, V.J. and Cobby, J.M., 1985. The effect on annual dry matter yield of using a hand-controlled mower or a plot harvester for cutting on small plot trials. Grass Forage Sci., 40: 103-107. Webb, G.W., Conrad, B.E., Hussey, M.A. and Potter, G.D., 1987. Growth of yearling horses under continuous or rotational grazing systems at three levels of forage-on-offer. Proc. 10th Equine Nutr. Physiol. Soc. Symp., pp. 31-36.