Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands

Animal Research, Jul 2018

Géraldine Fleurance, Patrick Duncan, Bruno Mallevaud

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Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands

Anim. Res. Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands FLEURANCE 0 2 k DUNCAN 2 Bruno MALLEVAUD 1 0 Present address: Les Haras Nationaux, Direction du Développement, Station Expérimentale des Haras , 19370 Chamberet , France 1 Parc Interrégional du Marais Poitevin, place de l'Église , 79510 Coulon , France 2 Centre d'Études Biologiques de Chizé , CNRS-UPR 1934, 79360 Beauvoir-sur-Niort , France - In spite of the importance of grazed forage in horse nutrition, little information is available on their daily intake at pasture. We determined the intake of 4 non-breeding mares of a heavy breed (average body weight = 674 kg), grazing during the summer in heterogeneous natural grasslands of the Marais Poitevin (France), an internationally important wetland where grazing is an essential process which maintains biodiversity. The mares ate large quantities of forage (21.9 ± 2.4 kg of organic matter per day, i.e. 166.2 ± 20.8 g of organic matter per kg LW0.75 per day) in comparison with previous published values and with the estimated requirements of these horses. The use of the vegetation was very selective, the mares spent about 70% of their feeding time on short grass lawns (sward surface £ 4 cm, biomass < 100 g.m-2), that represented only 10% of the area. This behaviour maintained the plants at young growing stages which are of better quality than ungrazed plants. These results are discussed in relation to the dynamics of the plant communities. - maintenait les plantes dans de jeunes stades de croissance, de meilleure qualité que celle d’une végétation non pâturée. Les résultats sont discutés en rapport aux dynamiques des groupements végétaux présents sur le site. cheval / ingestion / prairie / hétérogénéité / zone humide 1. INTRODUCTION Intake rates by horses of green or dried forages distributed ad libitum are well documented [ 2, 4, 5, 18 ]. In spite of the importance of grazed forages in horse nutrition (these can represent more than 70% of the annual food, [17]), little information is available on intake at pasture [17], principally because in the field it is difficult to estimate faecal output and diet digestibility accurately. Rittenhouse et al. [24], using neutral detergent fiber (NDF) as an external marker for non productive and lactating mares in a natural grassland, measured intakes of 113 and 138 g DM.LW–0.75.d–1 respectively. Nonetheless, the reliability of this method, and of others based on external markers such as chromic oxid (e.g. in yearling horses, 80 g DM.LW–0.75.d–1) [ 14 ] is debatable [ 13, 15, 26 ]. Mesochina et al. [18], in a recent study, demonstrated that fecal crude protein is a good predictor of the digestibility of forages by horses (dry matter digestibility: rsd: 0.038; r2 = 0.65; organic matter digestibility: rsd: 0.036, r2 = 0.74). Duncan [8] used this approach to determine the intake of lactating mares of the Camargue breed (intake: 170 g DM.LW–0.75.d–1). This study was part of a long term programme of research on the effects of different herbivores on the dynamics of the vegetation in a wetland of international importance, the Marais Poitevin. The site was in natural wet grasslands of the Communal des Magnils-Reigniers, which is of importance for the conservation of wetland plants (e.g. Ranunculus ophioglossus), birds (e.g. breeding Tringa totanus) and mammals (e.g. otters, Lutra lutra), and has traditionally been grazed by cattle. The experimental set-up consisted of 10 plots of 1–2 ha which were grazed by cattle, horses or both species. Under cattle at densities comprised between 341 and 1246 kg.ha–1, the plant community dynamics depended on the biomass density of the animals [1]: with high densities there are many rosette plants (hemicryptophytes), while at low animal densities these plants are replaced by competitive species (sensu Grime [12], mainly perennial graminoids). Horse grazing lead heterogeneous plant communities, as the two dynamics were observed whatever the density of the animals (345 to 900 kg.ha–1). Tall grasses appear in patches of lightly grazed plants, where the animals deposit faeces. This leads to a loss of plant species diversity in the tall grass patches, but an increase in structural diversity. Elsewhere, in heavily grazed lawns, halophytes and hemicryptophytes appear. The aim of this work was first to provide data on the rates of daily food intake in the horses on these pastures and then to contribute to this programme by describing the use of these grasslands by horses. We measured the time-budgets of the horses and their selective use of structural types (lawns and patches of tall grasses) and of the plant communities. 2. MATERIALS AND METHODS 2.1. Study site The Magnils-Reigniers common is part of an area of brackish grasslands where the topography leads to a specific pattern of plant communities [1]. The shallow natural drainage channels, which are flooded from late autumn to early spring, are characterized by hygrophile vegetation with subaquatic species (Tab. I). The top of the catena is occupied by graminoids, forming the mesophile community. The slopes, whose soils are saline, are favourable for sub-halophile species, forming the mesohygrophile community. We defined two structural vegetation types, which were represented in all communities, lawns (sward surface £ 4 cm) and taller grasses (mostly at the reproductive stage during the study) where the horses deposited all their faeces. 2.2. The animals We studied 4 non-reproductive mares (Mulassier Poitevin breed), 2 to 7 years old. They were distributed in 2 sets, homogeneous for weight, each grazing a 2 ha pasture (E2D or E2) from April to October 1997. The mean weight of the animals was 674 kg during July and August. 2.3. Measurements 2.3.1. Daily food intake The daily food intake was measured as OMI = F/(1-OMD) where OMI is the organic matter intake, F is the weight of faeces produced over 24 h (g) and OMD is the organic matter digestibility estimated by the crude protein (CP) content of the faeces [18]. The results are expressed in organic matter (OM) in order to avoid inaccuracies due to contamination by soil. Faecal output was measured by collecting the total amount of faeces produced 3 times a day over 4 successive days (29.VII.97 to 01.VIII.97) [ 15 ]. Faecal dry and organic matter content were measured in one sample per plot per day, dried at 80 °C to constant weight and by mineralization at 550 °C. Nitrogen was determined by the Kjeldahl method. The results were corrected for the mean weight of the animals, expressed as live or metabolic weight. 2.3.2. Spatio-temporal organization of the feeding behaviour The mares were observed for a total of 48 h, between 21.VII-08.VIII.97. Individual behaviours were recorded using scan sampling during periods of 3 h by day and by night in order to cover the 24 h in 4 days. The 4 mares, previously accustomed to the presence of the observer in the pasture and to the use of a lamp, were checked successively by the same person at 5 minute intervals. An animal was recorded in feeding activity if one of these actions was observed: searching (sorting or gripping food with the upper-lip), cutting, chewing or swallowing. The structural type and the community grazed were noted. In each plot, 100 measures of the sward surface were made (50 on each diagonal, with a measure every 4 m, on 21.VII.97, 28.VII.97, 06.VIII.97) using a 50 · 50 cm sward stick. At each measure, the structural type and the community were recorded. The mean of the 3 measurements enabled us to determine the area covered by each community and structural type in each plot, and to calculate for each plot a selection index S for the 2 mares of the pasture (the ratio of the percentage of the feeding time passed on the particular structural type or community, and the percentage of the area covered by that structural type or community). The null hypothesis was that the animals used the vegetation types evenly, i.e. S = 1.0. 2.3.3. Above-ground plant biomass, and forage quality In each plot, samples (Tab. I) were clipped with shears in a rectangle (50 · 25 cm) placed at random in each structual type of each community (mesophile, meso-hygrophile, hygrophile). The samples were dried at 60 °C to constant weight to calculate the plant biomass (dry weight/area clipped). Some samples were analyzed for nitrogen (Kjeldahl method), cellulose (Weende G.Fleuranceetal. p s e s s a r g l l a T s n w a L p n s o so is r r c a a p , s m s co rag )c ll ( a t d f n o a ) s e (b h ; tc s a se p s d .7 .2 .2 6 0 9 9 0 0 1 2 2 ± ± ± .4 .2 .9 5 3 1 3 0 6 5 4 4 ) ) ) 1 6 5 1 = = = n n n ( ( ( a P p C ( m o in C te ) a ro . ( p ts . Ie edu lop lb rc th a f o T o b n D a 2 2 e E E m ) 2 – s s .m a M m o D i g B ( ) ) a a ( ( method) and cell walls: Neutral Detergent Fiber (NDF) [28]. 2.4. Statistics The significance of selection for vegetation types was determined by chi-square tests, by comparing the numbers of feeding observations with the values expected if the animals used the vegetation types evenly. Kruskal-Wallis and Mann-Whitney tests were used to compare plant biomass and grass quality respectively, between plant communities (across structural types) and between structural types (across plant communities). Statistical tests were bilateral, for p < 0.05 using Statistica [25]. 3. RESULTS 3.1. Daily food intake at pasture The mares in both pastures selected diet of good quality (estimated organic matter digestibility: 61.8 ± 0.8%) with little variability. Intake was on average 21.9 kg OM. kg LW–0.75.d–1, i.e. 3.3% of liveweight, i.e. 166.2 ± 20.8 g OM.kg LW–0.75.d–1. The mean value in terms of dry matter was 172.5 ± 23.1g DM.kg LW–0.75.d–1. The digestible organic matter intake (nutrient assimilation) was 102.8 ± 13.8 g DOM.kg LW–0.75.d–1. 3.2. Spatio-temporal organization of the feeding behaviour The mares fed on average 60 ± 1% of the 24 h. This activity occupied about 3/4 of the night (74 ± 4%) and half of the daylight (54 ± 1%). The use of vegetation communities was not homogeneous (chi-square test, p < 0.01). The mares selected the mesohygrophile community, using this 2.5 times as much as expected from the hypothesis of even use and they avoided the mesophile community (Tab. II). At the time of the study, the structural heterogeneity of the cover was not very pronounced in the pasture E2, with small patches of lawns beginning to appear. The use of the structural types by the mares was measured only in the E2D pasture. Here the mares fed on both the structural types, but the use of the lawns was 7 times as much as expected (chi-square test, p < 0.01), their selectivity index being 20 times that on the taller grass. The preference for the lawns is verified in each community (M, MH, H), where the percentage of the area covered by the lawns were respectively: 9.4%, 37.5%, 10.7% and the selection index were 6.24, 2.58, 7.15 for the lawns and 0.36, 0.05, 0.36 for the tall grasses (chi-square test, p < 0.01). 3.3. Aerial biomass and vegetation quality The meso-hygrophile community, which was selected by the mares, had the least biomass (p < 0.05) (Tab. I), but the difference was not significant within pastures because the variances are high. The mean biomass of the patches of tall grasses was 5 times higher than the biomass of the lawns (p < 0.01, Tab. I). There were no significant difference between the plant communities for crude protein, cellulose and NDF contents (Tab. I). Nonetheless, the lawns which were strongly selected by the mares were characterized by a crude protein content 38% higher than the tall grasses. The cellulose and NDF contents tended to be lower for lawns (p < 0.06) (Tab. I). 4. DISCUSSION 4.1. Daily intake The daily food intake found here (172.5 ± 23.1 g DM.kg LW–0.75.d–1 or 166.2 ± 20.8 g OM.kg LW–0.75.d–1) is much higher than the few values available for non-productive horses at pasture (e.g. 113 g DM.kg LW–0.75.d–1 from Rittenhouse et al. [24]) and higher than those of Mesochina [19] and Ellis et Lawrence [10] for growing saddle horses (92 g OM.kg LW–0.75.d–1 and 80 g DM. kg LW–0.75.d–1). The values obtained in the Marais Poitevin are comparable to the values for breeding and growing wild equid species [ 2, 11, 21 ] and for lactating mares (e.g. in the Camargue, France [8]). Nonetheless, even if the needs for production seem to be the main determinant of food intake [ 2 ], Meyer [20] has shown that horses can have intakes higher than their needs, and maintain these for a long time. These intakes are also much higher than those of comparable beef cattle at pasture (3.3% of liveweight in horses compared with 2–3% in cattle (Dulphy et al. [7]). In view of the fact that there are considerable inter-individual variations in intake in some herbivores [6, 23], and the small number of animals used in this study, the measures should be repeated. Nonetheless the slight variations between days on food intake (coefficient of variation: 12%) show that data from four days allow estimation of daily intake for horses on these pastures with reasonable accuracy. An obvious improvement would be to individualise the faeces with colored plastic marks. 4.2. Duration of feeding activity The time spent feeding, 60 ± 1% (or 14 h 30) is consistent with previous published values (14 h–17 h, [8]) and the animals in the experimental plots appeared to have normal feeding behaviour. Nocturnal feeding comprised 38% of the total time spent feeding, which is typical for horses (20 to 50%, [16]), which is why it was necessary to conduct 24 h observations in this study of their feeding strategies. 4.3. Selection of vegetation types The horses showed more marked selection for lawns than for any of the vegetation communities, as has been observed previously in dry grasslands [ 3, 9, 22 ]. The mares maintained lawns where the quality of the grass was high in comparison with the remainder of the pasture (38% higher crude protein, lower fibre). In this pasture, the horses choose their food plants principally on the basis of their structure (growth stage) rather than their botanical species. In spite of the short period covered by this study, the feeding strategies of the mares described here clarify causes of the differences in the vegetation dynamics under cattle or horse grazing described by Amiaud [1]. The intense use of the meso-hygrophile community by horses explains the rejuvenation of sub-halophyte communities and the development of hemicryptophytes on the slopes. In the same way, the opening of the hygrophile community leads to the emergence of annual species (Galium debile, Ranunculus sardous) [1]. The development of highly competitive species at the top of the catena seems to be the result of low grazing pressure and perhaps favourable conditions of mineral and nitrogen availability brought by the faeces. Finally, the high intake levels compared with cattle and strong selection for short grass lawns explain why the horses create and maintain structural diversity in such grasslands, whatever their stocking level. The current hypothesis to explain the heterogeneous use of grasslands by horses proposes that the animals minimize their feeding time on patches of tall grasses where the faeces are deposited in order to minimize the intake of gastro-intestinal parasites [27]. The results of this study suggest another hypothesis: if the feeding strategy of these horses is to maximise daily nutrient assimilation, the maintenance of lawns of good quality could be a nutritional as well as an antiparasite tactic. ACKNOWLEDGEMENTS This work was funded by the Parc Interrégional du Marais Poitevin. We wish to thank D. Naudon for his support of the project and for his help in the collection of the data. We are also grateful to J.B. 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Géraldine Fleurance, Patrick Duncan, Bruno Mallevaud. Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands, Animal Research, 149-156, DOI: doi:10.1051/animres:2001123