Agr50172 238..244

Sward composition and structure influence herbage intake of grazing animals. We conducted a grazing study to examine how forage mixture complexity affected sward structure. Replicated 1-ha pastures (Hagerstown silt loam soil: fine, mixed, semiactive, mesic, Typic Hapludalf) were planted to either orchardgrass (Dactylis glomerata L.) and white clover (Trifolium repens L.) or a nine-species mixture [orchardgrass, tall fescue (Festuca arundinacea Schreb.), perennial ryegrass (Lolium perenne L.), Kentucky bluegrass (Poa pratensis L.), red clover (Trifolium pratenseL.), birdsfoot trefoil (Lotus corniculatus L.), white clover, alfalfa (Medicago sativa L.), and chicory (Cichorium intybus L.)]. Pastures were stocked rotationally with lactating dairy cows (Bos taurus). Herbage mass, sward height, vertical structure, and nutritive valueweremeasured preand postgrazing during four periods in each of 2002 and 2003. Pregrazing sward height was similar between the mixtures. Pregrazing herbage mass and sward bulk density were 30% less in the two-species mixture than the nine-species mixture in 2002 but similar in 2003. The nine-species mixture had more herbage drymatter in the upper sward layers than the two-species mixture. Bulk density of herbage dry matter increased and nutritive value decreased from the top to the bottom of both swards. Cattle grazed deeper into the two-species sward (18 cm) than the nine-species mixture (11 cm) in 2003; however, individual botanical components in the upper 25 cm of the canopy were removed similarly between treatments. We conclude that sward structure did not alter the pattern of herbage removal during grazing of simple or complex swards. PASTURES often contain complex plant communities. In the northeastern USA, for example, species richness in pastures ranges from 18 to 53 species per 0.1 ha (Tracy and Sanderson, 2000). Plant species diversity affects ecosystem functions such as primary productivity and resistance to invasion (Fridley, 2001). Some suggest that managing species-diverse (complex) mixtures of forages is a low-input way of increasing herbage productivity in pastures (Tilman et al., 1999; Minns et al., 2001; Sanderson et al., 2004). Complex mixtures of forages yielded more herbage than simple grass–legume mixtures when compared in grazed small plots (Deak et al., 2004) or pastures (Sanderson et al., 2005). Animal performance on pasture depends not only on herbage productivity, but also on how the grazing animal interacts with the sward through its ingestive behavior, including grazing time, bite rate, and bite size (Ungar, 1996; Gordon, 2000). Several structural characteristics of the sward affect grazing animal ingestive behavior and ultimately affect grazed herbage intake (Hodgson, 1985). These vertical structural features include sward height, bulk density, and the distribution of leaves, stems, dead material, and plant species within the canopy (Gordon, 2000; Laca and Lemaire, 2000). Herbage intake by grazing animals increases asymptoticallywith sward height (Allden and Whittaker, 1970) and with the proportion of green leaf in the canopy (Chacon and Stobbs, 1976). Bulk density influences herbage intake mainly through bite size (Stobbs, 1973). The vertical structure has been well described for homogeneous swards of temperate legumes and grasses such as perennial ryegrass and white clover. The upper sward layers consist mainly of live leaves, whereas the lower layers contain mostly stems, petioles, leaf sheaths, and dead material (Hodgson, 1985). As the sward ages, stem material becomes more uniformly distributed within the canopy.White clover leaves usually are arranged in a horizontal plane within the plant canopy, whereas grass leaves are more vertical. On these types of swards, cattle usually graze from the top of the canopy downward in successive layers with a minimum of selection (Hodgson, 1981). Less is known, however, about the structure of complex mixtures of forages with contrasting morphology and how the structure affects grazing behavior. Because of differences among species in height, leaf and stem arrangement, and growth patterns, we would expect swards composed of amixture of grasses, legumes, and forbs to be more complicated in vertical structure than a two-species grass–legume mixture. The differences in vertical structure between the two sward types would probably affect the ingestive behavior of grazing cattle because of the greater opportunity for selection among species within the complex mixture than in the grass–legume sward. For example, the ingestive behavior of grazing dairy cattle differed when grazed on swards of tropical grasses and legumes with contrasting height and morphology (Stobbs, 1973). On complex swards at high herbage allowance, cattle are vertically oriented in their grazing (Ungar, 1996). Previously, we reported on how forage mixture complexity (two-, three-, six-, and nine-species mixtures) affected herbage yield and botanical composition (Sanderson et al., 2005) and herbage intake andmilk production of dairy cows (Soder et al., 2006). Our objective in the current study was to determine how foragemixture complexity affected sward structure, including the vertical distribution of species. We compared a relatively simple mixture (orchardgrass–white clover) with a complex mixture of nine species that included grasses, legumes, and chicory. M.A. Sanderson, K.J. Soder, and K. Klement, USDA-ARS, Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Road, University Park, PA, 16802-3702; L.D. Muller, Dep. of Dairy and Animal Science, The Pennsylvania State Univ., University Park, PA, 16802; and N. Brzezinski, F. Taube, and M. Wachendorf, Univ. of Kiel, Germany. Received 9 June 2005. *Corresponding author ([email protected]). Published in Agron. J. 98:238–244 (2006). Pasture Management doi:10.2134/agronj2005.0172 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CP, crude protein; DM, dry matter; IVTD, in vitro true digestibility; NDF, neutral detergent fiber. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d . 238 Published online February 7, 2006