c07-01-0045 Brink.indd

Intake of animals grazing grass pasture is in part infl uenced by canopy density. Our objective was to determine the vertical distribution of dry matter and neutral detergent fi ber (NDF) within temperate perennial grass swards. The study was conducted in 2004 and 2005 on a Plano silt loam (fi ne-silty, mixed, superactive, mesic Typic Argiudolls). When leaf height of each grass reached 25 cm in spring, summer, and fall, canopy layers were harvested from 20 to 25, 15 to 20, and 10 to 15 cm. Differences in canopy density were usually not signifi cant when precipitation was below normal. With abundant spring precipitation, quackgrass [Elymus repens (L.) Gould] had greater upper canopy density (1.9 mg dry matter [DM] cm−3) than any of the other grasses studied. Grasses typically grazed at shorter heights such as bluegrass (Poa pratensis L.) and perennial ryegrass (Lolium perenne L.) had greater density in the lower canopy layer (1.0–1.3 g DM kg−1). While soft-leaf tall fescue (Festuca arundinacea Schreb.), reed canarygrass (Phalaris arundinacea L.), and meadow fescue (Festuca pratensis Huds.) had lower density in the upper canopy than timothy (Phleum pratense L.) and quackgrass in the spring, the reverse was true in the summer. Herbage NDF of grasses generally increased from the upper to lower canopy layer (mean difference of 50 g kg−1); perennial ryegrass and meadow fescue often had the lowest NDF throughout the canopy (range of 392– 452 and 418–469 g kg−1, respectively). Pasture intake may benefi t from a diverse species composition because animals can graze a mixture having optimum density and NDF throughout the canopy. U.S. Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706. Received 24 Jan. 2007. *Corresponding author ([email protected]). Abbreviations: DM, dry matter; NDF, neutral detergent fi ber. Published in Crop Sci. 47:2182–2189 (2007). doi: 10.2135/cropsci2007.01.0045 © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . CROP SCIENCE, VOL. 47, SEPTEMBER–OCTOBER 2007 WWW.CROPS.ORG 2183 frequent cutting was greatest in genotypes with greater leaf area index in the basal layers of the canopy (Rhodes, 1971b). Within a ryegrass canopy, dry matter (DM) and chemical composition vary as well. From the upper (>15 cm) to the lower layer (0–5 cm), Delagarde et al. (2000) reported vertical gradients in DM (increase of 80 g kg fresh grass), crude protein (decrease of 100 g kg organic matter), and neutral detergent fi ber (NDF, increase of 250 g kg organic matter). Depending on the ploidy (diploid vs. tetraploid) and maturity (early, intermediate, or late), Gilliland et al. (2002) noted signifi cant diff erences in canopy structure (proportion of lamina, green leaf mass, sward and tiller height, and bulk density) and herbage nutritive value (water-soluble carbohydrate concentration and proportion of linoleic and α-linolenic fatty acids). The relationship between canopy structure and intake by grazing animals has been established in both cattle and sheep. McGilloway et al. (1999) reported that sward height was the principal determinant of bite weight in lactating dairy cows grazing perennial ryegrass, but that the infl uence of sward bulk density on intake became greater as sward height declined. Casey and Brereton (1999) reported similar fi ndings when the bulk density of perennial ryegrass was altered by removing tillers; bite weight increased as sward height and density increased and the eff ect of height was greater than that of density. The eff ect of sward density, however, increased as sward height decreased. In sheep, pasture intake was related to sward height when tiller density was constant, and to bulk density at similar sward height (Black and Kenney, 1984). Although pasture intake is infl uenced primarily by DM allowance, NDF of available pasture has relevance in grazing-based dairy systems because it is negatively associated with potential intake (Vazquez and Smith, 2000). The general role of NDF in the diet of dairy cows was described by Mertens (1997): if the forage consumed has excessive NDF, energy density may be low and intake and productivity may be reduced. Thus, the amount of dietary fi ber can have an impact on pasture utilization. Depending on management goals and pasture composition, practitioners of managed intensive rotational grazing typically have a target sward height at which they begin and end grazing. For tall-growing temperate grasses, current recommendations suggest that grazing begin at a 25cm height and end when a 10-cm stubble is reached (Undersander et al., 2002). Given these criteria, the objective of this study was to determine seasonal diff erences in canopy density and NDF among temperate grasses common to the Midwest and Northeast USA. MATERIALS AND METHODS The study was conducted in 2004 and 2005 at the University of Wisconsin’s Arlington Agricultural Research Station (43.30°N, 89.35°W) on a Plano silt loam (fi ne-silty, mixed, superactive, mesic Typic Argiudolls). Mean and monthly precipitation during the experiment are presented in Fig. 1. Mean daily soil temperature at 5-cm depth is presented in Fig. 2. The site had a pH of 6.5, 52 mg kg P, and 117 mg kg K (Bray P1). In April 2003, ‘Park’ Kentucky bluegrass (Poa pratensis L., KBG), ‘Mara’ perennial ryegrass (PRG), ‘Bronc’ orchardgrass (Dactylis glomerata L., OGR), ‘Itasca’ timothy (Phleum pratense L., TIM), ‘Johnstone’ tall fescue (Festuca arundinacea Schreb., TFK), ‘Barolex’ soft-leaf tall fescue (TFS), ‘Bartura’ meadow fescue (Festuca pratensis Huds., MDF), ‘Lincoln’ smooth bromegrass (Bromus inermis Leyss., SBG), ‘Rival’ reed canarygrass (Phalaris arundinacea L., RCG), and common quackgrass [Elymus repens (L.) Gould, QGR] were broadcast seeded in 4.56by 6.08-m plots on a prepared seedbed. The seeding rate for each species was the following: bluegrass, 16.8 kg ha; perennial ryegrass, 28 kg ha; orchardgrass, 11.2 kg ha; timothy, 9.0 kg ha; tall and meadow fescue, 11.2 kg ha; smooth bromegrass, 17.9 kg ha; reed canarygrass, 6.7 kg ha; and quackgrass, 22.4 kg ha. Plots were separated by 1-m alleys of ‘Phoenix’ turf-type tall fescue and were arranged in a randomized complete block design with four replicates. Plots were clipped to a 10-cm stubble and fertilized with 56 kg N ha as NH 4 NO 3 in June and August 2003. Before spring grass growth began, plots were mowed to remove residue in early April 2004 and 2005 and fertilized with 56 kg N ha as NH 4 NO 3 . The canopy structure of primary spring growth was sampled in mid-May by a stratifi ed clipping method (Rhodes and Collins, 1993) using a 25by 100cm aluminum quadrat supported by four cylindrical legs. Each leg passed through a circular ring welded to the corners of the quadrat. The circumference of each leg was grooved at 5.0-cm Figure 1. Monthly precipitation during 2004 and 2005, and 30-yr mean at Arlington, WI. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 2184 WWW.CROPS.ORG CROP SCIENCE, VOL. 47, SEPTEMBER–OCTOBER 2007 cies and season were assumed to be fi xed eff ects. Means for each grass were compared using Fisher’s LSD (P ≤ 0.05). RESULTS AND DISCUSSION Canopy layer density and NDF were analyzed by year and by season (spring, summer, fall) due to signifi cant (P ≤ 0.05) year × grass and season × grass interactions. Not all grasses were present during each season of each year; precipitation, temperature, and apparent winter injury in two cases had marked eff ects on grass growth and persistence. Due to below-normal precipitation in August and September of 2004 (Fig. 1), Kentucky bluegrass, perennial ryegrass, timothy, and smooth bromegrass produced insuffi cient growth in the fall. Perennial ryegrass and smooth bromegrass also suff ered apparent injury during the 2004–2005 winter. Although smooth bromegrass stands were inadequate for sampling in 2005, perennial ryegrass plots were uniformly recovered by the fall of 2005. Timothy produced inadequate growth for sampling in the summer of 2005, and as others (Riesterer et al., 2000) observed for the Midwest region, quackgrass produced little growth during the late summer and fall, and was not sampled either year. Canopy Layer Density Quackgrass is regarded as a noxious weed in cropping systems, but it is considered a valuable early season forage in temperate grazing systems (Asay and Jensen, 1996). Its potential value in pasture settings was observed here as well; quackgrass had greater density (1.89 mg DM cm) in the upper canopy layer (20–25 cm) than any other grass in increments and a spring-loaded ball detent was imbedded in each corner ring, allowing the quadrat to slide vertically on the legs between grooves and remain stationary at a specifi c height. When the mean nonextended leaf height of each grass species reached 25 cm, the frame was placed in two random locations in the plot and the quadrat set at 20-cm height. Herbage within the quadrat above this height was grasped by hand and harvested with hand clippers; at no time did leaves of one canopy stratum extend into a lower stratum. The quadrat was subsequently lowered to 15 cm and then to 10 cm, and the harvesting process repeated. Herbage from the same canopy layer within a plot was placed in a paper bag, dried at 65°C for 48 h, and weighed. After all grasses were sampled in the spring, plots were clipped to a 10-cm stubble and permitted to grow for approximately 21 d, at which time plots were again clipped to a 10-cm stubble and fertilized with 56 kg N ha as NH 4 NO 3 . The canopy structure of the regrowth was sampled in midJuly (summer) as described above. Plots were managed after the summer sampling in a manner similar to that following the spring sampling before canopy structure was measured in late September (fall). Spring, summer, and fall sampling periods will be referred to as seasons. Herbage from each layer was ground to pass a 1-mm screen in a Wiley mill. Samples were analyzed for NDF concentration by calibrated near-infrared refl ectance spectroscopy. The NDF of calibration samples was measured by the method of Mertens (2002) using 600-mL beakers. Calibration statistics for NDF were the following: standard error of prediction corrected for bias [SEP(C)] = 1.1, R = 0.98. Canopy structure was expressed as canopy layer density (mg DM cm). Density and NDF were analyzed by analysis of variance using a split-plot-in-time model (Steel et al., 1997). Block and year were assumed to be random eff ects, while speFigure 2. Mean daily soil temperature at 5-cm depth during the experiment at Arlington, WI, and mean sampling date of grasses in 2004