Published May 15, 2015
Research
Changes in Forage Nutritive Value among Vertical Strata of a Cool-Season Grass Canopy Renata L. G. Nave,* R. Mark Sulc, David J. Barker, and Normand St-Pierre
ABSTRACT An understanding of the vertical distribution of nutritive value in cool-season grasses could support decisions regarding the management of residual mass and plant height to meet nutritional value targets. The objective of this study was to characterize the vertical distribution of nutritive value within a grass sward and to relate it to the morphological components of the herbage (lamina, stem + sheath, and dead matter). The research was conducted from April to October 2009 and 2010 in a mixed cool-season grass sward consisting primarily of tall fescue [Schedonorus arundinaceus (Schreb.) Dumort., nom. cons., formerly Festuca arundinacea Schreb.]. Growing periods were initiated in April, May, June, July, and August, during which forage was allowed to accumulate for the remainder of the growing season, with weekly sampling for nutritive value and morphological composition. Neutral detergent fiber (NDF) was significantly greater in the 5- to 15-cm strata than in the 15- to 25-cm in 32% of all initiation date ´ days after cutting combinations across years. The NDF concentration increased as the season progressed, which was associated with increasing dead matter and declining lamina content in the canopy. Differences in NDF digestibility among strata were less consistent than for NDF. For the May through August initiation dates, few nutritive value differences were found among strata, which were attributed to lower nutritive value in the 10-cm strata immediately above the stubble height. Consequently, we anticipate a small decrease in nutritive value of the diet of a grazing animal as it consumes tall fescue–dominant pasture.
Dep. of Horticulture and Crop Science and Dep. of Animal Science, The Ohio State Univ., Columbus, OH 43210. Salary and research support provided in part by state and federal funds appropriated to the Ohio Agric. Res. and Dev. Ctr. (OARDC) and The Ohio State Univ. Partial financial support was also provided by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service (now National Inst. for Food and Agriculture), grant number 2006-55618-17025. Published as OARDC Journal Article HCS14-02. Received 8 Jan. 2014. *Corresponding author (
[email protected]). Abbreviations: DAC, days after cutting; DM, dry matter; IVTD, in vitro true dry matter digestibility; NDF, neutral detergent fiber; NDFD, in vitro NDF digestibility.
S
easonal variation in forage growth rate is a fundamental constraint on grazing system management. Pasture stocking rates and supplemental feed requirements are influenced by the dynamic balance between forage growth and the amount of herbage consumed by the animals. There is considerable variation in growth rate of cool-season grasses from year to year and season to season; however, in the Appalachian plateau, the growth rate of tall fescue [Schedonorus arundinaceus (Schreb.) Dumort., nom. cons., formerly Festuca arundinacea Schreb.] is usually highest in late spring (May to early June), while autumn (September to October) growth rate is generally less than or equal to rates during the summer months (mid-June to August) with adequate rainfall (Denison and Perry, 1990). Grazing strategies should allow flexibility in adjusting to changes in growth of the pasture grasses over the season. While grazing strategies should account for the seasonal variations in growth rate of forage grasses, changes in nutritive value of the forage are also an important consideration in managing
Published in Crop Sci. 54:2837–2845 (2014). doi: 10.2135/cropsci2014.01.0018 © Crop Science Society of America | 5585 Guilford 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.
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grazing systems. The digestibility of forage decreases as the plants mature (Buxton et al., 1985). Fiber concentration, the most important factor affecting dry matter digestibility, also increases as plants mature (Buxton and Redfearn, 1997). Stems of most plant species have a greater fiber concentration than lamina, and grasses usually contain more fiber than legumes (Deinum et al., 1968). In general, cool-season grass herbage digestibility is greater in spring, decreases in summer (Deinum et al., 1968), and increases again in fall ( Jung et al., 1974). Balanced diets providing higher levels of in vitro neutral detergent fiber digestibility (NDFD) can increase energy intake in early lactating dairy cows (Oba and Allen, 1999). It is important to understand the factors that control the vertical distribution of nutritive value, to predict the potential nutritive value of the ingested herbage. In legumes, especially alfalfa (Medicago sativa L.), the lower portion of the canopy can have poor nutritive value. Stems of alfalfa, red clover (Trifolium pratense L.), and birdsfoot trefoil (Lotus corniculatus L.) decrease in digestibility with maturity at a higher rate than lamina and also increase in proportion of the total dry matter (DM) present (Buxton et al., 1985). Early harvests are recommended to achieve higher digestibility of the harvested legume forage (Buxton et al., 1985). Differences in vertical distribution of leaf area index of ryegrass (Lolium perenne L.) cultivars within the canopy strata can result in different cutting frequency responses (Rhodes, 1971). Dry matter and nutritive value differences also can be observed, with an increase in DM, crude protein, and neutral detergent fiber (NDF) for vertical layers of 10 cm in perennial ryegrass (Delagarde et al., 2000). There is limited information regarding vertical distribution patterns of nutritive value in cool-season grasses, especially for vegetative herbage (Burner and Belesky, 2004). More complete information could support decisions regarding cutting schedules, residual mass and heights to meet nutritional value targets, and the pasture allocation during grazing (Griggs et al., 2007). The objective of this study was to use clipping and regrowth to characterize the vertical distribution of nutritive value within a grass sward and to relate this to the morphological components of the herbage (lamina, stem + sheath, and dead matter). Our hypothesis was that forage nutritive value is highest in the upper strata of a grass canopy and declines from top to bottom. If that is the case, the vertical gradient in forage nutritive value should allow an opportunity for preferential grazing by livestock with higher nutrient requirements, while lower quality forage in the lower portions of the canopy might constrain animal performance, even with an adequate quantity of available forage.
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MATERIALS AND METHODS Site Description
The research was conducted at the Don Scott Airport of The Ohio State University (40°4´32˝ N, 83°5´2˝ W, 274 m altitude) in Columbus, OH, from April 2009 to October 2010 in a mixed sward comprised predominantly of tall fescue (73%) and Kentucky bluegrass (Poa pratensis L.) (15%). The soil was a Kokomo silty clay loam (a fine, mixed, superactive, mesic Typic Argiaquolls) with a pH of 6.8, 3.8% organic matter, 86 mg P kg-1 soil, and 233 mg K kg-1 soil. Urea was applied at 78 kg N ha-1 on 2 Apr. and 1 June 2009 and at 50 kg N ha-1 on 4 Apr. and 22 kg N ha-1 on 1 June and 13 July 2010. Rainfall was adequate for forage growth from April through July in both years and averaged 95 mm per month, which was equal to the 30-yr average (data not shown). Late summer and autumn forage growth was reduced by warm and dry weather in both years. In 2009, August to October rainfall averaged 72 mm per month, which was 4% below the 30-yr average, and in 2010 it averaged 42 mm per month, which was 44% below the 30-yr average. The mean air temperature from April to October was 1.2°C below and 0.9°C above the 30-yr average in 2009 and 2010, respectively. The April to October average air temperature was 17.2 and 19.3°C in 2009 and 2010, respectively.
Measurements Five initiation dates for herbage accumulation were imposed during 2009 and 2010. In 2009, initiation dates were 4 April, 6 May, 5 June, 6 July, and 4 August. In 2010, initiation dates were 6 April, 4 May, 4 June, 2 July, and 5 August. On each initiation date, the sward was clipped to a 5-cm stubble height and allowed to grow without harvesting until herbage accumulation ceased each autumn, which was 16 Oct. 2009 and 9 Sept. 2010 in the experiment. The earlier date for termination of sampling in 2010 was due to the dry and warm conditions in August and September that caused grass accumulation to cease by early September. The initiation dates were replicated four times in a randomized complete block design. Individual plots (experimental units) were 5 × 9 m. To characterize morphological composition and nutritive value of the sward canopy, random sampling without replacement of a 0.1 m 2 area within each experimental unit was used on a weekly basis. Sampled quadrats were identified with flags to avoid resampling of previously clipped areas. Forage samples were collected from each 10-cm vertical section of the forage present, above a 5-cm stubble height (i.e., 5 to 15, 15 to 25, 25 to 35, and >35 cm above ground level). When clipping each strata, the plants were extended to the full vertical height (i.e., strata represent the extended length rather than the natural vertical canopy distribution). The vertical strata subsamples were separated into three components: green lamina, green stem + leaf sheath, and dead material. Morphological components were dried at 60°C, reaching constant dry weight within 8 to 12 h, and dry mass for each component was recorded. Morphological components within each strata sample for each plot were then combined and ground through a 1-mm screen (Thomas-Wiley Laboratory Mill Model 4, H. Thomas Co.) for laboratory analyses of fiber and fiber digestibility using the ANKOM A200 Fiber Analyzer and DaisyII
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Incubator (ANKOM Technology Corporation). The NDF concentration was determined using the method of Van Soest and Robertson (1980), with modifications for the filter bag system according to Method 6 of ANKOM (2011), which included the use of α-amylase and Na2SO3 (Sigma no. A3306, Sigma Chemical Co.). For in vitro true dry matter digestibility (IVTD), we used a method similar to that described by Vogel et al. (1999), in that IVTD was determined according to Method 3 of ANKOM (2005), which uses Stage 1 of the procedure described by Marten and Barnes (1980), including the Kansas State buffer in a 48-h incubation with rumen fluid, and then undigested residues were treated with NDF solution. Rumen fluid was obtained from a nonlactating, fistulated dairy cow (Bos taurus) that was offered a diet of orchardgrass (Dactylis glomerata L.) hay. The NDFD was calculated using Eq. [1] (all units g kg-1): NDFD = {1– [(1000 – IVTD)/NDF]} ´ 1000. Equation
[1]
Statistical Analysis Data were analyzed as a mixed model using the MIXED procedure of SAS (V9.2, SAS Institute, 2008). The model included the fixed effects of initiation date treatment, sample date, strata, and all their two- and three-way interactions, and the random effects of block, block ´ initiation date, block ´ initiation date ´ sample date, and block ´ initiation date ´ strata. Data for each year were analyzed separately because the initiation dates and sampling dates were different for the 2 yr. Hence, a combined analysis would make the interpretation of the results far more difficult. In our model, error correlations were induced from two levels of repeated measures: the repeated measures across time (i.e., sampling dates) and across space (strata). The SAS offers only a limited set of potential error matrices for double repeated measurements. Because not all strata were present on each sampling date for all initiation date treatments, the number of potential error structures for strata was limited. The first-order autoregressive correlation structure [AR(1)] was used to model the repeated measures in space (strata) using the REPEATED statement. The errors for the repeated measures across time were modeled as a compound symmetry correlation structure using the RANDOM statement (i.e., modeled on the G matrix side as opposed to the R matrix). The KenwardRogers adjustments to the denominator degrees of freedom for the F tests and adjustments to the standard errors were used. When an effect was significant, pair-wise comparisons were conducted using Fisher’s protected least-significant difference. Differences were considered significant at P £ 0.05. Nutritive value traits were regressed on morphological components by strata to relate the vertical distribution of nutritive value to morphological components of the herbage. Slope coefficients were reported if significant at P £ 0.05.
RESULTS AND DISCUSSION
Differences Among Canopy Strata The interaction of initiation date ´ days after cutting (DAC) ´ strata was significant (P < 0.0001) in both 2009 and 2010, with differences occurring for NDF in about crop science, vol. 54, november– december 2014
half of all initiation date ´ DAC combinations across both years (Tables 1, 2). Concentration of NDF was significantly greater in the 5- to 15-cm strata than in the strata immediately above it (15 to 25 cm) in 10 of 32 comparisons in 2009 (+54 g kg-1 NDF) and in 11 of 33 comparisons in 2010 (+47 g kg-1 NDF). In general, when NDF differences among strata were present, the lowest strata (5 to 15 cm) had greater NDF concentration than higher strata (>15 cm). Brink et al. (2007) reported a similar trend of increasing NDF of cool-season grasses from the upper to lower canopy layer (mean difference of 50 g kg-1). After the period of reproductive development (April to June) was past, the NDF differences among strata were less pronounced owing to an increase in fiber concentration in all strata. The variation in NDF concentration among strata within any initiation date became less evident and less consistent as the sward matured and the season progressed (Tables 1, 2). For NDFD, the interaction of initiation date ´ DAC ´ strata was significant (P < 0.0001) in both 2009 and 2010, but these differences occurring for NDFD were less consistent than for NDF (Tables 3, 4). Significant NDFD differences among strata were found for 38% of all initiation date ´ DAC combinations across years. The NDFD of the 5- to 15-cm strata was significantly lower than for the 15- to 25-cm strata in 8 of 32 comparisons in 2009 (-76 g kg-1 NDFD) and in 9 of 33 comparisons in 2010 (-70 g kg-1 NDFD). The differences in NDFD were variable in the forage accumulated from early April. The lowest strata (5 to 15 cm) tended to have low NDFD compared with the other strata, but when the canopy was fully developed, the uppermost strata often had the lower NDFD (DAC = 52, 75, 89, 114, and 160 in 2009; DAC = 44, 59, and 65 in 2010), probably because it consisted primarily of inflorescence. For the May through August initiation dates, there were fewer significant NDFD differences among strata, and when they did occur, the lowest strata (5 to 15 cm) generally had lower NDFD than higher strata (Tables 3, 4). Our results generally agreed with Griggs et al. (2007), who reported that vertical distribution of digestibility in orchardgrass became less pronounced as the season progressed. In forage accumulated from early April and May, most differences in NDF concentration could be explained by the age and morphological composition of the canopy. When evaluating the entire sward canopy in this experiment, Nave et al. (2013b) reported that dead matter and lamina proportion were well correlated with either NDF or NDFD, while stem proportion was not as well correlated with those two nutritive value variables. A similar trend was observed when analyzing the relationships between nutritive value traits and morphological components by individual strata (Table 5). Dead matter content was most consistently related to nutritive value. When dead matter was used as the independent variable, slope
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Table 1. Least square means of neutral detergent fiber (NDF) in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2009. 4 April initiation DAC† d 12 18 25 31 38 45 52 62 75 89 101 114 146 160 173
5–15 cm
6 May initiation
15–25 25–35 35–45 cm cm cm DAC
——————— g kg-1 ——————— 569 . . . 475 . . . 523a‡ 480b . . 554a 511b 497b . 513a 490ab 465b . 544a 522a 547a . 581a 572a 581a 562a 591a 581a 560a 576a . . . . 627a 616a 601a 514b 628a 619ab 583bc 561c . . . . 632a 624a 604a 598a 642a 650a 627a 635a . . . . 634a 641a 635a 595b 616a 623a 608a 617a . . . . 635ab 639a 633ab 605b . . . . . . . .
5–15 cm
5 June initiation
15–25 25–35 cm cm DAC
———— g kg-1 ———— . . . . . . . . . . . . 583 . . 509 . . 525 . . 535 . . 607a 560b . 600a 527b . 597a 535b . . . . 598a 550b . 588a 574a . . . . 620a 588a . a 616 600a . . . . 616a 587a 553b . . . . . .
d . . . . 6 13 20 30 37 43 57 69 82 114 128 141 . .
d . . . . . . . . 7 13 27 34 39 52 84 98 111 137
5–15 cm
6 July initiation
15–25 cm DAC
—— g kg-1 —— . . . . . . . . . . . . . . . . 684 . 650 . 538 . 509 . 557a 520b 552a 569a . . 613a 612a 615a 602a . . 629a 600a . . 562a 527b
d . . . . . . . . . . . 3 8 21 53 67 73 80 106
5–15 cm
4 August initiation
15–25 cm DAC
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . 643 . 672 . 627 . . . 567 . 596 . 597a 572a 608a 608a . . 557a 522b
†
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for NDF within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
d . . . . . . . . . . . . . . 17 24 38 51 65 77
5–15 cm
15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622 . 578 . 567 . . . 558 . 549a 527a 521a 483b
Table 2. Least square means of neutral detergent fiber (NDF) in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2010. 6 April initiation DAC† d 7 14 21 28 37 44 49 59 65
84
107
135 148
5–15 cm
15–25 cm
4 May initiation 25–35 cm
35–45 cm DAC
——————— g kg-1 ——————— 611 . . . 565 . . . 577a‡ 524b . . 504a 483a . . a 622 584b . . 527b 500b 600a . 563a 532a 526a . 597a 605a 600a . 577a 556ab 535b 498c . . . . . . . . 567a 566a 535ab 511b . . . . . . . . 577a 570a 545ab 525b . . . . . . . . . . . . 589a 594a 571a 566a . . . . 611a 614a 614a 603a . . . .
d . . . . 9 16 21 31 37 42 56
79
107 120
5–15 cm
15–25 cm
4 June initiation 25–35 cm DAC
———— g kg-1 ———— . . . . . . . . . . . . 667 . . 512 . . 469 . . 538a 472b . 569a 495b . 548a 522a . . . . 526a 499ab 484b . . . . . . 567a 542ab 518b . . . . . . . . . 560a 547ab 514b . . . 594a 596a 592a . . .
d . . . . . . . . 6 11 20 25 33 41 48 56 62 76
97
5–15 cm
2 July initiation 15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . 570 . 545 . 545a 475b 500a 439b 513a 476b 531a 510a 520a 479b 537a 487b 535a 516a . . 561a 506b . . . . 572a 583a
DAC d . . . . . . . . . . . . 5 13 20 28 34 41 48 55 61 69
5–15 cm
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for NDF within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
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15–25 cm
——g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . 580 . 582 . 491 . 465 . 522a 472b 512a 478a 511a 475a 557a 534a 545a 544a 567a 551a
†
2840
5 August initiation DAC d . . . . . . . . . . . . . .
7 14 21 27 35
5–15 cm g kg-1 . . . . . . . . . . . . . . . . . 585 552 539 536 515
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Table 3. Least square means of neutral detergent fiber digestibility in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2009. 4 April initiation DAC† d 12 18 25 31 38 45 52 62 75 89 101 114 146 160 173
5–15 cm
6 May initiation
15–25 25–35 35–45 cm cm cm DAC
——————— g kg-1 ——————— 672 . . . 782 . . . 885a‡ 888a . . 695b 760a 820a . 657b 716a 695ab . 676a 721a 682a . 651a 688a 637ab 583b 619ab 662a 635a 572b . . . . 584b 602a 573a 489b 548a 549a 528ab 478b . . . . 518ab 574a 537ab 473b 534a 527a 525a 465b . . . . 371b 392b 490a 414ab 432a 478a 460a 367b . . . . 339a 337a 313a 344a . . . . . . . .
5–15 cm
5 June initiation
15–25 25–35 cm cm DAC
————— g kg-1 ————— d . . . . . . . . . . . . . . . . 627 . . . 694 . . . 696 . . . 695 . . . 712a 731a . 7 675a 665a . 13 681a 673a . 27 . . . 34 642a 683a . 39 a 651 603a . 52 . . . 544a 494a . 84 a 588 589a . 98 . . . 412b 512a 521a 111 . . . . . . 137
d . . . . 6 13 20 30 37 43 57 69 82 114 128 141 . .
5–15 cm
6 July initiation
15–25 cm DAC
—— g kg-1 —— d . . . . . . . . . . . . . . . . . . . . . . . . 662 . . 676 . . 724 . . 701 . 3 703a 710a 8 663a 674a 21 . . 622a 617a 53 608b 672a 67 . . 73 581b 656a 80 . . 551b 623a 106
5–15 cm
4 August initiation
15–25 cm DAC
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . 550 . 652 . 680 . . . 667 . 667 . 642a 733a 608b 717a . . 628a 674a
d . . . . . . . . . . . . . . 17 24 38 51 65 77
5–15 cm
15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 . 628 . 748 . . . 688 . 694a 756a 667b 728a
†
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for neutral detergent fiber within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
Table 4. Least square means of neutral detergent fiber digestibility in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2010. 6 April initiation DAC† d 7 14 21 28 37 44 49 59 65
84
107
135 148
5–15 cm
15–25 25–35 35–45 cm cm cm
——————— g kg-1 ——————— 683 . . . 724 . . . 771b‡ 849a . . 705a 759a . . 616b 671a . . 643a 687a 563b . 554a 593a 547a . 545a 561a 481b . 511b 533ab 573a 426c . . . . . . . . 422b 484a 471ab 478a . . . . . . . . 453a 466a 476a 452a . . . . . . . . . . . . 359a 381a 400a 364a . . . . 333a 386a 387a 390a . . . .
4 May initiation DAC d . . . . 9 16 21 31 37 42 56
79
107 120
5–15 cm
15–25 25–35 cm cm
————— g kg-1 ————— . . . . . . . . . . . . 559 . . 679 . . 708 . . 606a 606a . 623a 665a . a 611 641a . . . . 570a 602a 608a . . . . . . 482a 511a 491a . . . . . . . . . 460b 536a 457b . . . 425a 435a 431a . . .
4 June initiation DAC d . . . . . . . . 6 11 20 25 33 41 48 56 62 76
97
5–15 cm
2 July initiation 15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . 587 . 637 . 638a 672a 608b 671a a 594 595a 594a 645a 606a 647a 614a 648a 607b 676a . . 555a 532a . . . . 529a 553a
DAC d . . . . . . . . . . . . 5 13 20 28 34 41 48 55 61 69
5–15 cm
5 August initiation 15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . 515 . 639 . 723 . 681 . 634b 713a 653a 667a 549a 604b 626a 621a a 587 491b 536a 563a
DAC d . . . . . . . . . . . . . .
7 14 21 27 35
†
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for neutral detergent fiber within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
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5–15 cm g kg-1 . . . . . . . . . . . . . . . . . 582 621 606 601 621
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Table 5. Slope coefficients of nutritive value traits regressed on morphological components within individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) of a cool season grass sward during 2 yr. 6 April initiation Trait & Morphological Year components NDF† 2009
2010
NDFD† 2009
2010
4 May initiation
2 July initiation
5 August initiation
5–15 cm
15–25 cm
25–35 cm
35–45 cm
5–15 cm
15–25 cm
25–35 cm
5–15 cm
15–25 cm
5–15 cm
15–25 cm
5–15 cm
15–25 cm
n Lamina Stem + sheath Dead n Lamina Stem + sheath Dead
48 -0.21 NS 0.24 50 -0.08 NS 0.08
38 -0.25 -0.28 0.23 43 -0.08 NS 0.09
36 NS‡ -0.12 0.20 31 NS NS NS
24 NS NS NS 20 NS NS 0.10
34 NS NS NS 40 -0.10 NS 0.10
24 NS NS 0.07 28 -0.11 NS 0.12
4 NS NS NS 16 -0.10 NS 0.14
32 -0.15 NS 0.15 44 -0.10 NS 0.10
18 NS NS 0.18 36 -0.08 0.16 0.24
28 NS NS NS 40 -0.09 NS 0.12
10 NS NS NS 24 -0.10 NS 0.13
24 NS -0.94 NS 20 NS NS NS
8 NS NS NS . . . .
n Lamina Stem + sheath Dead n Lamina Stem + sheath Dead
46 0.66 NS -0.66 51 0.32 0.42 -0.42
38 0.72 0.66 -0.62 42 0.25 NS -0.39
35 0.38 0.22 -0.52 31 NS 0.14 -0.21
24 NS 0.20 -0.25 20 0.15 NS -0.12
38 0.31 -1.27 -0.31 40 0.34 NS -0.41
24 NS NS -0.15 28 0.29 NS -0.35
4 NS NS NS 16 0.22 NS -0.34
34 0.36 NS -0.36 44 0.13 NS -0.16
18 0.32 NS -0.32 36 NS NS -0.36
27 0.35 NS -0.35 40 0.23 NS -0.27
10 NS NS NS 24 0.23 -0.21 -0.20
24 0.30 NS -0.34 20 NS NS NS
8 NS NS NS . . . .
†
NDF, neutral detergent fiber; NDFD, neutral detergent fiber digestibility.
‡
NS, slope coefficient not significant at P = 0.05.
coefficients for NDF were positive and significant in 68% of the 25 comparisons, whereas for NDFD the slopes were negative and significant in 84% of the comparisons. When lamina was used as the independent variable, slope coefficients for NDF were negative and significant in 48% of the comparisons (mostly in 2010), whereas for NDFD the slopes were positive and significant in 68% of the comparisons. Stem + sheath was not consistently related to nutritive value within individual strata, with slope coefficients being significant in only 4 and 7 of the 25 comparisons for NDF and NDFD, respectively. From late April to mid-May, NDF tended to be lower in the 15- to 25- and 25- to 35-cm stratas owing to a lower percentage of dead material (Tables 6, 7) and high percentage of new leaves (data not shown). Mid-season increases in dead material (Tables 6, 7) and stem proportions (data not shown) in the April and May initiation dates resulted in elevating the forage NDF content. Delagarde et al. (2000) reported an increase in NDF concentration of 420 to 674 g kg-1 from the top to the bottom of the canopy in perennial ryegrass. In our study, an NDF concentration increase with canopy depth was expected as a function of leaf:stem proportion and maturity; however, the observed variation in NDF concentration through the vertical gradient was less than that reported for ryegrass (Delagarde et al., 2000). Early in the spring, the few NDFD differences found were attributed to a low proportion of dead matter (Tables 6, 7) and high proportion of leaves (data not shown) in the herbage. For April initiation, as the canopy reached maturity and entered the reproductive phase (DAC = 52 and 44 in 2009 and 2010, respectively), NDFD differences 2842
4 June initiation
were greater owing to higher percentages of dead matter and stems (data not shown), suggesting the importance of maintaining the canopy in a vegetative condition, especially for forage accumulation periods initiated in early spring (e.g., April initiation in our study), when the growth rate was the greatest (Nave et al., 2013a). Across all initiation dates, there was a general decrease in nutritive value as DAC increased (Tables 1–4), which corresponded with an increase in maturity related to lamina age and an increase in dead material (Tables 6, 7). According to Karn et al. (2006), lamina tissue digestibility decreased from 863 g kg-1 in the vegetative stage to 184 g kg-1 at anthesis, but NDF showed a less consistent pattern of change as maturity increased. Cool season grasses tend to decrease in digestibility with advancing maturity in the spring (Minson et al., 1960) in a consistent pattern to guide producers in predicting optimal grazing and cutting dates (Reid et al., 1959). Nave et al. (2013b) demonstrated with regression analyses applied to this experiment that there was a good relationship between NDF and DAC (age) for all strata within the April initiation date, but the relationship became weaker for the later growth periods (May to August initiation dates). They also reported a negative relationship between NDFD and DAC within the April initiation date; however, in contrast to the findings for NDF, the NDFD in the lower canopy strata showed a consistent negative relationship with DAC for most initiation dates. They concluded in that analysis that the changes in NDFD occurring in the bottom of the canopy with maturity are more consistent than the changes occurring at the top of the canopy.
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Table 6. Least square means of dead material in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2009. 4 April initiation DAC† d 12 18 25 31 38 45 52 62 75 89 101 114 146 160 173
5–15 cm
6 May initiation
15–25 25–35 35–45 cm cm cm DAC
——————— g kg-1 ——————— 267 . . . 112 . . . 12a‡ 0b . . 131a 0b 0b . 78a 0a 0a . 100a 3a 0a . 179a 14b 3b 0b 208a 30b 11b 0b . . . . . . . . 238a 113b 91b 21b . . . . . . . . 330ab 321ab 382a 264b . . . . 492b 502b 568b 775a . . . . . . . . 646a 627a 608a 690a . . . . . . . .
5–15 cm
15–25 25–35 cm cm DAC
———— g kg-1 ———— . . . . . . . . . . . . 320 . . 140 . . 61 . . 78 . . 65b 250a . 87a 20a . 65a 13a . . . . . . . 200a 150a . . . . 323a 398a . . . . . . . 560a 443b 584a . . . . . .
d . . . . 6 13 20 30 37 43 57 69 82 114 128 141 . .
d . . . . . . . . 7 13 27 34 39 52 84 98 111 137
5 June initiation
6 July initiation
5–15 cm
5–15 cm
15–25 cm DAC
—— g kg-1 —— . . . . . . . . . . . . . . . . 255 . 217 . 0 . 11 . 49a 5a 27a 47a . . 267a 207a . . . . 391a 221b . . 395a 224b
d . . . . . . . . . . . 3 8 21 53 67 73 80 106
4 August initiation 15–25 cm DAC
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . 335 . 333 . 80 . . . 90 . 125 . 171a 63a 214a 72b . . 361a 136b
d . . . . . . . . . . . . . . 17 24 38 51 65 77
5–15 cm
15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 . 174 . 33 . . . 31 . 148a 62a 198a 82b
†
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for neutral detergent fiber within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
Table 7. Least square means of dead material in regrowth of a cool season grass sward for individual canopy strata (5–15, 15–25, 25–35, and 35–45 cm above ground level) for sequential days after cutting (DAC) following five initiation (cutting) dates during 2010. 6 April initiation DAC† d 7 14 21 28 37 44 49 59 65
84
107
135 148
5–15 cm
15–25 25–35 35–45 cm cm cm
——————— g kg-1 ——————— 385 . . . 233 . . . 64a‡ 42a . . 145a 79a . . 115a 33a . . 103a 19a 5a . 137a 0a 0a . 179a 35b 0b . 169a 46ab 5b 15b . . . . . . . . 211a 82ab 65b 0b . . . . . . . . 305a 264a 124b 287a . . . . . . . . . . . . 676a 389b 473b 666a . . . . 596b 682ab 695ab 772a . . . .
4 May initiation DAC d . . . . 9 16 21 31 37 42 56
79
107 120
5–15 cm
15–25 25–35 cm cm
———— g kg-1 ———— . . . . . . . . . . . . 257 . . 99 . . 4 . . 72a 1a . 59a 0a . 91a 5a . . . . 76a 6a 22a . . . . . . 337a 301a 121b . . . . . . . . . 470a 307b 81c . . . 572a 548a 414b . . .
4 June initiation DAC d . . . . . . . . 6 11 20 25 33 41 48 56 62 76
97
5–15 cm
2 July initiation 15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . 142 . 44 . 85a 0a 36a 0a 179a 34b 51a 8a 16a 0a 81a 40a 115a 29a . . 240a 60b . . . . 308b 479a
DAC d . . . . . . . . . . . . 5 13 20 28 34 41 48 55 61 69
5–15 cm
5 August initiation 15–25 cm
—— g kg-1 —— . . . . . . . . . . . . . . . . . . . . . . . . 313 . 75 . 0 . 0 . 41a 0a 69a 84a a 63 74a 274a 152a 301a 179a a 474 424a
DAC d . . . . . . . . . . . . . .
7 14 21 27 35
†
DAC according to the initiation date shown in each main heading. Each line represents samples collected on the same calendar date.
‡
Means for neutral detergent fiber within the same sampling and initiation date that are followed by a different superscript are significantly different (P < 0.05).
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5–15 cm g kg-1 . . . . . . . . . . . . . . . . . 119 0 3 259 274
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Morphological Composition Effects Morphological composition varied across the canopy strata. The percentage of dead material consistently increased within strata for all initiation dates, with a higher percentage in the lowest strata (5 to 15 cm) for the April initiation (Tables 6, 7). For the April initiation, upper strata (25 to 45 cm) had more stem than lower strata (5 to 25 cm) (data not shown), explaining some of the differences observed in NDF and NDFD after reproductive development. Lamina percentage varied among strata for April initiation, but was constant among strata for forage accumulating from the May, June, July, and August initiation dates (data not shown). Initially, there was an increase in lamina (data not shown) and a decrease in dead material in the sward, followed by a period with little change (in most cases), after which there was a general trend of decreasing percentage of lamina (data not shown) and an increase in dead material in the canopy (Tables 6, 7). The percentage of stem was usually lower than percentage of lamina and dead material, increasing from beginning of May to the end of June for April and May initiation (data not shown). Our results were in general agreement with previous studies where stem mass increased during the spring and summer in legume canopies (Buxton et al., 1985). As the plant matures from the vegetative stage to the reproductive stage, the leaf:stem ratio tends to decrease, especially in drier seasons, and is a result of reproductive stem growth and leaf senescence (Albrecht et al., 1987).
SUMMARY We were able to characterize the vertical distribution of nutritive value within a grass sward and to relate this to the morphological components. Nutritive value of the canopy in the spring was highly affected by changes in morphological composition occurring as the sward aged; however, on any given sampling date the differences in NDF and NDFD among vertical strata of the sward were often smaller and less consistent than we hypothesized on the basis of previous findings reported for legumes that show a large and more consistent vertical gradient in nutritive value. Our results suggest there would be a relatively small change in nutritive value of the grazing animal’s diet when it is consuming tall fescue–dominant pastures near to the stubble height compared with the relatively large nutritive value changes that occur within a pure legume canopy. Brink et al. (2013) reported that changing the residual sward height from 4 to 24 cm above the soil surface had little effect on nutritive value of hand-sampled forage in a rotational stocking management system. On the basis of our results and those of Brink et al. (2007, 2013) it appears that residual height after grazing has little impact on nutritional value of harvested forage, but whether this translates to minimal impact on the animal should be confirmed by actual animal performance in pastures managed 2844
for varying residual heights, because selectivity exercised by the animal while grazing may affect the outcome. Our results do demonstrate the importance of maintaining the sward canopy in young, vegetative stages of development, as shown by the consistent negative relationship between dead matter and NDFD and the positive relationship between dead matter and NDF. Maintaining the sward in vegetative stages seems to be especially important during spring when there is rapid herbage accumulation due to stem elongation and reproductive development, which translates into accumulation of dead matter as the herbage matures. Early in the season, frequent harvests should be made to control the appearance of inflorescence and accumulation of dead material that generate an overall lower nutritive value. Vegetative pasture will produce forage with a constant leaf percentage throughout the vertical strata and consequently avoid the substantial decrease in NDFD caused by accumulation of dead material. Acknowledgments The authors thank Dale Geltier, Facilities Manager of Don Scott Airport, The Ohio State University, for access to the research site. We also thank John McCormick, Gina Roberts, and Jason Rethman for their assistance with data collection, laboratory analyses, and plot maintenance. We thank several anonymous reviewers for their excellent suggestions that improved the manuscript.
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