Animal Feed Science and Technology 171 (2012) 52–59
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In sacco degradation kinetics of fresh and field-cured peanut (Arachis hypogaea L.) forage harvested at different maturities J.L. Foster a,∗ , G.C. Lamb b , B.L. Tillman c , J.J. Marois d , D.L. Wright c , M.K. Maddox b a b c d
Texas AgriLife Research, Texas A&M University Department of Soil & Crop Science, 3507 Hwy 59 E, Beeville, TX, USA North Florida Research and Education Center, Department of Animal Sciences, University of Florida, 3925 Hwy 71, Marianna, FL, USA North Florida Research and Education Center, Agronomy Department, University of Florida, 3925 Hwy 71, Marianna, FL, USA North Florida Research and Education Center, Department of Plant Pathology, University of Florida, 155 Research Road, Quincy, FL, USA
a r t i c l e
i n f o
Article history: Received 6 January 2011 Received in revised form 21 September 2011 Accepted 28 September 2011
Keywords: Peanut Arachis Degradation kinetics Maturity Warm-season legume Nutritive value
a b s t r a c t There is interest in growing peanut (Arachis hypogaea L.) for forage, but little is known about the nutritive value and forage quality of modern cultivars. The objective of this study was to compare the chemical composition and in sacco degradation kinetics of three cultivars of peanuts (cv. ‘C99-R’, ‘Georgia-01R’, and ‘York’) at either stage 2 or 8 maturities when fresh and field-cured. Herbage yield was at least 3000 kg DM/ha for all cultivars at both maturities. Crude protein (CP) was greater (P < 0.0001) at R2 stage than at R8 stage; whereas, neutral detergent fiber (aNDF), acid detergent fiber, and Lignin (sa) were greater (P < 0.01) at R8 than R2 maturity stages. Water soluble carbohydrate and acid detergent insoluble nitrogen was not different (P > 0.07) among cultivars, maturity stage, or harvest forms. In vitro true digestibility was greatest (P < 0.02) for C99-R and least for York. Undegradable intake protein concentration was greatest (P < 0.04) in York and least for C99-R. Maturity had a greater effect on the degradation kinetics than harvest form or cultivar. The dry matter (DM) and CP in the soluble wash fraction (A) and insoluble but degradable fraction (B) and the effective ruminal degradability were greater among all cultivars and both harvest forms of the R2 maturity stage than the R8. The undegradable DM, aNDF, and CP in the undegradable fraction were greatest (P < 0.002) for all three cultivars at R8 maturity. The rate of degradation of DM and CP in the B fraction was faster (P < 0.001) at R2 stage than at R8 stage; whereas, rate of aNDF degradation was not different (P > 0.09) among treatments. Lag of DM, aNDF, or CP degradation was not different (P > 0.1) among treatments. The cultivars C99-R and Georgia-01R are recommended for further feeding trials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Legumes have a greater protein concentration and less structural fiber concentration than grasses. This and the reticulate venation of leaves allows legumes to be degraded more easily and rapidly by ruminal microbes (Waghorn et al., 1989; Wilson, 1994; Jung and Allen, 1995; Dewhurst et al., 2003; Frame, 2005). Despite the nutritional benefits of legumes, warm-season legumes are not commonly used in the United States. Most of the available cultivars are annuals which require seed purchase,
Abbreviations: A, soluble wash fraction; ADF, acid detergent fiber; ADIN, acid detergent insoluble nitrogen; B, insoluble but degradable fraction; C, undegradable fraction; CP, crude protein; CV, cultivar; DM, dry matter; E, effective ruminal degradability; HF, harvest form; IVTD, in vitro true digestibility; kd , fractional rate of degradation; kp , fractional passage rate; L, lag time; Lignin (sa), lignin determined by sulfuric acid method; M, maturity; aNDF, neutral detergent fiber; aNDIN, neutral detergent insoluble nitrogen; NLIN, non-linear; UIP, undegradable intake protein; WSC, water soluble carbohydrates. ∗ Corresponding author. Tel.: +1 361 358 6390; fax: +1 361 358 4930. E-mail address:
[email protected] (J.L. Foster). 0377-8401/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2011.09.019
J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
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land preparation, and planting each spring. There are very few warm-season perennial legumes in the southeast due to the length and high temperatures of the summer (Leep et al., 2002; Barnes and Nelson, 2003). One common summer legume crop is peanut (Arachis hypogaea L.), a crop grown on 0.5 million hectares in the United States and 1.7 trillion kilograms of peanuts are produced annually (NASS, 2007). Worldwide peanut is produced on 17 million hectares and it is the third most important oil seed crop after soybean and cotton (Singh and Singh, 1991). The use of fungicides preclude the feeding of peanut stover to ruminant animals in many countries, including the United States; however, seed is less expensive and more readily available compared to that of leguminous forage crops (Foster, 2008), planting is relatively easy and equipment readily available, and management practices are well documented. There is interest in growing peanut for hay, haylage, or silage. In north Florida, a stand of peanut was planted for forage and maintained forage production through self reseeding for 7 years (Foster et al., 2009a,b). There may be as large of a variation in forage nutritive value among cultivars as there is among species (Jung et al., 1997). Digestibility and intake by lambs was increased when cultivar ‘Florida MDR 98’ was stored as hay or haylage and supplemented to bahiagrass (Paspalum notatum Flügge) hay or haylage (Foster et al., 2009a,b). There is a need to evaluate and compare the nutritive value of peanut cultivars so that those with potential could be identified and evaluated for their use as a self reseeding forage crop. Three cultivars with potential as forage types in the southeastern US are ‘C99-R’, ‘Georgia-01R’, and ‘York’. All three are late maturing (150–155 d), runner type peanuts with an upright central stem and prostrate growth habit (Gorbet and Shokes, 2000; Branch, 2002; Wehtje and Grey, 2004). All three cultivars have resistance to tomato spotted wilt virus and to leafspot. The objective of this study was to compare the chemical composition and in sacco degradation kinetics of three cultivars of peanuts at two different maturities when fresh and field-cured. 2. Materials and methods 2.1. Materials and experimental site Three peanut cultivars, C99-R (Gorbet and Shokes, 2000), Georgia-01R (Branch, 2002), and York, were established at North Florida Research and Extension Center (30◦ 48 N, 85◦ 12 W; 33 m above sea level) near Marianna, FL, USA. Temperature and precipitation monthly averages during the experiment and 30-year average are presented in Fig. 1. Two rows of each cultivar were planted in 0.91 m row spacings in 42.5 m long plots on 3 June, 2008 with border rows on each side. Fields were managed according to Florida Cooperative Extension Service recommendations for peanut including overhead irrigation, however, no fungicide was applied. At R2 [beginning peg (one elongated gynophore); 21 Aug 2008] and R8 (harvest maturity; 21 Oct 2008) maturity stages (Boote, 1982), samples were collected from the two center rows (0.25 m2 quadrat) to a 5 cm stubble height. Samples were weighed, dried at 55 ◦ C until weight loss ceased, and weighed again for dry matter yield determination. Additional samples were taken randomly within the plots at R2 maturity stage. These samples were placed on plastic sheeting and allowed to sun-cure. At R8 maturity stage peanuts were harvested from the plots with a K.E.W. peanut plot combine (Kingaroy Engineering Works, Kingaroy, Queensland, Australia). Vegetative material was collected from the machine and placed on plastic sheeting to sun-cure. Dried fresh (R2 and R8) and sun-cured (R2 and R8) material was ground to at least 3 mm in a Wiley mill (Arthur H. Thomas Company, Philadelphia, PA) for in sacco incubation. A sub-sample (250 g) was then ground to 1 mm for laboratory analysis. 2.2. In sacco incubations Approximately 4.5 g of sample as fed was weighed into 10 cm × 20 cm polyester bags (53 ± 10 m pore size; Bar Diamond, Inc., Parma, ID) in triplicate. Bags were heat-sealed and incubated in the ventral rumen of each of 3 mature Brangus steers
Fig. 1. Temperature and precipitation monthly averages during the experimental period in 2008 and 30-year monthly averages.
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J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
(900 ± 25 kg) and removed after 4, 8, 16, 24, 48, and 96 h. Zero hour bags were not incubated in the rumen. Steers were housed in a pen (10 m × 20 m) in a covered barn and had ad libitum access to both bahiagrass and perennial peanut hays which were placed in separate feeders and water. Use of the animals was approved by the University of Florida Institutional Animal Care and Use Committee. After incubation, bags were placed in ice water, rinsed in cool tap water to remove rumen contents on the exterior, and frozen until all bags had been removed. All bags were then washed together in a cool-rinse cycle in a top-loading washing machine (Kenmore Series 70, Sears, Roebuck & Co., Hoffman Estates, IL) and dried at 55 ◦ C to constant weight in a forced-air oven. 2.3. Chemical composition Samples representing forages prior to incubation were ground to 1 mm and analyzed for dry matter (DM), crude protein (CP), neutral detergent fiber (aNDF), acid detergent fiber (ADF), lignin determined by sulfuric acid method (Lignin (sa)), acid detergent insoluble nitrogen (ADIN), undegradable intake protein (UIP), water soluble carbohydrate (WSC), and in vitro true digestibility (IVTD). After incubation, the in sacco residue was analyzed for DM, CP, aNDF, and aNDIN. All chemical constituents are reported on a DM basis. Total N concentration was determined by rapid combustion using procedure number 992.15 of AOAC (1995) in a macro elemental N analyzer (Elementar, vario MAX CN, Elementar Americas, Mount Laurel, NJ) and used to calculate CP (CP = N × 6.25). Neutral detergent fiber concentration was measured using the method of Van Soest et al. (1991) in an ANKOM 200 Fiber Analyzer (ANKOM Technologies, Macedon, NY). Heat stable alpha amylase was used for aNDF analysis and sodium sulfite was not. Feed samples were analyzed for ADF with procedure number 973.18 of the AOAC (1990) and Lignin (sa) with the method of Robertson and Van Soest (1981) in the ANKOM 200 Fiber Analyzer. The N concentration was measured on the ADF residues with the macro elemental N analyzer and ADIN calculated. Water soluble carbohydrates were measured using methodology described by Hall et al. (1999). The Van Soest et al. (1966) method was used to determine in vitro true digestibility (IVTD) in an ANKOM Daisy II Incubator whereby a subsample was incubated in a mixture of ruminal fluid and buffer for 48 h and then subjected to the aNDF procedure previously described. The aNDIN of in sacco residues was determined by measuring the N in aNDF residues and undegradable intake protein (UIP) was calculated from aNDIN according to Haugen et al. (2006). 2.4. Calculations of degradation kinetics In situ rumen DM, aNDF, and CP degradation data were fitted to thefirstorder exponential model with discrete lag (Mertens, 1977) using the iterative Marqardt method and the NLIN procedure of SAS (SAS Institute, Cary, NC). The model is of the form: R(t) = B × (e−kd (t−L) ) + C where ‘R(t) ’ is the total indigested residue at any time ‘t’, ‘B’ is the insoluble but degradable fraction, ‘kd ’ is the fractional disappearance rate (h−1 ) of digestion of ‘B’, ‘t’ is the time incubated in the rumen in h, ‘L’ is the discrete lag time in h, and ‘C’ is the fraction not digested after 96 h of incubation. The wash fraction ‘A’ was the substrate washed out of the bag at 0 h. Effective ruminal degradability (E) was calculated using the model of Orskov and McDonald (1979):
E =A+
B×
kd kd + kp
where ‘kp ’ is the fractional passage rate, assumed to be 0.05 h−1 . 2.5. Statistical analyses The data were analyzed as a 3 × 2 × 2 factorial experiment with three cultivars, two maturities, and two harvest forms. There were 3 replicates of each incubation series and data averaged prior to statistical analyses. The GLIMMIX procedure of SAS (SAS Inst. Inc., Cary, NC) was used to evaluate treatment effects on in sacco degradation kinetics and the fixed effects model was: Y = b0 + b1i × cultivar + b2i × maturity + b3i × harvest form + b4i × cultivar × maturity + b5i × cultivar × harvest form + b6i × maturity × harvest form + b7i × cultivar × maturity×, harvest form + e where ‘cultivar’, ‘maturity’, and ‘harvest form’ are categorical variables, ‘b0 is the intercept, ‘b1i ’ is the effect of cultivar, ‘b2i ’ is the effect of maturity, ‘b3i ’ is the effect of harvest form, ‘b4i –b7i ’ are the interactions, and ‘e’ is the error term. The model for herbage yield did not include harvest form as this was not measured. Because only one observation of chemical composition for the field-cured harvest form was collected, the model for chemical composition was analyzed using a fixed effect model which included cultivar, maturity, and harvest form. The interactions were not included in the model for analysis. Significant (P < 0.05) means were separated with a PDIFF statement. Means reported are LSMEANS and standard errors are the greatest error of the means. Tendencies were declared at P > 0.05 and ≤ 0.10.
J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
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Fig. 2. Herbage yield of three peanut cultivars at two maturity stages [R2 (beginning peg) and R8 (harvest maturity)]. Error bars indicate SEM.
3. Results 3.1. Yield and chemical composition Crude protein concentration was not different (P > 0.70) among cultivars or harvest forms; however, it was greater (P < 0.0001) at R2 than at R8 (Table 1). Neutral detergent fiber, ADF, and Lignin (sa) concentrations were also similar (P > 0.11) among cultivars and harvest forms. Concentrations of aNDF, ADF, and Lignin (sa) were greater (P < 0.01) at R8 than R2 maturity stages. Water soluble carbohydrate and ADIN was not different (P > 0.07) among cultivars, maturity stage, or harvest forms. In vitro true digestibility was only affected by cultivar and was greatest (P < 0.02) for C99-R and least for York, whereas, Georgia-01R had intermediate IVTD. Undegradable intake protein concentration was also affected by cultivar and was greatest (P < 0.04) in York and least for C99-R, whereas, the UIP concentration of Georgia-01R was intermediate between the other two cultivars. There was a forage × maturity interaction for herbage yield (P < 0.03) (Fig. 2). The greatest (P < 0.01) herbage yield was for the Georgia-01R cultivar at R2. Less (P < 0.01) herbage yield was measured for C99-R at R8 and York at both R2 and R8. The herbage yield of C99-R at R2 was intermediate. Even the lower yielding cultivars and maturities produced at least 3000 kg DM/ha. 3.2. In sacco degradation kinetics 3.2.1. DM degradation kinetics The DM in the ‘A’ fraction was affected by (P < 0.001) cultivar, maturity, and harvest form (Table 2). The DM in the ‘A’ fraction was greatest (P < 0.001) at R2 stage for C99-R and Georgia-01R when fresh and York when field-cured; whereas, it was least in Georgia-01R at R2 stage when field-cured and in both Georgia-01R and York at R8 stage when fresh. The potentially degradable DM in the ‘B’ fraction was affected by maturity (P < 0.001) and harvest form (P < 0.05). Potentially degradable DM was greater (P < 0.001) for field-cured and fresh harvest forms at R2 stage than either harvest form at R8 stage. The undegradable DM in the ‘C’ fraction was affected by (P < 0.01) all three factors and was greatest (P < 0.0001) for all three cultivars at R8 maturity when field-cured. Dry matter in the ‘C’ fraction was least (P < 0.0001) in all three cultivars at R2 stage when field cured and in fresh C99-R at R2 stage. The rate of degradation of DM in the ‘B’ fraction was faster (P < 0.001) for all cultivars and both maturities at R2 stage than at R8 stage. Lag was not different (P > 0.32) among treatments. There was a three-way interaction (P < 0.05) among the treatments for the effective ruminal degradability of DM. Effective ruminal degradability was greater (P < 0.001) at R2 stage for York when field-cured, C99-R when field-cured and fresh, and Georgia01R when fresh than that of R8 stage for Georgia-01R when field-cured and fresh, C99-R when field cured, and York when field-cured. 3.2.2. aNDF degradation kinetics Neutral detergent fiber in the ‘A’ fraction was affected (P < 0.001) by cultivar, maturity, and harvest form (Table 2). The aNDF in the ‘A’ fraction was greatest (P < 0.002) at R8 stage for C99-R when fresh or field cured, Georgia-01R when fieldcured, and York when fresh; whereas, it was least at R2 stage in C99-R when field-cured, Georgia-01R when field-cured, and York when fresh. The potentially degradable aNDF in the ‘B’ fraction was affected by maturity (P < 0.001) and harvest form (P < 0.05). Potentially degradable aNDF was greater (P < 0.001) for either harvest form at R2 stage than fresh harvest form at R8 stage, and least (P < 0.001) for field harvest form at R8 stage. The undegradable aNDF in the ‘C’ fraction was affected by cultivar (P < 0.01) and maturity (P < 0.001) and was greater (P < 0.0001) for all three cultivars at R8 stage than at R2 stage. At the R8 stage the aNDF in the ‘C’ fraction was greatest (P < 0.005) in York, followed by Georgia-01R, and least (P < 0.04) for C99-R. Rate of aNDF degradation and lag prior to aNDF degradation were not different (P > 0.09) among treatments. There was a three-way interaction (P < 0.001) among the treatments for the effective ruminal degradability of aNDF. Effective ruminal
56
Table 1 Chemical characteristics and IVTD (in vitro true digestibility) of three cultivars of fresh and field-cured peanuts harvested at two maturity stages [R2 (beginning peg) and R8 (harvest maturity)]. Maturity
Type
CP (g/kg DM)
aNDF (g/kg DM)
ADF (g/kg DM)
Lignin (sa) (g/kg DM)
WSC (g/kg DM)
ADIN (g/kg N)
IVTD (g/kg)
aNDIN(g/kg DM)
UIP (g/kg DM)
C99-R
R2
Fresh Field cured Fresh Field cured
175 232 114 77
314 312 379 460
253 221 321 384
52 71 59 95
95 71 81 63
84 70 128 145
0.85 0.76 0.84 0.78
11 15 8 5
10 11 13 16
Fresh Field cured Fresh Field cured
169 187 116 79
336 314 348 480
305 248 297 425
55 57 72 100
65 37 69 49
75 76 77 166
0.80 0.83 0.80 0.44
7 11 9 5
9 11 17 17
Fresh Field cured Fresh Field cured
188 187 132 100
308 327 367 390
253 284 310 352
58 42 94 104
43 66 72 46
105 106 71 142
0.52 0.57 0.72 0.43
11 10 9 8
16 10 20 27
SE
10.1
12.3
21.2
7.2
6.3
14.9
0.06
1.7
1.6
CV M HF
ns
ns
ns
ns
ns
*
**
**
**
*
ns
ns
ns
ns ns
*
ns
ns ns ns
*
***
ns ns ns
ns
ns
R8
Georgia-01R
R2 R8
York
R2 R8
Significance
CP, crude protein; aNDF, neutral detergent fiber; ADF, acid detergent fiber; Lignin (sa), lignin determined by sulfuric acid method; WSC, water soluble carbohydrate; ADIN, acid detergent insoluble nitrogen; IVTD, in vitro true digestibility (proportion of dry matter incubated); aNDIN, neutral detergent insoluble nitrogen; UIP, undegradable intake protein; C, cultivar; M, maturity; HF, harvest form. * P < 0.05. ** P < 0.01 *** P < 0.001.
J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
Forage
Table 2 In sacco degradation kinetic parameters of three cultivars of fresh and field-cured peanuts harvested at two maturity stages [R2 (beginning peg) and R8 (harvest maturity)]. Dry matter
Neutral detergent fiber
Crude protein
Maturity
Harvest
A
B
C
kd
L
E
A
B
C
kd
L
E
A
B
C
kd
L
E
C-99R
R2
Fresh Field cured Fresh Field cured
399 380 375 342
486 493 431 431
115 127 194 277
0.136 0.187 0.105 0.089
2.29 2.93 2.79 2.31
753 764 666 586
49.3 3.2 70.7 88.4
597 681 544 416
354 316 386 495
0.181 0.144 0.079 0.088
3.37 2.36 3.04 3.03
503 502 397 352
407 490 340 274
438 467 553 508
356 44 107 218
0.173 0.219 0.113 0.115
2.22 3.05 3.01 2.11
821 866 716 627
Fresh Field cured Fresh Field cured
392 364 362 318
496 381 465 434
143 140 204 290
0.143 0.124 0.078 0.08
2.39 2.51 1.27 3.1
736 713 609 559
34.1 4.3 41.5 17.2
663 633 481 341
303 366 478 487
0.098 0.107 0.146 0.057
2.16 4.00 2.68 3.44
473 426 340 353
461 451 358 322
477 495 512 444
63 54 130 235
0.223 161 0.104 0.115
1.51 2.61 1.35 3.28
842 827 702 676
Fresh Field cured Fresh Field cured
367 384 354 317
484 495 421 386
149 121 225 297
0.1507 0.309 0.096 0.093
3.14 3.32 2.75 1.69
727 795 630 568
1.3 43.0 64.4 29.3
620 620 419 391
379 286 516 572
0.128 0.391 0.0953 0.0582
3.34 3.47 3.58 4.25
447 506 339 240
420 374 372 433
520 568 515 363.5
60 48 113 204
0.185 0.173 0.106 0.179
3.09 3.34 1.91 3.97
828 822 721 708
SE
1
9.2
9
0.035
1.1
12.3
3.7
26.6
19.8
0.09
1.16
18.6
9.6
13.7
8.8
0.029
0.92
15.9
CV M HF CV × M CV × HF M × HF CV × M × HF
***
ns
**
ns
***
ns
**
**
**
*
ns
***
***
***
***
***
***
***
***
**
***
***
***
*
***
***
***
*
ns
*
ns
***
***
***
ns ns
ns ns
ns
***
**
***
***
***
*
***
ns ns
ns
*
ns
ns ns
***
***
***
***
***
***
**
**
*
***
***
***
ns
ns
ns ns ns ns ns
*
***
ns
***
***
***
***
ns
ns ns ns ns ns
ns ns ns ns ns ns ns
ns
***
ns ns ns ns ns ns ns
***
***
ns ns ns ns ns ns ns
***
R8
Georgia-01R
R2 R8
York
R2 R8
Significance
***
ns
ns ns ns ns ns ns ns
−1
A, soluble wash fraction (g/kg DM); B, insoluble but degradable fraction (g/kg DM); C, undegradable fraction (g/kg DM); kd , fractional rate of degradation of B (h (g/kg DM); CV, cultivar; M, maturity; HF, harvest form. * P < 0.05. ** P < 0.01. *** P < 0.001.
***
ns *
ns ** **
); L, lag time (h); E, effective ruminal degradability
J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
Cultivar
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J.L. Foster et al. / Animal Feed Science and Technology 171 (2012) 52–59
degradability of aNDF was greater (P < 0.001) at R2 stage for York when field-cured, C99-R when field-cured and fresh, and Georgia-01R when fresh than that of R8 stage for Georgia-01R when field-cured and fresh, C99-R when field cured, and York when field-cured or fresh. 3.2.3. CP degradation kinetics Crude protein in the ‘A’ and ‘B’ fractions were affected by (P < 0.001) cultivar, maturity, and harvest form (Table 2). The CP in the ‘A’ fraction was greatest (P < 0.001) at R2 stage for C99-R when field cured and Georgia-01R when field-cured or fresh; whereas, it was least at R8 stage in C99-R and Georgia-01R when field-cured or fresh. Potentially degradable CP in the ‘B’ fraction was greater (P < 0.03) for York at R2 stage when field-cured or fresh and fresh C99-R at both R2 and R8 stages than field cured R8 stage Georgia-01R and York, field-cured C99-R at R2 stage, and fresh Geaorgia-01R at R2 stage. The undegradable CP in the ‘C’ fraction was affected by cultivar (P < 0.05), maturity (P < 0.001) and harvest form (P < 0.001). Undegradable CP in the ‘C’ fraction was greater (P < 0.002) for all three cultivars at R8 stage than at R2 stage. At the R8 stage the DM in the ‘C’ fraction was greatest (P < 0.001) in field-cured Georgia-01R, followed by York, and that of C99-R was intermediate. There were no differences (P < 0.08) in CP in the ‘C’ fraction among cultivars when fresh and at R8 stage; however, the CP in the ‘C’ fraction was greater (P < 0.002) in these samples than among any of the cultivars at R2 stage, which were not different (P > 0.14) among each other. Rate of CP degradation was faster (P < 0.0001) at R2 stage than at R8 stage; whereas, lag prior to CP degradation were not different (P > 0.12) among treatments. There was a three-way interaction (P < 0.01) among the treatments for the effective ruminal degradability of CP. Among all cultivars and both harvest forms, effective ruminal degradability of CP was greater (P < 0.0002) at R2 stage than at R8 stage. At R2 stage CP effective ruminal degradability was not different (P > 0.10) among cultivars or harvest forms. At R8 stage, effective ruminal degradability of CP was least (P < 0.04) in C99-R when field-cured and there was no difference (P > 0.17) among the other cultivars and harvest forms. 4. Discussion 4.1. Yield and chemical composition Ambient temperature and precipitation were below 30-year averages in September and October during the experiment (Fig. 1). However, irrigation to 30-year average precipitation was applied as needed and the plants did not exhibit drought stress. Georgia-01R likely produced greater yield at R2 stage because of taller growth that then senesced by the R8 stage. Similar to most forages, the more mature peanut (R8 stage) had lower CP and greater fiber fraction concentrations than the younger peanut (R2 stage) (Minson, 1990; Jung and Allen, 1995; Sun et al., 2010). Although the CP and fiber fractions were affected by harvest maturity, the IVTD was not. This is likely because the difference between the ADF and Lignin (sa) concentrations at R2 and R8 maturities were relatively small and the WSC concentration was not affected by maturity. The IVTD was greater and the UIP lesser for C99-R indicating that this cultivar has the greatest nutritive value. However to maximize protein efficiency the UIP should be 35% of total CP in forage based diets (Broderick, 1995). Because the UIP of C99-R is approximately 0.06 of the total CP, the protein efficiency may be expected to be lower than when York is fed because the UIP is 0.14 of the total CP (Broderick, 1995). Although the CP, aNDF, and ADF concentrations are similar among cultivars, the IVTD of York was lesser than that of either C99-R or Georgia-01R. This difference cannot be explained by the chemical characteristics evaluated for this experiment and further is required. Field curing resulted in a decline of IVTD in both C99-R or Georgia-01R peanut cultivars and was likely due to the increased time it took to dry the field cure hay versus the fresh hay. Plant respiration continued for a longer period of time, which caused a numerically greater aNDF, ADF, and Lignin (sa) concentrations and numerically lesser WSC concentration for these cultivars at R8 maturity (Rotz and Muck, 1994). There was a forage × maturity interaction for herbage yield (P < 0.03) (Fig. 2). The greatest (P < 0.01) herbage yield was for the Georgia-01R cultivar at R2. Less (P < 0.01) herbage yield was measured for C99-R at R8 and York at both R2 and R8. The herbage yield of C99-R at R2 was intermediate. Even the lower yielding cultivars and maturities produced at least 3000 kg DM/ha. 4.2. In sacco degradation kinetics Maturity had a greater effect on the degradation kinetics than harvest form or cultivar (Danley and Vetter, 1973; Hoffman et al., 1993; Buxton, 1996). The DM and CP in the ‘A’ and ‘B’ fractions and the effective ruminal degradability were greater among all cultivars and both harvest forms of the R2 maturity stage than the R8. The difference in CP degradation is likely the result of a decrease of at least 38% in CP concentration and an increase of UIP of at least 38% for each cultivar among harvest forms as these plants matured. As occurred in this experiment, the CP concentration generally decreases and structural carbohydrates generally increase as forages mature (Danley and Vetter, 1973; Buxton, 1996). As the structural carbohydrate concentrations increase, the UIP increases; therefore, the UIP is greater in more mature forages (Cassida et al., 2000). The aNDF in the ‘A’ fraction decreased with maturity, whereas aNDF in the ‘B’ and ‘C’ fractions and effective ruminal degradability increased with maturity. This is likely a function of increased aNDF concentration in more mature forage. The aNDF concentration increased by at least 19%, the ADF concentration 22%, and the Lignin (sa) concentration 25% from R2 to R8 maturity stages. Similar to most forages, the structural carbohydrate concentration increased in peanuts as they matured. The changes of CP, UIP, aNDF, ADF, and Lignin (sa) all affected the degradation kinetics of DM.
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The fractional rate of degradation of DM and CP in the ‘B’ fraction were greater for R2 maturity stage hays than R8 of both harvest forms and all cultivars. The fractional rate of degradation of aNDF in the ‘B’ fraction was not different among treatments in this experiment. This is a function of the concentrations of CP, UIP, and aNDF among the forage cultivars, maturities, and harvest forms. The lag time prior to degradation by ruminal microbes was short (average of 2.80 h) and was not different among cultivars, maturities, or harvest forms likely because of the relatively good nutritive value of all of these samples. 5. Conclusion All of these cultivars produced at least 3000 kg DM/ha and while maturity did not affect yield of C99-R or York, Georgia01R should be harvested for forage at R2 stage to maximize yield. Because the CP concentration decreased and the UIP, aNDF, ADF, and Lignin (sa) concentrations increased with maturity these cultivars of annual peanut are greater quality at the R2 maturity than at R8 and should be harvested at R2 stage to maximize quality and quantity. The greater IVTD of C99-R indicates that this cultivar is the highest nutritive value; however, the UIP is low which may decrease protein efficiency of ruminant diets containing C99-R. York had a lower IVTD than either C99-R or Georgia-01R and the IVTD of York was similar to that presented in the literature for moderate to low quality grasses. The cultivars C99-R and Georgia-01R are recommended for further feeding trials as these peanut cultivars have the most potential as a degradable intake protein source as a supplement to poor quality forage or inclusion in total mixed rations. References Association of Official Analytical Chemists, 1990. Official Methods of Analysis, 15th ed. 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