The Professional Animal Scientist 22 (2006):71–79
Evaluation of Hay Feeding Strategies on Feed Sorting Behavior of Dairy Heifers Fed Mock Lactation Diets P. C. HOFFMAN,*1 C. R. SIMSON,* and K. J. SHINNERS† *Department of Dairy Science and †Department of Biological Systems Engineering, University of Wisconsin, Madison 53706
Abstract Five different physical methods of incorporating hay into mock lactating dairy cow diets were developed to explore possible differences in nutrient intake and feed sorting behavior. Diets were fed to 80 gravid Holstein heifers (433 ± 39 kg) that were randomly assigned to 10 pens in a 5 × 5 Latin square experimental design. Experimental diets included 1) incorporation of long hay (LH) in a total mixed ration (TMR) mixer; 2) incorporation of bale cut hay (BC) in a TMR mixer; 3) incorporation of chopped hay (CH) in a TMR mixer; 4) top-dressing (TD) LH without TMR incorporation, and 5) TD BC hay without TMR incorporation. Top-dressing LH (P < 0.10) or BC hay (P < 0.02) to heifers resulted in a suppression (0.5 kg/d) of DMI as compared with heifers fed TMR diets in which hays were incorporated in the TMR. Heifers fed TD diets refused (P < 0.002) particles approximately 10 mm longer than heifers fed diets where hays were incorporated in the TMR. The greatest particle sorting occurred for particles retained on screens >6.35 mm, indicat-
1
To whom correspondence should be addressed:
[email protected]
ing that heifers preferred particles of smaller size. When a TMR mixer capable of incorporating LH is available, fully incorporating LH in the diet resulted in greater DMI and less particle sorting by heifers. In situations where a TMR mixer is incapable of incorporating LH, using BC in a TMR produced similar and acceptable results. Trial results indicate that feed intake and feed sorting behaviors are influenced by dietary hay incorporation methods. Key words: baled hay, feed sorting, particle size
Introduction Incorporating ensiled forages and grains in a total mixed ration (TMR) is a common method to feed lactating dairy cows. Similarly, it is common for dairy producers to replace ensiled forages with legume or grass hay in TMR diets. Because hay typically contains forage particles of longer length as compared with finely chopped ensiled forages, forage particles of hay are often more physically effective (Mertens, 1997). Incorporation of hay in the diet is implemented as a way to improve rumen fermentation and cow health when dairy cows are fed diets with mini-
mum NDF concentrations (NRC, 2001). Failure to provide adequate NDF or physically effective NDF in the diet has been shown to decrease chewing activity, rumen pH, and milk fat concentration (Grant et al., 1990; Beauchemin et al., 1994; LeLiboux and Peyraud, 1999) in lactating dairy cows. Metabolic diseases such as acidosis and displaced abomasum in dairy cows have also been associated with low dietary NDF or physically effective NDF (Shaver, 1997). Incorporation of forages with long particle lengths in TMR for dairy cows has disadvantages. Leonardi and Armentano (2003) demonstrated that dairy cows consistently sort against longer dietary particles in favor of consuming finer particles in the diet. In a second study (Leonardi et al., 2005), increasing geometric mean particle size (GMPS) in the diet did not result in improved rumen pH or milk fat concentration because cows preferentially sorted against long particles in favor of consuming finer particles. In the 2 aforementioned trials (Leonardi and Armentano, 2003; Leonardi et al., 2005), particle sizes of the forages were altered by chopping or feeding unprocessed baled hay to achieve variable particle size and did not consider on-farm mechanical applications to
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feeding hay in TMR. Many on-farm mechanical systems to incorporate hay in TMR are commercially available. Baled hay can be placed in vertical auger TMR mixers fit with knives on the vertical auger that reduce the GMPS of baled hay as mixing occurs. In addition, hay can be baled using a baler with cutting rotors located prior to the bale chamber. Hay is cut to a theoretical length of cut of approximately 50 mm during the baling process. The cut bale makes possible the incorporation of baled hay into TMR mixers, which are incapable of processing long hay (LH), without secondary chopping. Finally, baled hay can be incorporated into dairy cow diets by chopping hay to facilitate TMR incorporation or by simply topdressing (TD) LH and foregoing TMR incorporation. The nuances of onfarm baled hay incorporation into dairy cattle diets and their potential effects on animal performance have not been thoroughly investigated. The objective of this study was to evaluate nutrient intakes and feed sorting behavior of group-fed dairy heifers fed mock lactating cow diets employing various on-farm hay feeding strategies.
Materials and Methods Pretrial. In previous research (Leonardi and Armentano, 2003; Leonardi et al., 2005), feed sorting behavior was evaluated by feeding diets of variable GMPS to individual lactating dairy cows. These data demonstrate that preferential sorting against long particles in the diet is variable between individual cows. However, these data do not define particle sorting behavior of cows fed under group-feeding dynamics, which is integral to on-farm TMR feeding systems. Therefore, feeding hay in mock lactation diets to replicated pens of gravid Holstein heifers was chosen as the experimental design. This experimental design and method were chosen for 2 reasons. First, because the trial objective was to evaluate the effects of on-farm baled hay feeding
Hoffman et al.
strategies on nutrient intake and, in particular, feed sorting behavior under group-feeding dynamics, the experimental design was implemented to appropriately replicate the experiment and increase our statistical inference. Second, we wanted to explore whether the trial design resulted in similar feed sorting behavior as compared with previous feed sorting research with lactating dairy cows because feeding replicated pens of gravid heifers in a Latin square design is a potentially efficacious experimental method. Trial Forages. Approximately 10 ha of second-crop alfalfa at the one-quarter bloom stage of maturity was cut with a disk mower-conditioner and placed in a wide swath to facilitate field drying. Two lots of hay were baled with a Case IH LBX 331 large square baler (CNH Global, New Holland, PA) fit with 11 cutting knives in the cutting rotor. Approximately 4 ha was baled with cutting knives in the cutter rotor operational to achieve bale cut hay (BC) with a theoretical length of cut of 50 mm. The remaining 6 ha of hay was baled with the same baling equipment, but the cutting knives and the cutting rotor were disengaged to produce LH. At the time of baling, hays were treated with 0.6 g of propionic acid/kg to aid preservation. One-half of LH was then reprocessed by chopping in a forage harvester (CNH Global) fit with 12 knives to a theoretical length of cut of 1 cm to produce chopped hay (CH). Hays were stored inside storage facilities for 6 mo prior to the feeding trial. Feeding Trial. Eighty gravid Holstein heifers (433 ± 39 kg) where randomly assigned to 10 pens containing 8 heifers per pen. Heifers were housed in 4.5- × 9.0-m pens with 30.5 m2 of resting area. The pens were bedded with sawdust, and heifers had access to 0.5 m of linear bunk space per heifer. Five pens of heifers were randomly assigned to 1 of 2 squares and treatment sequences in a 5 × 5 Latin square experimental design. The Latin square design con-
sisted of five 7-d periods with 5 d of adaptation followed by 2 d of data collection. Seven-day periods were chosen for the study because previous work (Tolkamp and Kyriasakis, 1997; Tolkanp et al., 1998; Leonardi and Armentano, 2003) demonstrated that preferential diet selection and intake variation occurs within 3 d after introduction of a new dietary regimen. All animal care and experimental procedures were approved by the Research Animal Resource Committee at the University of Wisconsin-Madison. Five experimental diets were developed to evaluate nutrient intake and feed sorting behavior of baled hay feeding systems. For all experimental diets, dry hay was fed at 20.0% of the dietary DM; only the physical method of hay inclusion in the diet varied. Other ingredients and nutrient compositions of the experimental diets were formulated to be identical. Experimental diets included 1) dietary incorporation of LH using a vertical TMR mixer (Supreme International, Wetaskiwin, AL), 2) dietary incorporation of BC using a vertical TMR mixer, 3) dietary incorporation of CH using a vertical TMR mixer, 4) TD LH without incorporation into the TMR, and 5) TD BC without incorporation into the TMR. A completely balanced set of treatments would have included a treatment including TD CH, but it was not utilized because TD CH in combination with feeding a partial TMR was deemed to have impractical field applications. For experimental diets that involved TD LH or BC, the remaining dietary ingredients were fed as a TMR mixed with the same vertical mixer used for the other treatments; therefore, only hay portions of experimental diets were TD. The amount of hay fed in diets that included TD LH or BC was kept to the same proportion as hay fed in the diets that included incorporation into TMR. Remaining ingredients in experimental diets included corn silage, alfalfa silage, and a grain mix at 25, 15, and 40% of the dietary DM, respectively. Nutrient composition and GMPS of feeds uti-
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Feeding Baled Hay
TABLE 1. Nutrient composition and geometric mean particle size of feeds utilized in experimental diets. Item
Alfalfa hay Corn silage Alfalfa silage Grain mixa
Nutrient DM CP NDF Ash NFCb
(% of DM) 83.2 16.6 44.8 6.8 25.9
35.4 8.0 42.4 5.8 37.8
Geometric mean particle size Long hay Bale cut hay Chopped hay
65.4 15.6 44.7 7.8 25.9
83.2 17.2 10.0 2.2 64.6
8.0 — —
1.9 — —
(mm) 46.7 20.0 6.1
10.9 — —
a Grain mix consisted of 76.8% ground shelled corn and 23.2% soybean oil meal on a DM basis. b NFC = nonfiber carbohydrate.
lized in experimental diets are presented in Table 1. The experimental diets were formulated to simulate a commonly fed (NRC, 2001) lactating cow diet containing approximately 30% NDF and 42% nonfiber carbohydrate (NFC) providing a robust set of feed ingredients with varying GMPS for potential preferential selection by the heifers. Protein contents of the experimental diets were maintained near 14.5% of DM to meet the needs of gravid Holstein heifers (NRC, 2001). Experimental diets were fed once daily at 0900 h and were fed to achieve 10% orts on a DM basis. Heifers were weighed at the beginning and end of the experiment, and a mean BW was calculated for the experiment. Sampling and Laboratory Analysis. Prior to feeding, triplicate samples of LH, BC, CH, corn silage, alfalfa silage, and the grain mix were obtained, and DM was determined by oven-drying for 48 h at 55°C and grinding through a Willey mill (Arthur A. Thomas Co., Philadelphia, PA) fit with a 1-mm screen. Residual DM determination was conducted on the dried ground samples by drying at 105°C for 2 h. Feed ingredients were evaluated for CP and ash by AOAC (1990) procedures, and NDF was evaluated by the procedures of
Goering and Van Soest (1970) with modifications by Mertens (1992). The NFC content of feed ingredients was calculated by the equation [100 − (CP + ash + NDF + 4.0). The factor of 4.0 was applied to represent the sum of fat and NDF CP (NRC, 2001), which are additional noncarbohydrate nutrients. The ingredient compositions of experimental diets were identical. Fat and NDF CP contents of feeds used in the experimental diets were typically low (NRC, 2001); therefore, no laboratory analyses of these nutrients were made. Particle size distribution of feed ingredients was determined using the Wisconsin particle size separator according to ASAE S424.2 protocol (ANSI, 1998); and GMPS was calculated using the actual mean length of particles remaining on the top screen. The particle separator just described is mechanically operated and has 5 square-hole screens with nominal openings of 19.1, 12.7, 6.4, 4.0, and 1.2 mm and a bottom pan. During each period, the amount fed and orts for each experimental diet for each pen of heifers were weighed and recorded. Diet and ort weights on d 1 through 5 of each period were used daily to achieve 10.0% orts. On d 6 and 7 of each period, a duplicate sample of TMR was taken for the diets in which hay was incor-
porated into the TMR, and duplicate ort samples were taken on d 7 and 1 of the next period, prior to feeding. For the diets involving TD, an individual sample of the partial TMR and hay (LH or BC) was taken, and a duplicate subsample of the diet proportional to the amounts of partial TMR and baled hay was created. Orts for heifers fed the TD diets were sampled as defined for the experimental TMR diets. Duplicate diet and ort samples for each experimental diet for each period were evaluated for DM, CP, NDF, and ash by previously defined procedures. Particle separations were made on the remaining set of diet and ort samples for each experimental diet for each period with the fraction of material remaining on each screen and pan recorded. The GMPS of diet and ort samples were also made by previously defined procedures with one exception. After completing the first ort particle separations, we observed heifers to be preferentially selecting against consumption of corn cobs within corn silage (unprocessed), which were primarily retained on the 19.1-mm (top) screen of the particle separator. As a result, we hand-separated corn cobs from other long forage particles remaining on the top screen of the particle separator and recorded the fraction remaining for both. The mean length of other long forage remaining on the top screen and a fixed 30-mm length for corn cobs remaining on the top screen was used in the GMPS calculation. Calculations and Statistics. Nutrient intakes were calculated as the sum difference between the amount of nutrient offered and the amount of nutrient refused. Possible preferential selection of grain over forage by heifers fed experimental diets was evaluated by calculating fed and consumed ratios of grams of NDF:grams of NFC and comparing the ratio fed to the ratio consumed. Particle sorting was calculated and expressed as the percentage of potential intake (as fed) of particles on each screen actually consumed on an as-fed basis. Per-
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centages 100, indicating that heifers ate more particles from the screen than actually fed. A particle sorting percentage >100 is a nuance of data collection, as some small feed particles are lost posterior or otherwise to the bunk area and cannot be perfectly accounted for. The overall effects of this nuance in data collection were minimal. The experimental unit was pen replicate, not individual heifer. Treatment means were calculated and are presented on an individual heifer basis for ease of readership. Nutrient intake, indexes, particle sorting percentages, and GMPS data were analyzed using GLM procedures of SAS (SAS Inst., Inc., Cary, NC) with the model Yijkl = µ + Si + Pj(S)i + Tk + PDl + Tk × Si + eijkl where µ = overall mean, Si = random effect of square i, PjSi = random effect of pen j nested within square i, Tk = random effect of treatment k, PDl = random effect of period l, Tk × Si = random effect of the interaction of treatment k and square i, and eijkl = residual. We observed some treatment by square interactions for nutrient refusal data, but upon examination, data did not represent a logical biological event. Therefore, inferences pertaining to treatment by square interactions are not presented. Treatment comparisons were made by a preplanned contrast scheme and were declared significant (P < 0.10) only when a significant F-test was determined in the overall model. The preplanned contrast scheme was developed to evaluate applied application of the data with reason and approach as follows. Because implications of treatment effects in a 5-treatment experiment can be unwieldy, data comparisons were divided into 2 practical scenarios: scenario A and scenario B. For scenario A, where a TMR mixer capable of mechanically incorporat-
ing LH into the diet is available, alternatives to feed hay would include placing LH or BC in the mixer or foregoing incorporation of baled hay in the mixer and TD LH or BC hay. When such a mixer is available, placing CH in the mixer was assumed to be illogical because a labor-intensive secondary mechanical processing of hay is integral to CH but not required by the mixer. A preplanned contrast for scenario A included identifying the TMR with incorporated LH as a pragmatic control and comparing TMR with incorporated BC or LH to the control. For Scenario B, where a TMR mixer incapable of mechanically incorporating LH into the diet is available, the alternatives to feeding hay would include placing BC or CH, of which particle size has been reduced to facilitate dietary hay incorporation, into the TMR mixer or foregoing full TMR incorporation of hay and TD LH or BC. Preplanned comparisons for scenario B included identifying TD LH as a pragmatic control and comparing a TMR incorporating BC or CH and a diet in which BC is TD to the control. The preplanned contrast creates some redundancy in data presentation but provides logical inferences to field application of the data. The reader is cautioned that we used the same TMR mixer as just described to incorporate all hays into the TMR diets and to mix the partial TMR portions of TD diets. Therefore, the preplanned contrasts defined represented only a basis for interpreting on-farm application. Our intent with the study was to create a robust set of particle sizes and particle size distributions in diets containing hay to study animal response. Because numerous TMR mixers are commercially available, evaluating all possible scenarios of mixers capable or incapable of incorporating hay into a TMR diet as they pertain to dietary particle size, particle size distribution, and particle sorting behavior by animals was infeasible.
Results and Discussion Ingredient and nutrient compositions of experimental diets are pre-
sented in Table 2. Because the only variable in the experimental feeding regimen was the physical method by which hay was fed to heifers, nutrient compositions of experimental diets were planned and observed to be nearly identical. The observed NDF, NFC, and CP contents of all experimental diets fed to heifers were near the preplanned objectives of 31.0, 42.5 and 14.5% of DM respectively, simulating a lactating cow diet (NRC, 2001). The effects of hay feeding strategies on nutrient intakes of gravid Holstein heifers fed mock lactating cow diets are presented in Table 3. Heifers fed diets involving TD of LH and BC were offered less DM (P < 0.10) compared with heifers fed TMR in which LH, BC, or CH was incorporated. This observation is an artifact of the experimental protocol. All experimental diets were overfed to heifers to achieve 10% orts, which was not different (P = 0.82) among treatments (Table 3). Exacting 10% orts was used in the experimental protocol to ensure heifers had equal potential for particle selection among the experimental diets. Because heifers fed diets that included TD LH or BC actually consumed 0.5 kg of DM/d less (P < 0.001) than heifers fed diets in which LH, BC, or CH was incorporated into the TMR, 0.5 kg less DM was offered to heifers fed the 2 diets with TD LH and BC only to maintain the 10.0% ort criteria. The observed reduction in DMI for heifers fed diets involving TD LH or BC was likely related to factors associated with GMPS of the diets. The GMPS of TD diets fed to heifers were approximately 1.5 mm longer compared with the GMPS of the TMR diets (Table 4). In lactating dairy cows, Leonardi et al. (2005) observed a 0.9-kg/d reduction in DMI for each 1-mm increase in dietary GMPS. The observations of Leonardi et al. (2005) suggest greater DMI suppression (0.9 kg/d) associated with increasing GMPS of the diet as compared with our observations (0.5 kg/ d). However, their observations were made on lactating dairy cows with
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TABLE 2. Ingredient and nutrient composition of experimental diets. Experimental dieta Item
TMR-LH
TMR-BC
20.0 — — 25.0 15.0 40.0
— 20.0 — 25.0 15.0 40.0
59.2 14.7 30.3 7.2 41.8 5.8
58.8 14.7 31.8 6.4 41.0 5.7
Ingredient compositionc Long alfalfa hay Bale cut alfalfa hay Chopped alfalfa hay Corn silage Alfalfa silage Grain mixd Nutrient compositione DM CP NDF Ash NFC Geometric mean particle size, mm
TD-LH
TD-BC
SEMb
— — 20.0 25.0 15.0 40.0
20.0 — — 25.0 15.0 40.0
— 20.0 — 25.0 15.0 40.0
— — — — — —
58.6 14.6 31.2 5.7 42.5 4.8
58.7 14.6 32.4 5.3 43.0 7.4
59.0 14.8 0.6 5.3 42.5 6.4
0.2 0.1
TMR-CH (% of DM)
0.2 0.6 0.2
a
TMR-LH = total mixed ration containing long hay; TMR-BC = total mixed ration containing bale cut hay; TMR-CH = total mixed ration containing chopped hay; TD-LH = partial mixed ration with top-dressed long hay; TD-BC = partial mixed ration with topdressed bale cut hay. b SEM = standard error of measurement. Includes sampling and laboratory error. c Mineral mix contained 4.3% Zn; 4.3% Mn; 7,800 mg of Cu/kg; 830 mg of I/kg; 320 mg of Co/kg; 220 mg of Se/kg; 5,500,000 IU of vitamin A/kg; 1,375, 000 IU of vitamin D/kg; 18,700 IU of vitamin E/kg. The mix was fed at the rate of 57 g/d per heifer. d Grain mix consisted of 76.8% ground shelled corn and 23.2% soybean oil meal on a DM basis. e Values listed are percentages of DM unless otherwise defined; NFC = nonfiber carbohydrate.
DMI >22.0 kg/d, which were approximately double the DMI of gravid Holstein heifers observed our study (11.0 kg/d). Expressed as a percentage of DMI, DMI was reduced approximately 5.0 percentage units both in our study and in the study of Leonardi et al. (2005). Despite commonality between the observations of Leonardi et al. (2005) and our study in relation to diet GMPS and DMI, other research regarding DMI and GMPS has been contradictory. Other researchers (Fisher et al., 1994; Mooney and Allen, 1997) have also reported a reduction in DMI when lactating dairy cows were fed diets containing forages with increasing GMPS, but earlier studies (Shaver et al., 1986; Woodford et al., 1986) have conversely reported no effect of GMPS on DMI in lactating dairy cows. The amount of DM, CP, NDF, and NFC refused by heifers was not different among experimental diets (Table 3). As previously described, DM con-
sumed (P < 0.03) by heifers and DMI as a percentage of BW (P < 0.03) were less for heifers fed diets involving TD LH or BC. As a result of decreased DMI, heifers consumed (P < 0.10) lower amounts of CP per day when fed diets that involved TD LH or BC compared with heifers fed diets in which LH, BC, or CH was incorporated into the TMR. No differences in NDF intake, NDF intake expressed as a percentage of BW, or NFC intake were observed between heifers fed experimental diets. Similarly, there were no differences in NDF:NFC (g of NDF:g of NFC) intake ratios for heifers fed experimental diets. These results suggest that no experimental hay feeding strategy resulted in a deleterious alteration of dietary nutrient intake when heifers were group fed, other than a modest depression in DMI with TD hay. Within our preplanned contrast to evaluate practical application of these data, the following conclusions can be made regard-
ing nutrient intake. When a TMR mixer capable of incorporating LH into the diet is available (scenario A) replacing LH with BC hay in a TMR diet had no effect on DMI. Foregoing incorporation of LH into the TMR mixer and TD LH (P < 0.05) or BC (P < 0.02) to potentially enhance GMPS of the diet resulted in a moderate decrease in DMI. The effects of hay feeding strategies on dietary particle sorting by gravid Holstein heifers are presented in Table 4. In general, group-fed gravid Holstein heifers exhibited similar dietary particle preferences as lactating dairy cows (Leonardi and Armentano, 2003; Leonardi et al., 2005). This inference is supported by 2 observations as they pertain to preferential particle selection. First, group-fed gravid heifers in our study preferentially selected to consume particles of fine size over particles of coarse size. Heifers consumed between 61.3 and 86.4% of particles retained (not in-
11.3 2.62 1.68 3.31 0.77 4.85 0.69
Nutrient intake DM DM, % of BW CP NDF NDF, % of BW NFC NDF:NFC, g of NDF:g of NFC
TMR-CH d
11.3 2.63 1.68 3.45 0.80 4.81 0.73
1.09 9.7 0.14 0.45 0.38
12.4 1.81 3.99 5.17 0.76
11.4 2.66 1.68 2.49 0.80 4.94 0.72
1.04 9.7 0.12 0.42 0.37
12.5 1.81 3.90 5.31 0.74
(kg/d per heifer)
TMR-BC
10.9 2.52 1.63 3.27 0.75 4.81 0.69
0.13 9.7 0.15 0.48 0.38
12.0 1.72 3.72 5.17 0.73
TD-LH
10.8 2.50 1.63 3.22 0.75 4.72 0.69
1.18 10.2 0.15 0.51 0.38
11.9 1.77 3.76 5.08 0.74
TD-BC
0.1 0.03 0.05 0.09 0.02 0.09 0.02
0.09 0.8 0.01 0.04 0.04
0.2 0.05 0.09 0.09 0.02
SE
0.001 0.001 0.07 — — — —
— — — — —
0.06 — — — —
TRT
— — — — — — —
— — — — —
— — — — —
TMR-LH vs. TMR-BC
0.03 0.03 0.10 — — — —
— — — — —
0.05 — — — —
TMR-LH vs. TD-LH
0.004 0.01 0.08 — — — —
— — — — —
0.02 — — — —
TMR-LH vs. TD-BC
— — — — — — —
— — — — —
— — — — —
TD-LH vs. TD-BC
0.004 0.004 0.04 0.05 — — —
— — — — —
0.07 — — — —
TD-LH vs. TMR-CH
0.02 0.02 0.07 0.07 — — —
— — — — —
0.10 — — — —
TD-LH vs. TMR-BC
TRT contrast: scenario B
a
TRT = treatment; only P < 0.10 values are listed. Scenario A = total mixed ration mixer capable of incorporating long hay into the diet; scenario B = total mixed ration mixer incapable of incorporating long hay into the diet. b TMR-LH = total mixed ration containing long hay; TMR-BC = total mixed ration containing bale cut hay; TMR-CH = total mixed ration containing chopped hay; TD-LH = partial mixed ration with top-dressed long hay; TD-BC = partial mixed ration with top-dressed bale cut hay. c NFC = nonfiber carbohydrate. d Unless otherwise listed.
1.22 10.6 0.15 0.48 0.42
12.5 1.81 3.81 5.26 0.73
TMR-LH
Nutrient refusal DM DM, % of DM offered CP NDF NFC
DM CP NDF NFC NDF:NFC, g of NDF:g of NFC
Nutrient offered
Itemc
Experimental dietb
TRT contrast: scenario A
Effect (P
