Abstract
Forage neutral detergent fiber (NDF) content and particle size are important factors that affect rumen function. The aim of the current study was to evaluate the effects on rumen health, NDF digestibility, and animal performance of pelleting a forage-based diet. Eight Holstein heifers (age 336 ± 30 d, body weight 346 ± 35 kg) were randomly assigned to a repeated crossover design. Animals were housed in tie-stalls and fed for ad libitum intake. The study included 4 periods of 3 wk, the first 2 wk for adaptation to the diet and the last wk for data collection. Diets had the same ingredients but had a different physical form: total mixed ration (TMR) and pellet (diameter = 8 mm). The physically effective NDF (peNDF) differed between the 2 treatments (39.8 and 11.8% of NDF in the TMR and pellet diets, respectively). During the trial, dry matter intake (DMI), water intake, rumination time, rumen temperature, and pH were evaluated daily. Fecal samples were collected in wk 3 of each period to determine total-tract digestibility of the potential digestible (pd)NDF. Average daily gain and feed conversion ratio were calculated at the end of each period. With the pellet diet, DMI, DMI/body weight, and water consumption were higher. We observed no significant difference in average daily gain or feed conversion ratio. Rumination time was lower for the pellet diet than for the TMR diet (241 vs. 507 min/d, respectively). Diet had no effect on rumen temperature or rumen pH. The total-tract digestibility of the pdNDF was greater with the TMR diet than with the pellet diet (90.25 vs. 86.82% pdNDF, respectively). The results of the current study suggest that a complete-feed pellet diet was well accepted by the animals, as demonstrated by higher DMI. Rumination time was reduced with the pellet diet, but rumen pH was not different. The pdNDF digestibility was high for both diets, but significantly higher for the TMR diet. Given that animal performance was similar between the 2 diets, although they differed with respect to DMI and fiber digestion, we hypothesize that the 2 diets had different retention times, related to their physical form. A complete-feed pellet diet formulated to provide a sufficient level of NDF from forages could be fed to growing ruminants without apparent negative effects on rumen health and animal productivity, at least for a short period. More research over a longer growing period is needed before recommending this feeding strategy for growing heifers.
Key words
Introduction
Fiber particle size and NDF content are important factors influencing rumen health (
Allen, 1997
; Krause et al., 2002
; Kononoff et al., 2003
). This has led to a new concept introduced by Mertens (2001)
who estimated that physically effective NDF (peNDF) was the product of NDF concentration and physical effectiveness factor. The latter is the percentage of particles retained on a 1.18-mm sieve, considered highly resistant to passage out of the rumen (Poppi et al., 1985
).Fiber particle size influences chewing time and saliva secretion, affecting ruminal pH. Particle size might also affect the retention time in the rumen, and the extent of rumen fermentation and fiber degradation (
Kaske et al., 1992
; Teimouri Yansari et al., 2004
; Kammes and Allen, 2012
).Reducing fiber particle size in feeds has been used as a way to increase DMI. Several studies have demonstrated the difference between 2 or more feed chop lengths for animal performance (
Yang and Beauchemin, 2006
, Yang, et al., 2009
; Kammes and Allen, 2012
), but few trials have focused on pelleting as a strategy to achieve this effect (McCroskey et al., 1960
; Cullison, 1961
; Burt, 1966
). Using this method, controlled amounts of pressure and heat are applied to the feed aggregate to increase its density (Mani et al., 2006
). Pelleting offers many technical advantages, including improved stability (owing to very low moisture content), and easier handling, storage, and transportation.Because pelleting reduces fiber particle size, it might promote an increased rate of passage out of the rumen, and a subsequent decrease in fiber digestibility (
Van Soest, 1994
). Conversely, reduced particle size might increase the surface area available for bacterial attachment (Miron et al., 2001
), thereby increasing fiber digestibility. Reduction of fiber particle size could also affect rumen fermentation and promote the development of SARA (Khafipour et al., 2009
).The objective of this study was to evaluate the effects of a complete pelleted diet, formulated for growing heifers, on eating behavior, rumen fermentation, fiber digestibility, and animal performance. The hypothesis was that peNDF is not the only factor involved in maintaining a healthy rumen, and that a diet high in NDF content can overcome the risks of low pH due to a lack of coarse forage.
Materials and Methods
Animals and Treatments
The experimental procedures were approved by the scientific ethical committee for animal experimentation at Bologna University. Eight Holstein heifers were used in a repeated crossover design. The duration of the study was 12 wk, made up of 4 periods of 3 wk. Heifers were adapted to a diet during the first 2 wk of each period, and samples and data were collected during the last wk (experimental wk). The heifers had similar age (336 ± 30 d) and BW (346 ± 35 kg) at the beginning of the experiment, and they were divided into 2 homogeneous groups.
Diet composition was the same for both treatments, but different in physical form (Table 1 and 2). Diet 1 was prepared as a TMR with a horizontal auger, trailer-type TMR feed-mixer (Zago 13 m3; Zago srl, Padua, Italy). Diet 2 was produced as a complete-feed pellet diet, with the forages (grass hay and barley straw) chopped at 12 mm theoretical length and then incorporated with the other ingredients (corn meal, sunflower meal, NaCl), mixed, and pelleted (8 mm diameter).
Table 1Ingredients and chemical composition of pellet and TMR diets fed to heifers for ad libitum intake; the diets were formulated to be similar in chemical composition but different in physical form (evaluated as physical effectiveness factor and physically effective NDF)
| Item | Diet | SEM | |
|---|---|---|---|
| Pellet | TMR | ||
| Ingredients, % as fed | |||
| Grass hay | 41.8 | 41.8 | — |
| Barley straw | 27.4 | 27.4 | — |
| Corn grain | 16.4 | 16.4 | — |
| Sunflower meal | 13.7 | 13.7 | — |
| Salt (NaCl) | 0.7 | 0.7 | — |
| Chemical composition, % of DM | |||
| DM, % as fed | 92.0 | 88.0 | 1.02 |
| CP | 8.7 | 9.0 | 0.36 |
| Ash | 9.6 | 7.9 | 0.38 |
| aNDFom | 58.8 | 60.2 | 0.66 |
| ADF | 40.7 | 41.4 | 0.80 |
| ADL | 8.1 | 8.4 | 0.42 |
| 24-h IVNDFD | 45.3 | 46.2 | 1.50 |
| 240-h IVNDFD | 78.4 | 77.3 | 0.73 |
| uNDF240 | 12.4 | 14.1 | 0.61 |
| Starch | 15.7 | 15.6 | 1.07 |
1 aNDFom = amylase- and sodium sulfite–treated NDF, corrected for ash residue; IVNDFD = in vitro NDF digestibility; uNDF240 = unavailable NDF estimated via 240-h in vitro fermentation.
Table 2Physical characteristics and particle size distribution of pellet and TMR diets fed to heifers for ad libitum intake; the diets were formulated to be similar in chemical composition but different in physical form (evaluated as physical effectiveness factor and physically effective NDF)
| Item | Diet | SEM | P-value | |
|---|---|---|---|---|
| Pellet | TMR | |||
| Particle size distribution, % | ||||
| 6.70 mm | 0 | 4.86 | 0.28 | <0.01 |
| 4.75 mm | 0 | 8.95 | 0.58 | <0.01 |
| 3.35 mm | 1.19 | 11.78 | 0.39 | <0.01 |
| 2.36 mm | 4.29 | 12.15 | 0.28 | <0.01 |
| 1.18 mm | 14.62 | 28.36 | 0.34 | <0.01 |
| 0.15 mm | 60.66 | 31.11 | 0.94 | <0.01 |
| Pan | 19.23 | 2.77 | 0.27 | <0.01 |
| Physical effectiveness factor | 20.1 | 66.1 | 5.90 | <0.01 |
| peNDF, % of DM | 11.8 | 39.8 | 3.58 | <0.01 |
1 Particle size was measured using the Tyler Ro-Tap (W. S. Tyler, Mentor, OH).
2 Physical effectiveness factor was determined as the proportion of fiber retained by the sieve with the 1.18-mm pore size.
3 peNDF = physically effective NDF, measured as the NDF content of the forages (DM basis) multiplied by the physical effectiveness factor.
Data and Sample Collection
Throughout the experiment, heifers were housed in tie-stalls bedded with sawdust, and fed their respective diets once daily in the morning (0830 h). The amount of feed offered and the refusals (orts) were weighed daily for each heifer. Feed was given ad libitum based on orts quantity (10% of the DMI of the day before). Feed samples and orts were collected twice a week for analysis of chemical and physical composition. Daily water consumption was also recorded.
Rumination time was recorded each day for each heifer using an acoustic sensor collar (RuminAct, SCR Heatime, Netanya, Israel).
Reticulorumen pH and temperature values were monitored continuously via an indwelling pH and temperature sensor (smaXtech Animal Care, Graz, Austria) instilled in the reticulorumen region of the stomach. Data were transmitted to an external receiver via Wi-Fi every 10 min.
Heifers were weighed at the beginning of the study and at the end of each 3-wk period. Fecal samples were collected every 6 h at d 5 (starting at 0000 h) and d 6 (starting at 0300 h) of the experimental wk, so that 8 samples were taken for each heifer in each period, representing every 3 h of a 24-h period to account for diurnal variation. In each period, fecal samples belonging to the same heifer were composited and then analyzed for nutrient composition.
Sample Analysis and Calculations
Feed and fecal samples were dried in a forced-air drying oven (M700-VF; MPM Instruments, Bernareggio, Italy) at 65°C for 48 h to determine DM content. The particle-size distribution of the dried diet was determined using a sieve-type shaker (Ro-Tap; WS Tyler, Mentor, OH), consisting of 6 sieves with apertures of 6.70, 4.75, 3.35, 2.36, 1.18, and 0.15 mm, plus a bottom pan. The fraction of DM retained on the 1.18-mm screen or larger was used to calculate the physical effectiveness factor of the diets.
For analysis, dried diets, individual feed ingredients, and fecal samples were each ground separately in a Cyclone mill (1-mm screen; model SM100; Resch GmbH, Haan, Germany). Feed samples were analyzed for ash, determined after 4 h combustion at 550°C in a muffle furnace (Vulcan 3–550; Dentsply Neytech, Burlington, NJ); amylase- and sodium sulfite–treated NDF, corrected for ash residue (aNDFom), in accordance with Mertens (2002); ADF and ADL (
AOAC, 1990
; method 973.18); and CP (AOAC, 1990
; methods 976.06 and 984.13).In vitro NDF digestibility (IVNDFD) at 24 h and 240 h was determined using the Tilley and Terry modified technique (
Tilley and Terry, 1963
; Robertson and Van Soest, 1981
). Rumen fluid was collected via a rumen cannula from 2 lactating cows fed a hay-based diet (milk production, 33.2 ± 1.7 kg/d; DIM, 251 ± 2), mixed, and placed in a thermally controlled bottle (Pyrex; SciLabware, Staffordshire, UK). Rumen contents were filtered through 4 layers of cheesecloth under constant O2-free CO2. We added 10 mL of rumen fluid to 150-mL Erlenmeyer flasks that had been placed in a heated (39.3°C) water bath under CO2-positive pressure to ensure anaerobiosis. Then, 0.5 g of a ground sample was weighed into each flask before the addition of 40 mL of buffer, as described by Goering and Van Soest (1970)
. Each sample was analyzed 3 times, in 2 separate in vitro incubations. The sample preparation, donor cows, and diets were the same for both assays. At the end of fermentation, the contents of each flask were analyzed to determine the aNDFom content of the residue, and filtered through crucibles (40-μm porosity) with microfiber glass filters. Residues were then treated following the procedure described by Goering and Van Soest (1970)
, after a 3 h drying in a forced-air oven (105°C), and the hot weight of the crucibles was recorded. Ash correction was made after the residue was incinerated at 495°C for 3 h, followed by a second crucible hot weight.Digestibility was then calculated as described in equation [1]:
where aNDFomr was the residual aNDFom, aNDFomb was the blank correction, and aNDFomi represented the initial NDF. All described terms were expressed in grams. The unavailable NDF fraction, uNDF240, was determined after 240 h of in vitro fermentation, and was calculated as expressed in equation [2]:
where IVNDFD240h was in vitro NDF digestibility at 240 h and aNDFom was the aNDFom content of the sample, on a DM basis. Potentially digestible (pd)NDF was calculated as the difference between aNDFom and uNDF240, on a DM basis. The total-tract digestibility of the pdNDF, TTdpdNDF, was then calculated as described in equation [3]:
where uNDF240 diet/feces was the ratio of dietary and fecal uNDF240, and pdNDF feces/diet represented the ratio of fecal and dietary pdNDF.
[1]
where aNDFomr was the residual aNDFom, aNDFomb was the blank correction, and aNDFomi represented the initial NDF. All described terms were expressed in grams. The unavailable NDF fraction, uNDF240, was determined after 240 h of in vitro fermentation, and was calculated as expressed in equation [2]:
[2]
where IVNDFD240h was in vitro NDF digestibility at 240 h and aNDFom was the aNDFom content of the sample, on a DM basis. Potentially digestible (pd)NDF was calculated as the difference between aNDFom and uNDF240, on a DM basis. The total-tract digestibility of the pdNDF, TTdpdNDF, was then calculated as described in equation [3]:
[3]
where uNDF240 diet/feces was the ratio of dietary and fecal uNDF240, and pdNDF feces/diet represented the ratio of fecal and dietary pdNDF.
Length of fermentation was based on previous studies indicating 240 h as the maximum extent of fiber digestion in an anaerobic environment in vitro (
Fox et al., 2004
; Raffrenato and Van Amburgh, 2011
; Palmonari et al., 2014
, Palmonari et al., 2016
). For these fermentations, both rumen fluid and buffer were re-inoculated after 120 h to preserve microbial activity during the entire process, as described by Palmonari et al., 2014
. A final volume of 100 mL was treated to determine aNDFom as described above.Rumination-time data (rumination/DMI, rumination time/aNDFom intake, rumination time/forage-aNDFom intake, rumination/peNDF intake) were used to calculate average daily rumination time in each period.
Rumen pH data were used to evaluate mean pH, area under the curve (the area between the observed pH and a line drawn at pH 5.8 and 5.5), and time (min) under pH 5.8 and 5.5. A rumen pH of 5.8 was chosen as the threshold for initial SARA (starting fibrolytic bacteria depression), and 5.5 for actual SARA. The duration (min/d) and total area (pH × min, area under the curve) that pH was below each SARA threshold were calculated to evaluate the severity of rumen acidosis. Area under the curve was calculated by adding the absolute value of negative deviations in pH from 5.5 or 5.8 for each 10-min interval (
Dohme et al., 2008
).We used BW to calculate ADG using the formula reported below:
This calculation was made at the end of each of the 4 periods.
This calculation was made at the end of each of the 4 periods.
Feed efficiency was computed as feed consumption adjusted for differences in gain (feed conversion ratio).
Statistical Analysis
Data recorded in wk 3 of each period were analyzed using JMP-12 (SAS Institute Inc., Cary NC). We analyzed DMI, water intake, rumination time, rumen pH and temperature, and NDF digestibility using a mixed-effects model for repeated measures:
where Y is the dependent variable; μ is the overall mean; T is the fixed effect of treatment (i = 1, 2); P is the fixed effect of period (j = 1, 2); D is the fixed effect of day (k = 1, …, 7); H is the random effect of heifers (l = 1, …, 8); and e is the residual error. As we found no effects for period, day, treatment × period, treatment × day and heifer, we reported only the treatment effect in the tables. We analyzed ADG and feed conversion ratio using a post hoc Tukey’s adjustment.
where Y is the dependent variable; μ is the overall mean; T is the fixed effect of treatment (i = 1, 2); P is the fixed effect of period (j = 1, 2); D is the fixed effect of day (k = 1, …, 7); H is the random effect of heifers (l = 1, …, 8); and e is the residual error. As we found no effects for period, day, treatment × period, treatment × day and heifer, we reported only the treatment effect in the tables. We analyzed ADG and feed conversion ratio using a post hoc Tukey’s adjustment.
Data were considered significant if P < 0.01.
Results and Discussion
Diet Characteristics, Fiber Particle Size, and Intake
The 2 diets used in this trial were similar in chemical composition (Table 1). The CP (% of DM) content was lower than that suggested by
NRC (2001)
for 300 kg heifers. However, diets in the present study were formulated using the Cornell Net Carbohydrate and Protein System (Higgs et al., 2015
; Van Amburgh et al., 2015
), in which MP and ME requirements were covered (656.4 g/d and 16.1 Mcal/d, respectively, with a DMI of 8.4 kg/d). The 2 diets had different distributions of fiber particles (Table 2). The number of particles retained by a 1.18-mm screen was greater in the TMR diet than in the pelleted diet (66.12% and 20.12%, respectively). We used the threshold of 1.18 mm to distinguish particles that were highly resistant to passage and consequently were able to stimulate rumination (Cardoza, 1985
; Poppi et al., 1985
; Mertens (2001)
). The peNDF was 39.78 and 11.82% of DM in the TMR and pellet diets, respectively. Measurement of peNDF is important to determine the size of particles retained in the rumen. The minimum peNDF recommendation is 21% of the ration DM (Mertens (2001)
), based on a previous study in which approximately 19.7% peNDF was needed to maintain the milk fat percentage in Holstein cows at 3.4%, and 22.3% peNDF was needed to maintain an average rumen pH of 6.0 (Mertens, 1997
). The values of peNDF recorded in the TMR diet in the present study were more than adequate to guarantee good chewing activity, saliva production, and rumen health. In contrast, the pellet diet was created intentionally to have a low peNDF (11.82%) compared with recommendations. Because the study involved only pre-primiparous growing animals, observations were limited primarily to rumination time and rumen pH.A treatment effect was noted for DMI (Table 3). Differences were observed in DMI at 3 h after feeding (2.70 vs. 3.25 kg in the pellet and TMR diets, respectively; P < 0.01). Greater daily DMI was noted with the pellet diet than with the TMR diet (10.80 vs. 8.40 kg; P < 0.01). This difference was still significant even when the DMI was normalized for animal BW (2.88 vs. 2.23% of BW; P < 0.01). This higher DMI was above that suggested by the
NRC (2001)
for 300 kg growing heifers. As well, this finding was not in line with the literature, in which low protein content was found to negatively affect DMI (Tedeschi et al., 2000
). This increased intake may have been to partially compensate for the lower protein content in the pellet diet, to reach a protein intake similar or higher to that of the NRC (2001)
guidelines (756 vs. 972 g of CP/d in the TMR and pellet diets, respectively).Table 3Intake characteristics of heifers (daily average) fed for ad libitum intake with pellet and TMR diets; the diets were formulated to be similar in chemical composition but different in physical form (evaluated as physical effectiveness factor and physically effective NDF)
| Item | Diet | SEM | P-value | |
|---|---|---|---|---|
| Pellet | TMR | |||
| DMI | ||||
| 3 h post-feeding, kg/d | 2.70 | 3.25 | 0.439 | <0.01 |
| 24 h post-feeding, kg/d | 10.80 | 8.40 | 0.451 | <0.01 |
| % of BW | 2.88 | 2.23 | 0.100 | <0.01 |
| aNDFom intake | ||||
| kg/d | 6.34 | 5.03 | 0.267 | <0.01 |
| % of BW | 1.69 | 1.34 | 0.059 | <0.01 |
| uNDF240 intake | ||||
| kg/d | 1.33 | 1.18 | 0.059 | <0.01 |
| % of BW | 0.36 | 0.32 | 0.013 | <0.01 |
| peNDF intake | ||||
| kg/d | 1.21 | 3.45 | 0.118 | <0.01 |
| % of BW | 0.32 | 0.92 | 0.024 | <0.01 |
| Water intake | ||||
| L/d | 55.00 | 45.00 | 3.229 | <0.01 |
| L/kg of DMI | 5.01 | 5.13 | 0.245 | 0.31 |
1 aNDFom = amylase- and sodium sulfite–treated NDF, corrected for ash residue; uNDF240 = unavailable NDF estimated via 240 h in vitro fermentation; peNDF = physically effective NDF, measured as the NDF content of the forages (DM basis) multiplied by the physical effectiveness factor.
Water intake (L/d) was higher for the pellet diet (55.0 vs. 45.0 L; P < 0.01), but the difference disappeared after correction for DMI, suggesting that water intake was related to DMI, not a treatment effect.
The aNDFom intake was greater with the pellet diet than with the TMR diet (6.34 vs. 5.03 kg/d, 1.69 vs. 1.34% of BW; P < 0.01), as was the corresponding uNDF intake (1.33 vs. 1.18 kg/d, 0.36 vs. 0.32% of BW; P < 0.01). The peNDF intake, consistent with previous findings, was higher with the TMR diet (3.45 vs. 1.21 kg/d, 0.92 vs. 0.32% of BW; P < 0.01). These results confirm that an increase in fiber particle size has a negative effect on DMI, as reported in other studies (
Allen, 2000
; Kononoff et al., 2003
; Kammes and Allen, 2012
). Reducing dietary fiber particle size could be considered as a way of decreasing the DMI-limiting fill effect in the reticulum-rumen when diet fiber composition could, otherwise, prevent animals from attaining an adequate DMI to reach their energy requirements (Montgomery and Baumgardt, 1965
).The ratio between uNDF240h intake and BW (0.36 vs. 0.32% of BW in the pellet and TMR diets, respectively) was similar to values reported by other authors (
Cotanch et al., 2014
) for dairy cows, where the ratio was 0.36 for a diet based on grass hay and 0.48 for alfalfa hay. Based on these data, it is possible to hypothesize a minimum uNDF240h requirement to ensure rumen health and function.Animal Performance
The ADG was similar for the 2 treatments (1.1 vs. 1.0 kg for the pellet and TMR diets, respectively; P = 0.94), within the normal range for the breed, age, sex, and size of the cattle used in this study, even though the study duration was short. The optimal ADG for growing heifers is 0.8 kg/d (
NRC (2001)
). Higher values are associated with a delay in age at first conception and calving. Other authors (Gardner et al., 1977
) have reported that high ADG (1.1 kg/d) is associated not with reproductive problems, but with lower milk production, primarily in the first lactation. Still other studies have reported no negative effect on milk production with an ADG higher than 0.9 to 1 kg/d (Gardner et al., 1988
; Van Amburgh et al., 1998
).The feed conversion ratio was similar for the 2 diets (11.0 vs. 10.6 kg for the pellet TMR diets, respectively; P = 0.33).
Rumination Time and Rumen pH
Rumination time and rumen attribute data are reported in Table 4. Cows fed the pellet and TMR diets ruminated for 241 and 507 min/d (P < 0.01), respectively. Rumination time decreased during administration of the pellet diet (−52%), as expected because of the reduced fiber particle size. We observed this effect on rumination related to DMI, aNDFom, or forage-aNDFom (23.3 vs. 58.5 min/kg; 41.0 vs. 94.0 min/kg; 23.0 vs. 58.5 min/kg with the pellet and TMR diets, respectively; P < 0.01). Rumination time is closely related to the physical and chemical characteristics of the diet (
Grant et al., 1990
). The differences in rumination time observed in the present study could be related to different physical effectiveness factors (20.12 and 66.12% for the pellet and TMR diets, respectively); this relationship has been reported in other studies (Woodford and Murphy, 1988
; Mertens (2001)
; Krause et al., 2002
). Another study (Teimouri Yansari et al., 2004
) evaluated the effect of reduced fiber particle size on chewing, rumination time, and rumen pH, but that study was conducted on dairy cows fed diets based on alfalfa hay. In the present study, reduction in rumination time had no effect on rumen pH. Recorded values were similar between the 2 diets (6.10 vs. 6.11 for the pellet and TMR diets, respectively; P = 0.79). This effect may have been related to the fact that the diets were low in starch, high in fiber, and adequate in uNDF intake (Yang and Beauchemin, 2007
; Cotanch et al., 2014
).Table 4Rumination time and rumen condition of heifers (daily average) fed for ad libitum intake with pellet and TMR diets; the diets were formulated to be similar in chemical composition but different in physical form (evaluated as physical effectiveness factor and physically effective NDF)
| Item | Diet | SEM | P-value | |
|---|---|---|---|---|
| Pellet | TMR | |||
| Rumination | ||||
| Time, min/d | 241.00 | 507.00 | 17.20 | <0.01 |
| Time/DMI per d, min/kg | 23.30 | 58.50 | 1.86 | <0.01 |
| Time/NDF intake per d, min/kg | 41.00 | 94.00 | 1.56 | <0.01 |
| Time/forage NDF intake per d, min/kg | 23.00 | 58.50 | 0.96 | <0.01 |
| Rumen condition | ||||
| Mean rumen pH | 6.10 | 6.11 | 0.07 | 0.79 |
| Mean rumen temperature, °C | 38.87 | 38.84 | 0.07 | 0.34 |
| Time below pH 5.8, min/d | 188.00 | 176.00 | 124.90 | 0.33 |
| Time below pH 5.5, min/d | 3.40 | 4.60 | 5.58 | 0.67 |
| Area below pH 5.8, min × pH units/d | 24.40 | 22.80 | 15.60 | 0.51 |
| Area below pH 5.5, min × pH units/d | 0.21 | 0.31 | 0.34 | 0.59 |
1 pH values evaluated as described by
Kleen et al. (2003)
.Short particle size, as well as reduced rumination time and saliva production, is usually associated with metabolic disorders such as SARA. The definition of SARA is based on rumen fluid pH (
Plaizier et al., 2008
). For the present study, we considered 2 thresholds of suboptimal pH, pH <5.8 as an indicator of fibrolytic bacteria depression (initial SARA) and pH <5.5 as a cutoff for actual SARA, in accordance with Kleen et al. (2003)
. In our study, the average daily pH values (recorded every 10 min) were >6.0 throughout the experimental week. Furthermore, pH values, expressed either as min under the critical pH thresholds (5.5 and 5.8) or the corresponding areas under the curve, did not demonstrate any significant differences between the 2 diets or indicate any risk of SARA, defined as likely to occur when the rumen pH remained below 5.5 for at least 180 min/d (Kleen et al. (2003)
; Plaizier et al., 2008
).NDF Digestibility
Data reported in Table 5 specify the chemical composition of feces and corresponding calculations of fiber digestibility in the gastrointestinal tract.
Table 5Fecal composition and fiber digestibility of heifers fed for ad libitum intake with pellet and TMR diets; the diets were formulated to be similar in chemical composition but different in physical form (evaluated as physical effectiveness factor and physically effective NDF)
| Item | Diet | SEM | P-value | |
|---|---|---|---|---|
| Pellet | TMR | |||
| Chemical composition, % of DM | ||||
| aNDFom | 69.59 | 69.21 | 0.397 | 0.26 |
| ADF | 57.13 | 54.94 | 0.454 | 0.24 |
| ADL | 26.82 | 27.88 | 0.707 | 0.26 |
| uNDF240 | 47.38 | 52.12 | 0.748 | <0.01 |
| pdNDF | 22.18 | 17.14 | 0.817 | <0.01 |
| NDF digestibility, % of aNDFom | ||||
| 24-h IVNDFD | 11.41 | 10.70 | 0.724 | 0.51 |
| 240-h IVNDFD | 31.82 | 24.72 | 1.128 | <0.01 |
| TTdpdNDF, % of pdNDF | 86.82 | 90.25 | 0.652 | <0.01 |
1 aNDFom = amylase- and sodium sulfite–treated NDF, corrected for ash residue; uNDF240 = unavailable NDF estimated via 240 h in vitro fermentation; pdNDF = potentially digestible NDF.
2 IVNDFD = in vitro NDF digestibility; TTdpdNDF = total-tract digestibility of the pdNDF.
The fecal chemical composition of the 2 diets showed similar aNDFom, ADF, and ADL content, but uNDF240 content was higher for the TMR diet than for the pellet diet (52.12 vs. 47.38% of DM; P < 0.01). The pdNDF results were lower for the TMR diet than for the pellet diet (17.14 vs. 22.18% of DM; P < 0.01).
We assessed IVNDFD at 24 h and 240 h. The 24-h IVNDFD was not different between diets (11.41 vs. 10.70% of aNDFom for the pellet and TMR diets, respectively; P = 0.51), but the 240-h IVNDFD was higher for the pellet diet than for the TMR diet (31.82 vs. 24.72% of aNDFom; P < 0.01). Considering that the fecal 24-h IVNDFD was the aNDFom fraction with potential rapid digestibility, the difference between diets observed in 240-h IVNDFD rates could be assigned to a slowly degradable fraction of the diets. This result suggests that fiber particle size influenced the digestibility of the slowly digestible aNDFom, but had no effects on the rapidly digestible aNDFom.
The slowly digestible fraction represents fibrous material that is not digested in the gastrointestinal tract. Given that this fraction was lower in the TMR diet, higher total-tract digestibility would be expected, and indeed, total-tract digestibility of the potentially digestible aNDFom was higher for the TMR diet than for the pellet diet (90.25 vs. 86.82% of pdNDF; P < 0.01). Observations from the present study are consistent findings from a study by
Kammes and Allen, 2012
, in which the animals were fed a forage-based diet chopped at 2 different lengths (19 vs. 10 mm). The calculated total-tract digestibility of the potentially digestible NDF in that study was 90.6 and 88.7% for long- and short-particle diets, respectively, but no treatment effect was observed. In the present study, the pelletizing process could have had an effect on particle structure and density, increasing their respective passage rate.Fiber particle size influences many aspects of rumen function and digestion kinetics. The passage rate of particles is related to their size and density. The dynamic relationship between these factors defines their egress from the forage mat and flow out of the rumen (
Sutherland, 1988
). By experimental design, the fiber particle size was higher in the TMR diet. This size difference could have resulted in an increase in rumen retention time, improving de facto fiber digestion (Sejrsen et al., 2006
). The shorter particles in the pellet diet could have increased the surface area available for microbial attachment, leading to more extensive rumen degradation, but the same attribute of size may also have increased the escape rate from the rumen, limiting potential degradation (Kaske et al., 1992
; Lammers et al., 1996
).Conclusions
This study demonstrates that reduction of fiber particle size is a potential way to increase DMI in young ruminants. The shorter particle size led to a reduction in rumination time without causing an adverse effect on rumen pH. Furthermore, the use of a pelleted diet did not affect ADG. The different particle size of the treatments would be expected to affect the rate of passage from the rumen, being faster for the pellet treatment. Because of this, the total-tract digestibility of pdNDF was remarkably high in both treatments, although the effect of the larger particle size in the TMR diet resulted in a significant increase. We can conclude that a complete pelleted diet, well-designed to provide an adequate amount of NDF, could be fed to growing ruminants without apparent negative effect on rumen health and animal productivity, at least for a short period of time. More research is needed over a longer growing period before recommending this feeding strategy for growing heifers. Future studies are also needed to evaluate the effectiveness of this strategy in dairy cows, particularly during the transition period, when DMI is not sufficient to meet increasing animal requirements.
Acknowledgments
The authors express their appreciation to AIFE (Ravenna, Italy) and CRPA (Reggio Emilia, Italy) for their technical support and for funding the study.
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Article info
Publication history
Published online: September 28, 2016
Accepted:
August 19,
2016
Received:
February 15,
2016
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© 2016 American Dairy Science Association®.
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