Effects of Dietary Neutral Detergent Fiber on Intakes of Dry Matter and Net Energy by Dairy and Beef Cattle: Analysis of Published Data

INTRODUCTION

Voluntary consumption of food is generally recognized as one of the most important factors defining cattle performance. Intake of a particular diet is physiologically regulated at both digestive and metabolic levels. In general, physiological regulation of feed intake includes reticulo-rumen fill, osmolality, and kinetics, as well as specific digestive hormones and blood metabolites derived from feed digestion or catabolism (Forbes, 2000). Various external factors also affect feed intake by ruminants such as chemical composition of the diet, animal breed, age, physiological status, environmental conditions (NRC, 1996; 2001), and animal learning abilities (Forbes and Provenza, 2000). Some of these factors can be controlled by feeding procedures to a greater extent than others, which limits the ability to predict feed intake and consequently to predict performance. Nonetheless, the chemical composition of the diet has a major effect on voluntary intake. Among various chemical components, NDF has been proposed as a reliable predictor of voluntary consumption under certain conditions (Minson, 1990; Van Soest, 1994).

Although meta-analysis of published literature is sometimes criticized, it has proved useful to quantitatively review studies with a common background (Lipsey and Wilson, 2000). Mixed model regression methods allow for data from various experiments to be adjusted for random effects associated with trials (e.g., trial-adjusted means) and weighted for differences in variability (e.g., pooled SE of treatment means) associated with a particular study (St-Pierre, 2001). Our objective was to apply meta-analysis procedures to evaluate the relationships between dietary NDF concentration and voluntary DM and NE intakes of high-producing cattle from 2 previously compiled literature databases.

MATERIALS AND METHODS

Dairy and Beef Cattle Databases

The 2 databases used were configured from literature publications for dairy and feedlot beef cattle. The experimental treatments and means of these databases were those reported by Galyean and Abney (2006) for dairy and Galyean and Defoor (2003) for beef cattle. The dairy cattle database included 18 experiments published in the Journal of Dairy Science with a total of 80 observations (treatment means). The beef cattle database included 11 experiments mostly from the Journal of Animal Science, with a total of 48 treatment mean observations. These experiments were selected to meet the criterion that no fiber was supplied from by-product feeds with a high digestible fiber concentration (e.g., soybean hulls, beet pulp, etc.). Thus, the NDF content had only 2 possible sources: forage and concentrates. Many of these experiments involved changes in roughage concentration and source. The number of trials, literature references, and treatment mean numbers for the 2 databases are shown in Table 1.

Characteristics of Databases

Data not related to treatment effects and with common features for all trials were obtained and processed for descriptive purposes. For dairy cattle, the average and SD of cow (ruminally cannulated or not) initial live and metabolic BW, replications per treatment, and DIM were calculated for the 18 trials included in the database. For the beef cattle database, the average and SD (based on treatment means) of animal numbers, initial live and metabolic BW, animals per experimental unit, replications per treatment, and length of feeding periods were calculated across the 11 trials.

Data Processing 

For the dairy data, the relationship between total NDF (% of dietary DM) and intake was studied for DMI (% of animal BW and kg/d per animal), NE^sub 1^ intake above maintenance (kcal/ kg of BW^sup 0.75^), NE^sub 1^ intake above maintenance relative to DMI (Meal/ kg of DMI), and milk fat content (%). For the beef cattle database, the relationship between total NDF (% of dietary DM) and intake was studied for DMI (% of animal BW and kg/d per animal), NE^sub g^ intake (kcal/kg of BW^sup 0.75^), and NE intake relative to DMI (Meal/kg of^DMI). Although efficiency of energy use for maintenance and lactation are considered to be the same for dairy cattle, we calculated NE^sub 1^ intake above maintenance to represent energy deposited in product (e.g., as a proxy for milk production). Thus, NE^sub 1^ intake above maintenance in dairy cattle should be a comparable value to NE intake in beef cattle (both representing energy deposited in product), thereby allowing us to present relationships in the dairy and beef database on an equivalent basis. When the dietary NE^sub 1^ concentration was not reported in an experiment, it was calculated from NRC (2001) values for the dietary ingredients. For the beef cattle database, dietary NE^sub m^ and NE^sub g^ values for all experiments in the database were calculated from NRC (1996) or NRC (2001) values for the dietary ingredients. When dietary NDF data were not reported for the dairy database, values were calculated from feed tabular values of NRC (2001). For the beef data, in which the original publications generally did not report NDF, all NDF values were calculated from tabular values for feed ingredients reported by NRC (1996) or NRC (2001). We recognize that the use of tabular values results in changes in concentrations of certain feeds being confounded with changes in dietary NDF; this would be true, however, even if measured NDF values were available. Moreover, given the range in sources of NDF in the articles that comprised the 2 databases, we believe the likelihood of the results reflecting the effects of changes in the dietary concentration of selected feeds is small.

Both sets of data were checked for outliers. An observation was declared an outlier when df betas calculated from SAS procedures (Freund and Littell, 2000) were larger than 2/ [the square root of]number of observations. A few observations were found to be outliers using this procedure; however, by reviewing the original information, no fundamental reasons were found to reject these data, so they were left in the database.

RESULTS AND DISCUSSION

Descriptive Database Data

Descriptive features for the dairy and beef cattle databases are reported in Tables 2 and 3, respectively. In the dairy cattle database, the 80 treatment mean observations included a total of 301 Holstein cows in 18 trials, with an average of 8 replicates/treatment. Most cows were multiparous, with an average of 85.6 DIM. The average BW or BW^sup 0.75^ did not exhibit large variability among trials, being 619 and 124 kg with 7.6 and 5.7% CV, respectively. In the beef cattle database, the 48 treatment mean observations had an average of 148 animals of different breed types per trial. Except in one study (Defoor et al., 2002) in which individual animals were used, most means were based on pens averaging 6 replicates/treatment. British crosses were represented to the greatest extent, with 48% of the total number of animals, followed by British × Continental (21.8%), with the remainder (30.2%) divided in almost equal portions between Brahman and unidentified breeds. The average BW and average BW0 75 were 408 and 91, with CV of 7.4 and 5.5%, respectively. The calculated CV of BW for beef and dairy were almost identical, indicating that variability in average BW among studies was fairly small, yielding a relatively homogeneous set of data as far as BW was concerned for both databases.

Relationships Between DMI and NDF-Dairy Cattle

Regression equations are depicted graphically in Figures 1, 2, 3, 4, and 5 for total dietary NDF (% of DM) vs. the trial-adjusted dependent variables DMI (% of BW and total kg/d per animal), NE^sub 1^ intake (kcal/ kg of BW^sup 0.75^), NE^sub 1^ intake per unit DMI (Mcal/kg of DMI), and milk fat (%), respectively. The corresponding regression equation and the r^sup 2^ for each of the dependent variables are reported within each figure. Dietary NDF concentration ranged from 22.5 to 45.8% across the trials included in the dairy database. The DMI as a percentage of BW, as well as DMI expressed as kilogram per animal daily, were negatively related to increasing dietary NDF concentration (Figures 1 and 2). Somewhat surprisingly, total DMI (kg/d) seemed better related to NDF than DMI expressed as percentage of BW, with r^sup 2^ values of 0.672 and 0.595, respectively. This finding might be a result of much less variability than expected in average BW among trials, thereby negating effects of expressing DMI relative to BW. Over the range of NDF values in the database, trial-adjusted daily NE^sub l^ intake above maintenance (kcal/ kg of BW^sup 0.75^; Figure 3) decreased sharply with increasing dietary NDF concentration (r^sup 2^ = 0.815). Similarly, NE^sub l^ intake per unit of DMI decreased with increasing dietary NDF concentration (Figure 4). Because feeds high in NDF are generally lower in NE concentration than feeds with a low NDF concentration (NRC, 2001), a negative relationship between NE intake above maintenance and dietary NDF concentration would be expected. Galyean and Abney (2006) reported that the DM (% of BW) and NE intakes in this data set were more closely related to total dietary NDF than to NDF from the roughage component of the diet alone. Therefore, Galyean and Abney (2006) suggested that if the NDF from the roughage component of the diet accounts for physical effectiveness of NDF, physically effective NDF would not likely be more highly associated with DMI than NDF alone, at least for the concentrations and sources of NDF used in these studies.

Similar to our findings, Allen (2000) also reported a negative relationship between dietary NDF concentration (resulting from modifying the proportions of forage and grain) and DMI by lactating dairy cows. Undoubtedly, NDF concentration is a major factor determining ruminai fill, which might explain decreased DMI with increasing NDF concentration. Nonetheless, the filling effect of a particular diet also would depend on factors affecting rate of digestion and flow from the reticulo-rumen, such as size and density of digesta particles, motility, reticulo-omasal orifice size, and rate of emptying of the abomasum (Allen, 2000).

The trial-adjusted milk fat percent increased linearly with increasing dietary NDF, with r^sup 2^ = 0.617 (Figure 5). This finding agrees with previous observations indicating a positive association between both forage and nonforage NDF and milk fat percent (Armentano and Pereira, 1997). Recently, a meta-analysis of 25 experiments with dairy cows in early lactation indicated that the acetate-to-propionate ratio was positively correlated to dietary NDF content (Tafaj et al., 2007). Thus, if acetate production is greater with increasing NDF intake, synthesis of milk fat should be favored, although related factors such as ruminai pH and biohydrogenation reactions involving long-chain fatty acids can have marked effects on milk fat percent (Bauman and Griinari, 2000). Similarly, Holstein cows fed diets with NDF-to-starch ratios of 0.74, 0.95, and 1.27 (all diets had 18% forage NDF but total dietary NDF varied from 24.7 to 32.2%) resulted in changes in ruminai VFA with increasing dietary NDF-to-starch ratio that accounted for the observed increases in milk fat (Beckman and Weiss, 2005).

Although other factors in this database might have influenced milk production, changes in milk fat percent did not seem to be related to 4% FCM production (r^sup 2^ = 0.019; P = 0.217; data not shown). Therefore, over the total dietary NDF range in this database of 22.5 to 45.8%, NE^sub l^ intake decreased, milk fat percent increased, and 4% FCM was not changed. The NE^sub l^ intake was positively related to 4% FCM production (r^sup 2^ = 0.247; data not shown). Increased milk fat concentration as a result of greater dietary NDF might partially offset the negative effect of increased NDF on NE^sub l^ intake above maintenance. Galyean and Abney (2006) indicated that total dietary NDF seemed more important to sustain or increase milk fat than NDF from roughage.

Relationships Between DMI and NDF-Beef Cattle

Regression equations generated with weighted and trial-adjusted beef cattle data are shown for DMI (% of BW and kg/d per animal), NE intake (kcal/kg of BW^sup 0.75^), and NE^sub g^ intake per unit DMI (Meal/kg of DMI) in Figures 6, 7, 8, and 9, respectively. As with the dairy database, the regression equations and the r2 for each of the dependent variables are reported within the figures. The dietary NDF values ranged from 7.5 to 35.3% across the trials in the beef database. The DMI expressed either as a percentage of BW or in kilograms per day of DM per animal were positively related to increasing dietary NDF concentration. Similar to the results with the dairy database, total DMI (% of BW; Figure 6) seemed almost equally related to NDF as DMI expressed as kilograms per day per animal (Figure 7), with r^sup 2^ values of 0.954 and 0.965, respectively. As noted previously, this similarity in the 2 methods of expressing DMI presumably reflects low variability in BW among trials (Table 3).

Galyean and Defoor (2003) reported that NDF supplied by roughage was highly related to DMI (% of BW) in feedlot cattle; however, they did not evaluate total dietary NDF. Present results suggest that total dietary NDF concentration in beef feedlot diets has a similar relationship to DMI as does NDF supplied by the roughage portion of the diet. For the NDF range in the feedlot diets in our database, the overall effect was for DMI to increase with increasing dietary NDF concentration. In contrast, greater NDF intakes were associated with decreased DMI in the dairy cattle database. This difference between the dairy and beef databases possibly reflects a lower total NDF in beef feedlot diets, the relative contribution of different ingredients to the total dietary NDF (e.g., proportion of NDF supplied by roughage vs. NDF supplied by other dietary ingredients), and the greater starch content and thereby presumably greater fermentability of feedlot diets. Galyean and Defoor (2003) suggested that NDF supplied by roughage would be an effective means of exchanging roughages in beef feedlot diets to achieve equal DMI. Present data suggest that total dietary NDF also should be an effective basis for roughage exchange in feedlot diets.

The NE^sub g^ intake (kcal/kg of BW^sup 0.75^; Figure 8) was highly related to dietary NDF concentration (r^sup 2^ = 0.859); however, total dietary NDF (r^sup 2^ = 0.001) did not account for variation in intake of NE^sub g^ per unit of DMI (Figure 9). This finding is not consistent with the results with the dairy cattle database. These differences in NE intake per unit of DMI between the databases could be attributable to the fact that the average total dietary NDF content was 16.0 ± 5.7 and 32.7 ±5.1 for the beef and dairy cattle databases, respectively. Thus, at low dietary NDF supply, as in the feedlot database, DMI was most likely regulated by physiological mechanisms (Forbes, 2000) rather than by critical distention of the reticulorumen. As a result, small increments of added NDF in feedlot diets might stimulate salivary secretion and ruminai function and kinetics, which could favor increased DMI. Therefore, increased DMI with increasing NDF at relatively low fiber levels in the beef database did not affect the NE intake per unit DMI as was noted for the dairy cattle database. Given that DMI was highly related to dietary NDF concentration in our beef cattle database and NE^sub g^ intake per unit DMI was not related to NDF concentration, one could infer that DMI would not be highly related to NE^sub g^ intake per unit of DMI or to other measures of feed efficiency. In support of this inference, Gill et al. (1986) evaluated DMI and performance data from 300 pens of 295- to 340-kg feedlot cattle fed ad libitum in one commercial feedlot. Greater DMI was associated with increased ADG (r^sup 2^ = 0.50); however, G:F ratio did not change (r^sup 2^ = 0.001) with increasing DMI. Thus, our findings suggest that when it is economically feasible, small increments in roughage concentration (e.g., increasing NDF concentration) in beef feedlot diets would likely increase total DMI with limited negative effects on gain efficiency and potential benefits in ruminai function and for disorders related to a high starch supply (e.g., acidosis). Nonetheless, this scenario assumes that maintenance energy costs do not increase with increased DMI. If maintenance costs increase with increasing DMI, or if efficiency of ME use for gain changes with DMI or dietary NDF concentration, gain efficiency would likely be negatively affected by increased NDF concentration.

IMPLICATIONS

From a practical standpoint, chemical determination of NDF is reasonably simple, fast, and it also is highly associated with dietary quality and DMI independent of feed sources. Based on our findings, one might expect, however, different outcomes in total DMI at low vs. high NDF dietary concentration for high-producing animals. In the beef database we evaluated, with a range in dietary NDF concentration of 7.5 to 35.3%, increased DM and NE^sub g^ intakes were observed with increasing dietary NDF concentration. Conversely, total DM and NE^sub l^ intakes decreased sharply in the dairy database as NDF concentration increased over the range of 22.5 to 45.8% dietary NDF. In dairy cattle, increased dietary NDF concentration would likely increase milk fat percent, which might partially compensate, from an economic standpoint, for lower milk production associated with decreased NE intake. The cost of energy from roughages is frequently greater than the cost of energy from grains. Thus, the cost of the additional NE^sub l^ or NE^sub g^ intake resulting from added roughage must be considered in determining optimal roughage concentrations in dairy and beef feedlot diets.

ACKNOWLEDGMENTS

Appreciation is expressed to Universidad Nacional del Sur and Comisión de Investigaciones Científicas de la Provincia de Buenos Aires for allowing H. M. Arelovich to participate in this project at Texas Tech University. Funds from the Jessie W. Thornton Chair in Animal Science Endowment at Texas Tech University supported this research.

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