Ryegrass Intake and Ruminal Characteristics Related to Gain in Yearling Steers

Author: H. Lippke, T.D.A. Forbes, P.G. Soderstrom, S.R. Baxter, C.M. Hensarling, and S.S. Sieckenius

Texas Agricultural Experiment Station, Uvalde 78801

Abstract

Ruminally fistulated, yearling Hereford steers grazing ryegrass pasture were used to examine the relationship of ADG to DM intake and digestibility and to various characteristics of ruminal fluid. Two different regimes of dietary transition from hay to ryegrass were used to expand the range of observations. The n-alkanes, C₃₂ and C₃₃, were used as markers for determination of intake and digestibility on pasture. Average daily gain among animals ranged from .83 to 1.30 kg during 37 days on immature ryegrass pasture. Digestibility of DM averaged 744 g/kg (SD = 10). Dry matter intake ranged from 24.0 to 30.6 g/kg BW. Variations in DM intake explained most of the variation in ADG. We found that ADG was also positively related to density of ruminal protozoa and negatively related to ruminal ammonia concentration. The results of this experiment are consistent with the hypothesis that reduced growth rates by young cattle grazing highly digestible forages are at least partially caused by inadequate protein metabolism acting to reduce dry matter intake.

Introduction

Ryegrass and small grain forages in vegetative stages of growth usually have DM digestibility values greater than 70%. The CP content of these forages ranges from 15 to 30%. In some instances, young cattle grazing lush small grain or ryegrass forages have demonstrated ADG values of 1.25 kg or more. More commonly, however, gains have ranged from .5 to .7 kg daily during the first month after turnout onto pasture. Such low growth rates are not consistent with the high digestibility and protein values of the diets being consumed, leaving abnormally low forage intake as the probable cause of poor animal performance. If herbage mass is adequate, the basis for low DM intake is not clear. We hypothesize that it is the end result of feedback mechanisms that stem from high rates of deamination and low rates of N recapture by ruminal microbes, depriving the animal's tissues of sufficient amino acids for high rates of growth. Based on the results of previous experiments, we further hypothesize that this condition is exacerbated by digestive upset collateral to large, abrupt dietary change.

The objectives of this experiment were to examine the relationships among ruminal fluid characteristics, DM intake, digestibility, and weight gain by yearling steers grazing ryegrass and to determine the effects of dietary transition regime on these variables.

Methods

Ten yearling Hereford steers (BW = 237 kg) that had been ruminally fistulated and fitted with Jarret-type cannulas (5 cm diameter) were paired by weight and randomly assigned within pairs to two groups. They were group-fed a mixture of coarsely chopped bermudagrass and alfalfa hays at the rate of 7.6 g/kg BW twice daily in drylot during the pre-grazing period. During this period, treatment for internal and external parasites was applied, and Synovex S was implanted into one ear of each animal. On the day of turnout onto immature ryegrass pasture (day 1), group 1 steers were released to pasture at mid-morning without a feeding of hay. Steers in group 2 were fed hay ad libitum on day 1 until they were released to pasture at mid-afternoon. The cattle were weighed on day -2 and on day 45, each time after 14 hr without feed or water. On day 38, the steers were removed from pasture and returned to drylot and to the ration used before the grazing period, in order to establish weighing conditions similar to those on day -2.

At 1100 and 1700 on day 2 and at 0700, 1100, and 1700 on days 3, 4, and 5, samples of ruminal fluid were taken from each steer via fistula by a dipper that was opened in the ventral sac of the rumen. Samples of ruminal fluid were also taken at 0800 and 1700 on day 8, at 0800 on day 12, and at 0800 and 1700 on day 26. Ammonia, pH, and VFA were determined on all ruminal fluid samples. For 11 of the ruminal sampling events, a subsample of ruminal fluid from each animal was examined microscopically, and the density of protozoa was scored on a scale from 0 to 5.

On days 2 to 12, each animal was dosed via ruminal fistula twice daily with the n-alkane, C₃₂, adsorbed onto cellulose fiber in a gelatin capsule. Fecal samples for determination of the n-alkanes, C₃₂ and C₃₃, were taken from each animal once on day 2 and twice daily on days 8 to 12. Hand plucked samples of ryegrass forage were gathered separately by two of us on day 8. All samples were frozen and subsequently lyophilized and ground through a 1-mm screen for determination of protein and fiber in forage samples and determination of alkanes in all samples. Procedures for preparation of n-alkane dose materials, alkane extraction from forage and feces, and n-alkane analysis were adapted from the procedures of Dove (H. Dove, personal communication), Mayes (R. W. Mayes, personal communication), Mayes et al. (1986), and Vulich et al. (1995).

Results and Discussion

The chopped hay fed during the pre-grazing and post-grazing periods contained 567 g/kg of NDF, 341 g/kg of ADF, and 158 g/kg CP. The plucked ryegrass gathered on day 8 had average NDF, ADF, and CP concentrations of 355, 171, and 269 g/kg, respectively. These values are typical of early season ryegrass forage from fertilized, irrigated pastures in southwest Texas. Differences in concentrations due to sward sampling were < 13 g/kg for any of these measures. The concentrations of C₃₂ and C₃₃ in ryegrass forage were 5 and 49 mg/kg, respectively.

Dietary transition regime did not have a significant effect on any of the variables measured in this experiment. Average daily gains were 1.06 (SD = .20) and .97 (SD = .10) kg for treatment groups 1 and 2, respectively. Ad libitum hay intake by group 2 steers was 26.6 g/kg BW on day 1. One steer that had been assigned to group 2 was removed from the experiment due to illness. Among the remaining animals, ADG ranged from .83 to 1.30 kg (mean = 1.02). Based on the ratio of C₃₃ in forage and feces, digestible DM ranged from 725 to 757 g/kg (mean = 744). Estimated DM intake, based on dosed C₃₂ concentration in feces and forage indigestibility, ranged from 24.0 to 30.6 g/kg BW (mean = 26.9). The alkane marker technique provided precision equal to that of standard collection digestion trials. The magnitudes of both DM intake and digestibility were consistent with observed ADG.

Average daily gain while on pasture was related to estimated DM intake during days 8 to 12 (r = .66, P = .054, Figure 1). The data for one animal appear to lie outside this relationship. The line superimposed on Figure 1 depicts the relationship with that data point omitted (r = .85, P = .008). When ADG was regressed on both DM intake and digestibility, R² = .54 for the full data set, and R² = .97 for the reduced data set.

Ruminal fluid pH for samples taken on d 3 to 12, averaged within animal, ranged from 5.85 to 6.10 (mean = 5.98) among animals. Average ruminal ammonia concentration ranged from 23.8 to 35.2 meq/L (mean = 29.1), and average protozoal density score (PDS) ranged from 2.7 to 3.9. The average ruminal acetate:propionate ratio within animal in samples taken on d 4 to 12 ranged from 2.8 to 3.4 (mean = 3.0), having declined from a mean of 3.8 for all steers at the first sampling on day 2.

Average daily gain on pasture was related positively to PDS (P = .023) and negatively to ruminal ammonia concentration (P = .056). Figure 2 shows the relationship of ADG (adjusted for ruminal ammonia) to PDS. The decision to include at least a gross measure of protozoal activity in the experimental protocol was based on the hypothesis that protozoa would engulf proteins and peptides coming from plant cell degradation, providing a measure of protection from bacterial deamination and presenting a substantial mass of cell protein to the lower gut, thereby increasing amino acid supply to the intestines and improving ADG. Our observations were consistent with this hypothesis.

Figure 3 shows the negative relationship of ADG (adjusted for PDS) to ruminal ammonia concentration. Increased rates of amino acid degradation in the rumen would hypothetically be associated with greater ruminal ammonia concentration and lower animal growth rate. The relationships depicted in Figures 2 and 3 support the hypothesis that inadequate protein metabolism is a component of the reduced growth rates frequently observed in young cattle consuming highly digestible pasture forages.

Conclusions

The naturally occurring n-alkane, C₃₃, provided excellent precision in determining digestible DM of immature grazed ryegrass.

The dosed n-alkane, C₃₂, provided a range of DM intake values that were consistent with the literature and highly related to ADG.

The positive relationship between ADG and ruminal protozoa density and the negative relationship between ADG and ruminal ammonia concentration found in this experiment are consistent with the hypothesis that reduced growth rate by young cattle grazing highly digestible pasture forages is at least partially caused by inadequate protein metabolism acting to reduce dry matter intake.

Literature

Vulich, S.A., J.P. Hanrahan, and B.A. Crowley. 1995. Modification of the analytical procedures for the determination of herbage and faecal n-alkanes used in the estimation of herbage intake. J. Agric. Sci., Camb. 124:71-77.

Mayes, R.W., C.S. Lamb, and P.M. Colgrove. 1986. The use of dosed and herbage n-alkanes as markers for the determination of herbage intake. J. Agric. Sci. Camb. 107:161-170.

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