Nutrient requirements of beef cattle

Forage quality can be assessed in terms of the animal performance that is elicited when the forage is offered to the animal. Animal performance is the product of supply, nutrient and energy concentration, intake, digestibility, and metabolism. Typically, forage quality is measured assuming that the forage is freely available and that deficient nutrients other than protein and energy are provided in a supplement. Of the remaining factors affecting forage quality, intake is the most important one affecting animal performance. Reid (1961) listed the amount of forage eaten as the first factor limiting the usefulness of forage-testing schemes. His list also included the effect of concentrates on the amount of forage eaten, degree of selective feeding, and effect of physical form as important information not provided by the forage testing systems used at that time. Unfortunately, intake, substitution rates, selection, and physical form still are not a part of routine forage evaluation systems in the 1990's. Animal performance depends on the intake of digestible and metabolizable nutrients. Of the variation in digestible dry matter (DDM) or digestible energy (DE) intake among animals and feeds, 60 to 90% is related to differences in intake, whereas only 10 to 40% is related to differences in digestibility (Crampton et al., 1960; Reid, 1961). Intake generally accounts for twice as much variability in DDM intake as does digestibility (Milford, 1960; Ingalls et al., 1965). Milford and Minson (1966) observed that DDM intake is more closely correlated with dry matter intake (DMI) than any other feed or animal characteristic. Similarly, high correlations would occur between DMI and metabolizable (ME) or net energy (NE) intake, and between DMI and overall animal performance when forages are fed. The primary importance of intake in assessing forage quality does not mean that digestibility or metabolizability of nutrients are invariant or unimportant. Metabolizability can vary depending on the amounts and ratios of absorbed nutrients, the individual and interacting biochemical pathways of nutrient conversion and the type of animal production (MacRae et al., 1985; Blaxter, 1989). However, differences in the efficiency of converting DE to ME are small compared to differences in intake and digestibility, and the metabolic conversion of DE to ME or NE is typically considered to be relatively constant within type of production, such as maintenance, growth, pregnancy, or lactation (Moe and Tyrrell, 1976; NRC, 1989). Digestibility of forages is more variable than metabolizability, and feces typically are the greatest loss of ingested nutrients and energy. Extensive research has been devoted to measuring digestibility and relating it to feed characteristics because digestibility can be accurately measured with relative ease compared to DMI. Although intake is more important than digestibility in assessing forage quality, progress in understanding the basic factors that affect intake has been hampered by our inability to measure it accurately and to separate the influences of animal and diet on intake. Thus, we routinely use availability information (DE, ME, andNE)

Regulation of Forage Intake

Even under the intensive concentrate feeding systems of ruminant animal production in the United States, forages continue to represent the single most important feed resource. Cell-wall concentration and digestibility limit the intake potential and energy availability of forage crops in beef and dairy production. Identification of cell-wall characteristics that should be targets of genetic modification is required if plant breeders and molecular biologists are to successfully improve forages for livestock feeding. As the forage plant cell develops, phenolic acids and lignin are deposited in the maturing cell wall in specific structural conformations, and in a strict developmental sequence. Lignin is the key element that limits cell-wall digestibility, but cross-linkage of lignin and wall polysaccharides by ferulic acid bridges may be a prerequisite for lignin to exert its affect. Lignin composition and p-coumaric acid in the wall are less likely to affect digestibility. Voluntary intake of forages is a critical determinant of animal performance and cell-wall concentration is negatively related to intake of ruminants consuming high-forage diets. Cell walls affect intake by contributing to ruminal fill. A simple model of cell-wall digestion and passage in which ruminal fill is a function of rates of digestion and passage, as well as the indigestible fraction of the cell-wall indicates that cell-wall concentration and rate of passage are the most critical parameters determining ruminal fill. Plant factors that affect rate of passage include those that affect particle size reduction by chewing and those that affect particle buoyancy in the rumen. The latter is primarily affected by 1) the ability of the particulate matter to retain gases, which is probably related to plant anatomy and rate of digestion of the plant tissue, and 2) the rate at which the gas is produced, which is affected by the potentially digestible fraction of the particulate matter and the rate of digestion of this fraction. Increasing rate of digestion should increase rate of passage by diminishing the gas produced and increasing density over time. A reduction in the indigestible cell-wall fraction is beneficial because this will decrease fill and increase digestibility. Animal production and economic benefits from reduced cell-wall concentration and increased digestibility are significant. Because of the high cell-wall concentration and large digestible cell-wall fraction of grasses, reduction in cell-wall concentration would probably be of greater value than improving digestibility in these species. Legumes represent the opposite situation and may benefit more from improvements in the digestibility of their cell walls.

Characteristics of plant cell walls affecting intake and digestibility of forages by ruminants.

The Veterinary Clinics of North America

In situ and Other Methods to Estimate Ruminal Protein and Energy Digestibility: A Review

Low live weight gain of cattle in the wet season of tropical areas was identified as a major limitation to achieving annual growth rates from tropical pasture systems sufficient to meet new market specifications of young animals of high carcass weight Both protein and energy are limiting nutrients for growth Net transfer of feed protein to the intestines is often not complete, and losses occur with grasses and legumes when CP content exceeds 210 g of CP/kg of digestible OM This protein loss is important because a collation of experimental data indicated that cattle consuming low- and high-quality pasture and silage-based diets all responded to extra protein The response was less for the higher-quality forage The role of legumes in supplying this protein was investigated and, unless legumes can increase total DMI by at least 30%, they will not supply sufficient intestinal protein to increase live weight gain by about 300 g/d The problem with legumes and some grasses is the loss of protein from the rumen, and increasing energy supply to the rumen, either through improved digestibility or energy supplements, is a strategy that could be used to reduce this Strategies to increase the proportion of escape protein would be successful, but incorporation of lowly degradable protein fractions into legumes may be more difficult because of the level of expression of these protein fractions required for a significant live weight gain response Cattle entering the wet season usually exhibit compensatory growth and are exposed to high ambient temperatures and often to high humidity Intestinal protein above that stipulated in feeding standards may be beneficial in these circumstances, and more emphasis should be placed on the ability of legumes to supply protein postruminally At present the protein delivery capacity of agronomically competitive legumes seems to be inadequate for the higher growth rates required in production systems, and supplements of energy and protein will be needed to achieve these higher targets until new cultivars appear

Protein and energy utilization by ruminants at pasture.

Three grasses and two legumes, each at two stages of maturity, were fed to three fistulated sheep and samples of the feed, and faeces and contents of the reticulo-rumen, abomasum and ileum were collected for the determination of particle size by wet sieving. Modulus of fineness (MF) was calculated for all particulate matter that failed to pass a 0.15 mm screen. Resistance to flow from the rumen of feed particles of different sizes was also calculated. The mean MF of the reticulo-rumen contents was 2.56 compared with 5.72 for the feed. Material leaving the rumen and found in the abomasum had a MF of 1.67 and subsequent changes were small and not significant; ileum 1.63, faeces 1.80. Particles greater than 1.18 mm passed out of the reticulo-rumen although the quantity was small (1-3%). The resistance to flow of particles of different sizes from the reticulo-rumen was closely related to particle size with no difference between grasses and legumes or between young and mature forages. It was concluded that in modelling the flow of particles from the reticulo-rumen a non-compartmentalized approach should be adopted, but if a simple two-compartment model is required then a critical sieve size of about 1.18 mm may be useful, since less than 5 % of the particulate material is retained on this sieve size.

The validity of the critical size theory for particles leaving the rumen

Pangola grass (Digitaria decumbens) and Rhodes grass (Chloris gayana) were cut as 6 and 12 week regrowths, dried, chopped, and separated into leaf and stem fractions by using a gravity separator. Each of the eight diets was offered to four cattle and eight sheep fitted with a ruminal cannula to measure voluntary intake and digestibility of the dry matter (DM), organic matter (OM) and neutral detergent fibre (NDF). Possible deficiencies of protein and minerals were minimized in this study by feeding casein and a mineral supplement. The total DM, OM and NDF contents of the rumeno-reticulum (rumen) were measured by manually emptying the rumen and the values used to estimate retention times of DM, OM and NDF. Cattle consumed 35% more leaf than stem fraction. Sheep consumed 21 % more leaf. When the daily voluntary intake was expressed as g/kg0.9 the intake was similar for cattle and sheep. The higher intake of the leaf fraction was associated with the shorter time that it was retained in the rumen of both cattle and sheep. Leaf and stem fractions were digested to the same extent by cattle. There was also no difference in the digestibility of leaf and stem fed to sheep. The digestive efficiency of sheep was 0.033 lower than that of cattle for both DM and NDF. Cattle retained all diets for longer periods in the rumen than did sheep. This difference was associated with the higher digestive efficiency of the cattle. It was concluded that both cattle and sheep consumed more leaf than stem fractions of grasses, and that the higher intake of leaf was associated with the shorter time that it was retained in the rumen and not by differences in digestibility as such.

Studies of cattle and sheep eating leaf and stem fractions of grasses. 1. The voluntary intake, digestibility and retention time in the reticulo-rumen

The effect of post-ruminal infusion of protein or energy on the pathophysiology of Trichostrongylus colubriformis infection and body composition in lambs was investigated using the comparative slaughter technique. Of 52 Dorset DownxCoopworth wether lambs (mean body weight 30.5, s.e. 0.44 kg), 8 comprised an initial slaughter group (IC), 36 were infected daily, per os, with 3000 Trichostrongylus colubriformis larvae per animal, and the remainder (C) were parasite-free controls. Groups of 12 infected animals were given either a continuous infusion (via abomasal catheter) of 50.4 g/day crude protein as sodium caseinate (PI), glucose (EI) in isocaloric amounts to PI, or a solution containing P and Na at the same rate as the caseinate infusions (SI). Uninfected controls (C) were also infused with this solution. Animals were slaughtered after 6 and 12 weeks for estimation of worm count and body composition. All animals were offered, ad libitum, meadow hay (118 g crude protein/kg dry matter (DM); 930 g organic matter/kg DM; 0.65 organic matter digestibility). DM intake of infected animals decreased by approximately 22% in PI and EI and by 32% in SI animals. Mean total metabolizable energy (ME) intake during the experiment (inclusive of infusions) was C, 0.607; PI, 0.687; EI, 0.704; SI, 0.617, s.e.d. 0.0308 MJME (kg W)-0.75 day-1. Nitrogen retention in infected sheep was increased fourfold by protein infusion and twofold by energy infusion. The rate of energy retention was reduced by infection, increased by protein infusion but only partly by energy infusion. Protein infusion reduced mean faecal egg output and mean total parasite count. Increased endogenous plasma loss into the gastrointestinal tract was similar in all infected groups. It was concluded that a major limiting factor in efficient food utilization in animals with gastrointestinal nematode infections is a parasite-induced protein rather than energy deficiency, possibly the result of increased endogenous protein losses into the gastrointestinal tract. The debilitating effect of internal parasites could be markedly reduced by increasing duodenal protein supply.

The effect of post-ruminal infusion of protein or energy on the pathophysiology of Trichostrongylus colubriformis infection and body composition in lambs

Pangola grass (Digitavia decumbens) and Rhodes grass (Chlovis gayana) cut as 6 and 12 week regrowths were separated into leaf and stem fractions and fed ad libitum to four cattle and eight sheep fitted with ruminal fistulae to determine the importance of particle size in controlling the retention time of feed in the rumeno-reticulum (rumen). Particle size was determined by using a wet sieving technique, and based on the cumulative dry matter distribution on the sieves of faeces from cattle and sheep; all particles >1.18 mm were described as large particles. The proportion of large particles was measured in the chopped diet offered, the masticated diet, the ruminal contents and the faeces. Chopped leaf and stem fractions contained 0.85 and 0.86 g/g large particles respectively. Mastication by cattle reduced the proportion of large particles in leaf and stem to 0.58 and 0.76 (P 0.05) and in cattle 0.272 and 0.345 (P 0.05) for leaf and stem, and in sheep 0.993 and 0.991 (P > 0.05). Large particles in leaf and stem were retained in the rumen of the sheep for 11.0 and 11.7 h (P > 0.05). Cattle retained large particles in all diets for a longer time (P 0.05). Differences in retention time of large particles in the rumen did not appear to be the only factor controlling the retention of dry matter in the rumen and voluntary intake. A model was developed to describe the flow of large and small particles through the rumen. Intake simulation studies indicated that the most important factor influencing dry matter retention time in the rumen was the retention time of small particles (> 1.18 mm). Changes in the rate of breakdown of large particles had a small effect on dry matter retention time.

Dennis P. Poppi

Papers

Protein and energy utilization by ruminants at pasture.

The validity of the critical size theory for particles leaving the rumen

Studies of cattle and sheep eating leaf and stem fractions of grasses. 1. The voluntary intake, digestibility and retention time in the reticulo-rumen

The effect of post-ruminal infusion of protein or energy on the pathophysiology of Trichostrongylus colubriformis infection and body composition in lambs

Studies of cattle and sheep eating leaf and stem fractions of grasses. 3. The retention time in the rumen of large feed particles