Acute and chronic acidosis, conditions that follow ingestion of excessive amounts of readily fermented carbohydrate, are prominent production problems for ruminants fed diets rich in concentrate. Often occurring during adaptation to concentrate-rich diets in feedyards, chronic acidosis may continue during the feeding period. With acute acidosis, ruminal acidity and osmolality increase markedly as acids and glucose accumulate; these can damage the ruminal and intestinal wall, decrease blood pH, and cause dehydration that proves fatal. Laminitis, polioencephalomalacia, and liver abscesses often accompany acidosis. Even after animals recover from a bout of acidosis, nutrient absorption may be retarded. With chronic acidosis, feed intake typically is reduced but variable, and performance is depressed, probably due to hypertonicity of digesta. Acidosis control measures include feed additives that inhibit microbial strains that produce lactate, that stimulate activity of lactate-using bacteria or starch-engulfing ruminal protozoa, and that reduce meal size. Inoculation with microbial strains capable of preventing glucose or lactate accumulation or metabolizing lactate at a low pH should help prevent acidosis. Feeding higher amounts of dietary roughage, processing grains less thoroughly, and limiting the quantity of feed should reduce the incidence of acidosis, but these practices often depress performance and economic efficiency. Continued research concerning grain processing, dietary cationanion balance, narrow-spectrum antibiotics, glucose or lactate utilizing microbes, and feeding management (limit or program feeding) should yield new methods for reducing the incidence of acute and chronic acidosis.

Acidosis in cattle: a review.

Relationship Between Fermentation Acid Production in the Rumen and the Requirement for Physically Effective Fiber

Starch is the major energy component of grains. Wheat contains 77% of DM as starch, corn and sorghum contain 72%, and barley and oats contain 57 to 58%. In vitro systems have provided valuable data on kinetic aspects of starch digestion. Molecular biological techniques have provided a clearer picture of the ruminal microbial milieu. Proportions of starch fermented in the rumen can be predicted satisfactorily for a variety of grains and processing methods. Compared with dry rolling, steam processing (flaking or rolling) increases ruminal digestibility of starch (percentage of intake) from 52 to 78% for sorghum, from 75 to 85% for corn, and six percentage units or less for other grains. Recent research provides new insight into pancreatic function and intestinal glucose transport systems. The capacity to digest starch in the intestine ranges from 45 to 85% of starch entering the duodenum, with that capacity apparently limited by the supply of pancreatic amylase. There is evidence that amylase secretion may be enhanced by increasing duodenal entry of protein. Capacity for active transport of glucose across of gut wall does not seem to limit the amount of starch digested that is absorbed as glucose. For ruminants eating medium- to high-concentrate diets, about 30% of their total glucose need comes from glucose absorption, 50% from organic acid absorption (substrates for hepatic gluconeogenesis), and 20% from other sources. When glucose absorption from the gut increases, ruminants generally adjust (decrease) gluconeogenesis to meet their need; that need is directly linked to DE intake. In terms of overall ME yield, grain starch is best used when it is fermented in the rumen. However, close coordination of protein and starch supply to the duodenum may improve capture of starch in the form of glucose.

Starch utilization by ruminants: from basics to the bunk.

Saponins occur widely in plant species and exhibit a range of biological properties, both beneficial and deleterious. This review, which covers the literature to mid 1986, is concerned with their occurrence in plants and their effects when consumed by animals and man. After a short discussion on the nature, occurrence, and biosynthesis of saponins, during which the distinction between steroidal and triterpenoid saponins is made, the structures of saponins which have been identified in a variety of plants used as human foods, animal feedingstuffs, herbs, and flavorings are described. Many of these compounds have been characterized only during the last 2 decades, and modern techniques of isolation, purification, and structural elucidation are discussed. Particular consideration is given to mild chemical and enzymatic methods of hydrolysis and to recent developments in the application of NMR and soft ionization MS techniques to structural elucidation. Methods currently used for the quantitative analysis of saponins, sapogenols, and glycoalkaloids are critically considered; advances in the use of newer methods being emphasized. The levels of saponins in a variety of foods and food plants are discussed in the context of the methods used and factors affecting these levels, including genetic origin, agronomic, and processing variables, are indicated. Critical consideration is given to the biological effects of saponins in food which are very varied and dependent upon both the amount and chemical structure of the individual compounds. The properties considered include membranolytic effects, toxic and fungitoxic effects, adverse effects on animal growth and performance, and the important hypocholesterolemic effect. A final section deals briefly with the pharmacological effects of saponins from ginseng, since use of this plant is increasing in certain sections of western society as well as being traditional in the Orient.

The chemistry and biological significance of saponins in foods and feedingstuffs.

Protein and Energy as an Integrated System. Relationship of Ruminal Protein and Carbohydrate Availability to Microbial Synthesis and Milk Production

Growth is defined as an increase in tissue mass. Mass increases by hyperplasia early in life and hypertrophy later in life, although hyperplasia of adipose tissue continues throughout life. The growth curve, being mass or cumulative weight plotted against age, is sigmoid, consisting of a prepubertal accelerating phase plus a postpubertal decelerating phase. Mathematically, this curve can be described as a function of mature mass, fractional growth rate, and age. At a specific fraction of mature mass, body composition seems to be constant, but the degree to which nutrition can alter mature mass is not certain. If mature mass is altered, body composition at any given mass will be altered. Mature mass can be decreased by starvation or protein deficiency early in life. Alternatively, retarding the deposition of fat or the administration of estrogenic compounds may increase mature protein mass. Many of the advances in rate and efficiency of growth and in reduced fat of meat cuts can be explained by increased mature protein mass of ruminants. Animals with higher mature weight require more energy for maintenance and reach puberty later in life, so a larger mature mass is not desirable for the breeding herd. Indeed, smaller replacement heifers would prove economical if reproduction were not decreased. A period of restricted growth and fat deposition (as on pasture) can increase the slaughter weight of small cattle into a more desirable range, presumably through increasing mature protein mass. However, calves with retarded growth often make less efficient feedlot gains than do calves finished immediately after being weaned. For growing large-framed heifers, pasture alone often provides an inadequate energy supply for early puberty, but excessive amounts of supplemental feed can enhance fat deposition in the udder, which subsequently decreases milk production. By manipulating the supply of specific nutrients and hormones, it may prove feasible in the future to reduce fat deposition in specific tissues and to alter mature body protein mass.

Factors that alter the growth and development of ruminants.

The impact of respiratory disease during a 150-d feedlot finishing period on daily gain, carcass traits, and longissimus tenderness was measured using 204 steer calves. Feedlot health status was monitored in two ways. First, clinical signs of respiratory infection were evaluated each day; treatment with antibiotic was based on degree of fever (if rectal temperature exceeded 40 degrees C then calves were treated). Steers that were treated (n = 102) had lower (P<.05) final live weights, ADG, hot carcass weights (HCW), less external and internal fat, and more desirable yield grades. Steers that were treated had a higher prevalence of carcasses that graded U.S. Standard than steers that were never treated. Second, as an alternative index of health status, lungs of all steers were evaluated at the processing plant using a respiratory tract lesion classification system; this health index included presence or absence of preexisting pneumonic lesions in the anterioventral lobes plus activity of the bronchial lymph nodes (inactive vs active). Lung lesions were present in 33% of all lungs and were distributed almost equally between treated (37%) and untreated cattle (29%). Steers with lesions (n = 87) had lower (P<.05) daily gains, lighter HCW, less internal fat, and lower marbling scores than steers without lesions. Compared to steers with lesions but inactive bronchial lymph nodes (n = 78), steers with lung lesions plus active lymph nodes had lower (P<.01) ADG and dressing percentage. Longissimus shear force values for steaks aged 7 d were lower (P = .05) from steers without lung lesions than those for steaks from steers with lung lesions. Overall, morbidity suppressed daily gains and increased the percentage of U.S. Standard carcasses. Compared to health assessment by clinical appraisal (based on elevated body temperature), classification based on respiratory tract lesions at slaughter proved more reliable statistically and, thereby, more predictive of adverse effects of morbidity on production and meat tenderness.

Health of finishing steers: effects on performance, carcass traits, and meat tenderness.

External and internal markers for appraising site and extent of digestion in ruminants

Ytterbium Chloride as a Marker for Particulate Matter in the Rumen

Two hundred fifty-six British and Continental crossbred yearling steers (329 kg initially) were fed to study the effect of time on feed on live performance, carcass characteristics, tissue composition, and steak tenderness. Steers were divided into four slaughter groups and fed for either 105, 119, 133, or 147 d. Daily gains (carcass weight-adjusted basis) increased in a quadratic manner (P < .05; maximum at 119 d), whereas feed intake tended to increase linearly (P < .08) as cattle were fed longer. Feed conversion (carcass weight-adjusted basis) for steers fed 119 d was superior to that of steers fed for 147 d. Carcass weight, s.c. fat thickness, kidney, pelvic, and heart fat, overall carcass maturity, and yield grade increased linearly (P < .01) with time on feed. Marbling score and the percentage of cattle grading U.S. Choice increased (linearly; P < .01) with time on feed but at a decreasing rate (quadratic; P < .05). Cholesterol and total lipid concentrations in longissimus muscle increased linearly (P < .01) as time on feed increased, whereas the percentage of protein and moisture tended to decrease linearly (P < .10 and P < .01). Tenderness of ribeye steaks tended to increase linearly (P < .07) with time on feed, primarily due to a linear decrease (P < .03) in the percentage of steaks considered tough (shear force above 4.50 kg). Performance and carcass characteristics indicate that a feedlot finishing period of 119 to 133 d is ideal for British Continental crossbred yearling steers (329 kg).

Fredric N. Owens

Papers

Factors that alter the growth and development of ruminants.

Health of finishing steers: effects on performance, carcass traits, and meat tenderness.

External and internal markers for appraising site and extent of digestion in ruminants

Ytterbium Chloride as a Marker for Particulate Matter in the Rumen

Effect of time on feed on performance of feedlot steers, carcass characteristics, and tenderness and composition of longissimus muscles