The Cornell Net Carbohydrate and Protein System (CNCPS) has a submodel that predicts rates of feedstuff degradation in the rumen, the passage of undegraded feed to the lower gut, and the amount of ME and protein that is available to the animal. In the CNCPS, structural carbohydrate (SC) and nonstructural carbohydrate (NSC) are estimated from sequential NDF analyses of the feed. Data from the literature are used to predict fractional rates of SC and NSC degradation. Crude protein is partitioned into five fractions. Fraction A is NPN, which is trichloroacetic (TCA) acid-soluble N. Unavailable or protein bound to cell wall (Fraction C) is derived from acid detergent insoluble nitrogen (ADIP), and slowly degraded true protein (Fraction B3) is neutral detergent insoluble nitrogen (NDIP) minus Fraction C. Rapidly degraded true protein (Fraction B1) is TCA-precipitable protein from the buffer-soluble protein minus NPN. True protein with an intermediate degradation rate (Fraction B2) is the remaining N. Protein degradation rates are estimated by an in vitro procedure that uses Streptomyces griseus protease, and a curve-peeling technique is used to identify rates for each fraction. The amount of carbohydrate or N that is digested in the rumen is determined by the relative rates of degradation and passage. Ruminal passage rates are a function of DMI, particle size, bulk density, and the type of feed that is consumed (e.g., forage vs cereal grain).

A net carbohydrate and protein system for evaluating cattle diets: II. Carbohydrate and protein availability.

Increasing atmospheric concentrations of methane have led scientists to examine its sources of origin. Ruminant livestock can produce 250 to 500 L of methane per day. This level of production results in estimates of the contribution by cattle to global warming that may occur in the next 50 to 100 yr to be a little less than 2%. Many factors influence methane emissions from cattle and include the following: level of feed intake, type of carbohydrate in the diet, feed processing, addition of lipids or ionophores to the diet, and alterations in the ruminal microflora. Manipulation of these factors can reduce methane emissions from cattle. Many techniques exist to quantify methane emissions from individual or groups of animals. Enclosure techniques are precise but require trained animals and may limit animal movement. Isotopic and nonisotopic tracer techniques may also be used effectively. Prediction equations based on fermentation balance or feed characteristics have been used to estimate methane production. These equations are useful, but the assumptions and conditions that must be met for each equation limit their ability to accurately predict methane production. Methane production from groups of animals can be measured by mass balance, micrometeorological, or tracer methods. These techniques can measure methane emissions from animals in either indoor or outdoor enclosures. Use of these techniques and knowledge of the factors that impact methane production can result in the development of mitigation strategies to reduce methane losses by cattle. Implementation of these strategies should result in enhanced animal productivity and decreased contributions by cattle to the atmospheric methane budget.

Methane emissions from cattle.

The Cornell Net Carbohydrate and Protein System (CNCPS) has a kinetic submodel that predicts ruminal fermentation. The ruminal microbial population is divided into bacteria that ferment structural carbohydrate (SC) and those that ferment nonstructural carbohydrate (NSC). Protozoa are accommodated by a decrease in the theoretical maximum growth yield (.50 vs .40 g of cells per gram of carbohydrate fermented), and the yields are adjusted for maintenance requirements (.05 vs .150 g of cell dry weight per gram of carbohydrate fermented per hour for SC and NSC bacteria, respectively). Bacterial yield is decreased when forage NDF is < 20% (2.5% for every 1% decrease in NDF). The SC bacteria utilize only ammonia as a N source, but the NSC bacteria can utilize either ammonia or peptides. The yield of NSC bacteria is enhanced by as much as 18.7% when proteins or peptides are available. The NSC bacteria produce less ammonia when the carbohydrate fermentation (growth) rate is rapid, but 34% of the ammonia production is insensitive to the rate of carbohydrate fermentation. Ammonia production rates are moderated by the rate of peptide and amino acid uptake (.07 g of peptide per gram of cells per hour), and peptides and amino acids can pass out of the rumen if the rate of proteolysis is faster than the rate of peptide utilization. The protein-sparing effect of ionophores is accommodated by decreasing the rate of peptide uptake by 34%. Validation with published data of microbial flow from the rumen gave a regression with a slope of .94 and an r2 of .88.

A net carbohydrate and protein system for evaluating cattle diets: I. Ruminal fermentation.

Nitrogen Metabolism in the Rumen

In addition to the bacteria in the rumen there are many larger (5–250/mm long) organisms which at various times have been designated protozoa Of these the ‘ovals’ (Quin’s and Eadie’s) are now known to be large bacteria (Orpin, 1976) and the ‘flagellates’ Neocallimastix frontalis, Piromonas communis and Sphaeromonas communis are the zoospores of phycomycete fungi (Orpin, 1977a, b) There are genuine flagellates in the rumen, eg Trichomonas spp, Monoceromonas sp and Chilomastix sp, but almost nothing is known about their metabolism (Jensen and Hammon, 1964) The largest, most obvious and most important protozoa are the ciliates, of which there are two groups both in the subclass Trichostomatia The so called ‘holotrich’ protozoa belong to the order Vestibuliferida and the entodiniomorphs to the order Entodiniomorphida, suborder Entodiniomorphina and family Ophryoscolecidae As the properties and metabolism of these two protozoal groups are different, they will be considered separately below

The Rumen Protozoa

Twelve grain mixtures, one lucerne (Medicago sativa) hay and one maize silage which had been used in mixed diets for which dietary nitrogen undegraded in the rumen (UDN) had been estimated with duodenally-cannulated cows, were studied. Total N in the feeds was fractionated into pool A (N soluble in borate-phosphate buffer), pool B (total N--(pool A + pool C)) and pool C (acid-detergent-insoluble N or residual N after 24 h incubation in protease solution). N solubilization in protease solution containing 6.6 units/ml (substrate-saturating enzyme concentration) indicated the presence of subfractions in pool B, with different rates of solubilization. Such subfractions were not detectable from in situ, Dacron bag, estimates of N solubilization. UDN was estimated using a dynamic mathematical model and rate-constants obtained from N solubilization in protease solution or in situ. For three grain mixtures tested using the protease technique the model predicted UDN values of 7, 10 and 12% compared with values of 47, 66 and 59% estimated in vivo. The full range of experimental feeds was tested using the in situ technique and UDN values predicted by the model were used to derive UDN values for twelve mixed diets. The latter values were significantly but not closely correlated with those determined in vivo (r2 0.41, P less than 0.05). An attempt was made to simulate rumen proteolysis in vitro by choosing a protease enzyme concentration (0.066 units/ml) providing a proteolytic activity similar to that of whole rumen fluid. The experimental samples of feed were subjected to simulated rumen proteolysis for 18 or 48 h to resemble the mean retention times in the rumen for grain mixtures and roughages respectively. The residual N at the end of incubation was considered as an estimate of UDN. The UDN values estimated from simulated rumen proteolysis and those determined in vivo for twelve mixed diets were in close agreement (r2 0.61, P less than 0.01). Simulated rumen proteolysis can serve as a simple, rapid and sensitive method to estimate UDN in a variety of feedstuffs.

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Evaluation of a mathematical model of rumen digestion and an in vitro simulation of rumen proteolysis to estimate the rumen-undegraded nitrogen content of feedstuffs

Four experiments were conducted to examine the effect of various factors on in situ digestion of feedstuffs in the rumen of Holstein cows. In Exp. 1, the effect of various days, animals and animal diets on in situ digestion was evaluated. Measured disappearance of dry matter (DM) and nitrogen (N) from soybean meal (SBM) suspended in dacron bags was lowest when the host animal was fed a high grain diet. No effect due to animals, day of incubation or period of experimentation was observed. The effect of bag porosity and substrate particle size on DM and N disappearance from bags was studied in Exp. 2. Rip-stop nylon, a material with small pores, appeared to decrease the influx of digesting agents into bags and limited efflux of digested residues from bags when compared with bags made from dacron polyester material. Disappearance of DM and N from SBM and distillers grains (DG) in rip-stop nylon bags was greater when these feedstuffs were in the pulverized form rather than in the commercially processed form. In Exp. 3, dacron and Acropore bags, with pore sizes of 52 and 5 microns, respectively, were used to examine DM and N disappearance, in the rumen of cows, from total mixed diets containing various dietary N sources. Lower DM and N disappearance from Acropore bags was observed for each of the diets regardless of time spent in the rumen; much of the difference was established within the first hour. The effect of formaldehyde treatment on degradation of amino acids from SBM suspended in dacron bags in the rumen for 16 h was studied in Exp. 4. Increased resistance of SBM amino acids to degradation in the rumen was observed as a result of formaldehyde treatment levels as low as .3% by weight. A possible increase in protection of lysine from degradation relative to other amino acids was also observed.

Factors Affecting Disappearance of Feedstuffs from Bags Suspended in the Rumen

Four lactating Holstein cows fitted with a rumen cannula and T-type cannulae in the duodenum and ileum were used in an experiment with a 4 X 4 Latin square arrangement of treatments. Diets containing soybean meal (SBM), corn gluten meal (CGM), wet brewers grains (WBG) and distillers dried grains with solubles (DDG) were compared with regard to protein degradability in the rumen and amino acid flow and absorption in the small intestine. The test protein sources provided about 50% of the dietary protein. Spot samples of digesta were collected during a 96-h period and lanthanum (La) was used as an indigestible marker to estimate flow and digestibility of nutrients. Apparent organic matter digestibility in the rumen was 31 +/- 4.8%, while true digestibility based on measurements of microbial organic matter averaged 55 +/- 5.1%. Ruminal ammonia-nitrogen (NH3-N) concentrations (mg/100 ml) were 10.1, 5.6, 6.9 and 6.4 for the SBM, CGM, WBG and DDG diets, respectively. Using diaminopimelic acid as a microbial marker, protein degradation in the rumen was higher for the SBM diet (70%) than for the CGM (45%), WBG (52%) and DDG (46%) diets. Flow of amino acids to the small intestine was generally higher than amino acid intake for all diets. Apparent absorption of amino acids from the small intestine was 70, 77, 71 and 66% of the amino acids entering the duodenum for the SBM, CGM, WBG and DDG diets, respectively. Actual amounts absorbed (g/d) were lowest for the SBM diet. It was concluded that diets containing CGM, WBG or DDG will generally supply more total amino acids to the intestine than a diet containing SBM. Net amounts of amino acids available for absorption in the intestine were also higher for these diets because protein availability in the intestine was not impaired even though protein was more resistant to microbial breakdown in the reticulorumen.

M.D. Stern

Papers

Evaluation of a mathematical model of rumen digestion and an in vitro simulation of rumen proteolysis to estimate the rumen-undegraded nitrogen content of feedstuffs

Factors Affecting Disappearance of Feedstuffs from Bags Suspended in the Rumen

Protein degradation in the rumen and amino acid absorption in the small intestine of lactating dairy cattle fed various protein sources.

Ruminal Protein Degradation of Corn Gluten Meal in Lactating Dairy Cattle Fitted with Duodenal T-Type Cannulae

Effect of Ruminal Protein Degradation of Blood Meal and Feather Meal on the Intestinal Amino Acid Supply to Lactating Cows