Nitrogen and carbon flows between the caecum, blood and rumen in sheep given chopped lucerne (Medicago sativa) hay

A mechanistic model for predicting intake of forage diets by ruminants

Accurate voluntary feed intake (VFI) prediction is critical to the productivity and profitability of ruminant livestock production systems. Simple empirical models have been used to predict VFI for decades, but they are inflexible, restrictive, and poorly accommodate many feeding conditions, such as those of developing countries. We have developed a mechanistic model to predict VFI over a range of forage diets (low- and high-quality grasses and legumes) by wild and domestic ruminants of varying physiological states (growth, lactation, gestation, nonproductive). Based on chemical reactor theory, the model represents the reticulorumen, large intestine, and blood plasma as continuous stirred-tank reactors and the small intestine as a plug flow reactor. Predicted VFI is that which 1) fulfills an empirical relationship between chemostatic and distention feedback observed in the literature, and 2) leads to steady-state conditions. Agreement between observed and actual VFI was great (generally R(2) >0.9, root mean square prediction error <1.4 kg/d, CV <25%). Root mean square prediction error for our model was only 67% that of the Beef NRC (2000) model, the leading empirical prediction system for cattle. Together, these results demonstrate that our model can predict ruminant VFI more broadly and accurately than prior methods and, by consequence, serve as a crucial tool to ruminant livestock production systems.

Effects of diet quality on urea fates in sheep as assessed by refined, non-invasive [15N15N]urea kinetics

The effect of diet quality on urea production, entry into the gastrointestinal tract (GIT) and subsequent diversion to anabolic or catabolic fates was examined in four sheep (mean live weight 49.5 kg). The animals received, in a crossover design, each of two rations, hay-grass pellets (1:1 HG) and a mixed concentrate-forage (CF). Measurements were made of N balance and urea kinetics based on a 4 d continuous intravascular infusion of [15N15N]urea. Enrichments of [15N15N]- and [14N15N]urea in the urine, and faecal 15N content were determined each day. After 24 h of infusion, urinary [15N15N]urea enrichments reached constant enrichment but a further 24 h was required before [14N15N]urea enrichment was at plateau. The latter is derived from hydrolysis of urea to 15NH3 in the digestive tract with subsequent absorption and reconversion to urea. The diets were not isonitrogenous (14.3 v. 17.1 g N supplied daily for HG and CF respectively) but showed no difference in N balance. Urea-N production was much greater (16.3 v. 11.1 g/d; P = 0.011) for CF compared with HG and more urea-N entered the GIT (9.9 v. 7.7; P = 0.07). A larger proportion of GIT entry was returned to ureagenesis (51 v. 42%; P = 0.047) for the CF diet but a smaller fraction was lost in the faeces (3.3% v. 7.1%; P = 0.013). In consequence, most of the additional urea-N which entered the GIT on the CF diet was returned to the ornithine cycle (probably as NH3) and the absolute amount available for anabolic purposes was similar between the rations (3.9 v. 4.5 g N/d).

Protein metabolism in the rumen of silage-fed steers: effect of fishmeal supplementation

1. Ryegrass (Lolium perenne cv. Cropper) silage was given to four Friesian steers (initial live weight (LW) 172 kg) alone or with a fishmeal supplement (150 g fresh weight/kg silage dry matter (DM)) in a balanced two-period change-over design. The dietary components were the same as those used in a recent experiment by Gill et al. (1987). All diets were offered hourly at 24 g DM/kg LW. 2. Fishmeal supplementation increased dietary nitrogen intake (P less than 0.01) and significantly increased the flow of total N (P less than 0.01), non-ammonia N (NAN) (P less than 0.01) and amino acids (P less than 0.05) at the duodenum. The increased supply of NAN to the duodenum was due largely (67%) to increased flow of undergraded dietary protein. 3. Microbial protein production was estimated simultaneously with 15N, diaminopimelic acid (DAPA) and a novel technique using L-[4,5-3H]leucine. Estimates varied with the marker and source of microbial isolate but mean values indicated that microbial N flow was significantly increased by fishmeal supplementation (P less than 0.05). The use of L-[4,5-3H]leucine as a microbial marker is justified and its possible advantages over other markers are discussed. 4. The efficiency of microbial protein synthesis was significantly increased from 30.8 g N/kg organic matter apparently digested in the rumen (OMADR) to 54.3 g N/kg OMADR by fishmeal supplementation (P less than 0.01). However, this indicates that relatively high efficiencies can be achieved with unsupplemented high quality silage supplied continuously. Rumen degradable N (RDN) supply was significantly increased by fishmeal supplementation (P less than 0.05) but apparent efficiency of capture of RDN by rumen microbes was not significantly increased. 5. Attempts were made to investigate the source of N utilized by the microbes on the two diets by intrarumen infusions of (15NH4)2SO4 and L-[4,5-3H]leucine but these were confounded by rumen-mixing problems. Findings obtained suggest that a lower proportion of microbial N may have been derived from rumen ammonia when the silage was supplemented with fishmeal but no differences in the extent of direct incorporation of leucine into microbial protein were observed. This could indicate an increase in microbial peptide uptake on the fishmeal-supplemented diet. However, evidence was also obtained suggesting that the improvement in microbial protein synthetic efficiency with supplementary fishmeal was also due to the provision of a more continuous supply of nitrogenous substrates for microbial growth, as a result of hourly feeding.(ABSTRACT TRUNCATED AT 400 WORDS)

Ruminal nitrogen metabolism in steers as affected by feed intake and dietary urea concentration

Four multiple-cannulated steers (340 kg) were used in a 4 X 4 Latin square design with a 2 X 2 factorial arrangement of treatments. Steers were fed a diet of 50% ground hay and 50% concentrate at two intakes (1.4 and 2.1% of BW), with urea and 15N-enriched ammonium sulfate infused continuously into the rumen at .4 or 1.2% of diet DM. Ratios of purines and diaminopimelic acid-N to N in fluid-associated and particulate-associated bacteria and in protozoa were similar among treatments but were lower for protozoa than for bacteria. Diaminopimelic acid-N:N was higher for fluid-associated vs. particulate-associated bacteria. Enrichment of 15N was similar between bacteria among treatments and was 30% lower for protozoa. Turnover rates of 15N in bacteria, NH3N, and non-NH3N pools were faster for steers infused with 1.2 than those infused with .4% urea, indicating less efficient usage of ammonia with higher urea. A method is described to estimate the proportion of duodenal nitrogen comprising bacterial and protozoal nitrogen.