Abstract
Nitrogen (N) isotopic discrimination (i.e. the difference in natural 15N abundance between the animal proteins and the diet; Δ15N) is known to correlate with N use efficiency (NUE) and feed conversion efficiency (FCE) in ruminants. However, results from the literature are not always consistent across studies, likely due to isotopic discrimination pathways that may differ with the nature of diets. The objective of the present study was to assess at which level, from rumen to tissues, Δ15N originates and becomes related to NUE and FCE in fattening yearling bulls when they are fed two contrasted diets. Twenty-four Charolais yearling bulls were randomly divided into two groups and fed during 8 months, from weaning to slaughter, either 1) a high starch diet based on corn silage supplying a balanced N to energy ratio at the rumen level (starch) or 2) a high fiber diet based on grass silage supplying an excess of rumen degradable N (fiber). All animals were slaughtered and samples of different digestive pools (ruminal, duodenal, ileal and fecal contents), animal tissues (duodenum, liver and muscle), blood and urine were collected for each animal. Ruminal content was further used to isolate liquid-associated bacteria (LAB), protozoa and free ammonia, while plasma proteins were obtained from blood. All samples along with feed were analyzed for their N isotopic composition. For both diets, the digestive contribution (i.e. the N isotopic discrimination occurring before absorption) to the Δ15N observed in animal tissues accounted for 65 ± 11%, leaving only one third to the contribution of post-absorptive metabolism. Concerning the Δ15N in digestive pools, the majority of these changes occurred in the rumen (av. Δ15N = 2.12 ± 0.66‰), with only minor 15N enrichments thereafter (av. Δ15N = 2.24 ± 0.41‰), highlighting the key role of the rumen on N isotopic discrimination. A strong, significant overall relationship (n = 24) between Δ15N and FCE or NUE was found when using any post-absorptive metabolic pool (duodenum, liver, or muscle tissues, or plasma proteins; 0.52 < r < 0.73; P ≤ 0.01), probably as these pools reflect both digestive and post-absorptive metabolic phenomena. Fiber diet compared to starch diet had a lower feed efficiency and promoted higher (P ≤ 0.05) Δ15N values across all post-absorptive metabolic pools and some digestive pools (ruminal, duodenal, and ileal contents). The within-diet relationship (n = 12) between Δ15N and feed efficiency was not as strong and consistent as the overall relationship, with contrasted responses between the two diets for specific pools (diet x pool interaction; P ≤ 0.01). Our results highlight the contrasted use of N at the rumen level between the two experimental diets and suggests the need for different equations to predict FCE or NUE from Δ15N according to the type of diet. In conclusion, rumen digestion and associated microbial activity can play an important role on N isotopic discrimination so rumen effect related to diet may interfere with the relationship between Δ15N and feed efficiency in fattening yearling bulls.
Highlights
Improving N use efficiency (NUE), the animal’s ability to transform feed N into animal proteins, is a key concern in animal production
In this regard, when the available N is in excess, the resulting increase in rumen ammonia concentration [14] is associated to higher Δ15N values in the rumen microbes [12, 15] and in animal proteins [15]
Yearling bulls were divided into two homogenous groups according to body weight and age and randomly allocated to two diets: 1) a high starch diet based on corn silage supplying a balanced N to energy ratio at the rumen level and so leading to an estimated rumen protein balance close to 0 or 2) a high fiber diet based on grass silage supplying rumen degradable N in excess and so leading to a highly positive rumen protein balance
Summary
Improving N use efficiency (NUE), the animal’s ability to transform feed N into animal proteins, is a key concern in animal production. The enzymatic isotopic discrimination results in a relatively higher excretion of 14N in the end-product (urea) and higher retention of 15N in body proteins [6] As these enzymes are involved in amino acid catabolism and ureagenesis [8], Δ15N is biologically linked to NUE. Variable Δ15N values have been reported in single cell organisms ranging from negative [11, 12] to positive [9] values, likely reflecting differences in the balance between N and energy supplied by substrates [11, 13] In this regard, when the available N is in excess, the resulting increase in rumen ammonia concentration [14] is associated to higher Δ15N values in the rumen microbes [12, 15] and in animal proteins [15]. Microbial activity in the rumen could contribute to the diet-dependent relationship previously found between Δ15N and NUE [4]
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