Nitrogen partitioning and isotopic discrimination are affected by age and dietary protein content in growing lambs

Abstract

It is difficult to separate an age-dependent fall in nitrogen use efficiency (NUE; N balance/N intake) in growing ruminants from a progressively decrease in animal protein requirements over time. This study examined the effect of dietary protein content on N partitioning, digestibility and N isotopic discrimination between the animal and its diet (Δ15Nanimal-diet) evaluated at two different fattening periods (early v. late). Twenty-four male Romane lambs (age: 19 ± 4.0 days; BW: 8.3 ± 1.39 kg) were equally allocated to three dietary CP treatments (15%, 17% and 20% CP on a DM basis). Lambs were reared with their mothers until weaning, thereafter housed in individual pens until slaughter (45 kg BW). During the post-weaning period, lambs were allocated twice (early fattening (30 days post-weaning) and late fattening (60 days post-weaning)) to metabolic cages for digestibility and N balance study. When diet CP content increased, the average daily gain of lambs increased (P < 0.05) while the age at slaughter decreased (P = 0.01), but no effect was observed on feed efficiency (P > 0.10). Diet CP content had limited effect on lamb carcass traits. Higher fibre digestibility was observed at the early v. late fattening period (P < 0.001). The N intake and the urinary N excretion increased when diet CP content increased (P < 0.001) and when shifting from early to late fattening period (P < 0.001). Faecal N excretion (P = 0.14) and N balance (P > 0.10) were not affected by diet CP content. Nitrogen digestibility increased (P < 0.001) as the diet CP content increased and on average it was greater at late v. early fattening period (P = 0.02). The NUE decreased (P = 0.001) as the diet CP content increased and as the lamb became older (P < 0.001). However, the age-dependent fall in NUE observed was lower at high v. low dietary CP content (CP × age interaction; P = 0.04). The Δ15Nanimal-diet was positively correlated (P < 0.05) with N intake (r = 0.59), excretion of faecal N (r = 0.41), urinary N (r = 0.69) and total manure N (r = 0.64), while negatively correlated with NUE (r = −0.57). Overall, the experiment showed NUE was lower in older lambs and when lambs were fed high diet CP content, and that Δ15Nanimal-diet was a useful indicator not only for NUE but also for urinary N excretion, which is a major environmental pollution factor on farm.

Relationship Between Nitrogen Isotopic Discrimination and the Proportion of Dietary Nitrogen Excreted in Urine by Sheep Offered Different Levels of Dietary Non-Protein Nitrogen

“…Moreover, when NI increased, manure N increased, which agrees with studies conducted in lactating dairy cows (Colmenero and Broderick, 2006;Kidane et al, 2018) and in mature rams Khanaki et al, 2021). The UN/NI ranged between 0.3 and 0.4 g/g, which was lower than reported by Brand et al (1992) and within the range observed in other studies (Bernard et al, 2020;Khanaki et al, 2021) increasing NI. This result is consistent with the other reports (Niu et al, 2016;Kidane et al, 2018), which showed that as animals consume more N, there is less of an increase in FN than UN.…”

“…In contrast, the main source of N in the urine is urea, which is reported to be depleted in 15 N (Steele and Daniel, 1978;Cheng et al, 2016). The range of plasma D 15 N values in this study is higher than the range stated in previous studies (Cheng et al, 2013a;Cheng et al, 2016;Bernard et al, 2020;Khanaki et al, 2021). We have no explanation for the differences between these previous studies and our current study, as all sheep were healthy and unstressed throughout the experiment.…”

“…Previous studies showed that digestion, rumen function, and metabolism may differ between large and small ruminants (Lapierre and Lobley, 2001;Doreau et al, 2003;Kawashima et al, 2007). Since most cited studies are with large ruminants, more research is still required (Lavery and Ferris, 2021;Khanaki et al, 2021) to explore this relationship, particularly as only three studies can be found (Cheng et al, 2013a;Bernard et al, 2020;Khanaki et al, 2021). The NI measurement is hard to measure in grazing systems, though it is needed to quantify N use efficiency (NUE) and UN/NI.…”

Dietary Supplementation of Yeast Culture Into Pelleted Total Mixed Rations Improves the Growth Performance of Fattening Lambs

“…In most YC supplement studies where even yeast is active, slaughter performance does not change (5,65). An increased ADG, as occurred in this study, does not necessarily translate to improved slaughter performance (66,67). Another reason is that in most slaughter trials, six to eight lambs are applied to detect treatment differences, which do not have enough power to detect a moderate response.…”

Effect of Dietary Crude Protein on Productive Efficiency, Nutrient Digestibility, Blood Metabolites and Gastrointestinal Immune Markers in Light Lambs

“…This represents a dietary CP supply between 16.2% and 15.1% on a DM basis, respectively [7]. Recently, it has been observed that selected Romane lambs (with ADG > 320 g/day) can respond to increases up to 20% of dietary CP (on a DM basis) by improving their apparent CP digestibility and growth performance, although their FCR is similar to lambs fed 15% CP [28]. Thus, our findings suggest that FCR remains steady when the dietary CP is kept within the range of 13% to 18% on a DM basis, in agreement with studies performed in other breeds [25,28,29].…”