The effect of dietary non-protein nitrogen content on the performance of finishing lambs

 

 

O.B. Einkamerer^I, ; A.V. Ferreira^II; M.D. Fair^I; A. Hugo^I

^IDepartment of Animal Science, P.O. Box 339, University of the Free State, Bloemfontein, 9300, South Africa
^IIPresent address: Molatek, P.O. Box 47, Malelane, 1320, Republic of South Africa

 

 

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ABSTRACT

The effect of increasing non-protein nitrogen content on nutrient digestibility, performance, and carcass characteristics of South African Mutton Merino wether lambs on low-fibre finishing diets containing similar degradable protein was investigated. The production and digestibility studies consisted of 60 and 32 lambs, respectively. Four similar dietary treatments were formulated with different non-protein nitrogen contents (16.6 g/kg, 28.3 g/kg, 40 g/kg and 51.7 g/kg) on a dry matter basis. The production study was conducted over 71 d, and the digestibility study over 7 d. Crude protein digestibility increased with non-protein nitrogen content. Organic matter (40 g/kg and 51.7 g/kg), metabolizable energy content (51.7 g/kg), as well as the digestibility of neutral detergent fibre (40 g/kg) and acid detergent fibre (28.3 g/kg and 51.7 g/kg) also increased with a higher non-protein nitrogen content. A lower non-protein nitrogen content within the 16.6 g/kg and 28.3 g/kg treatments resulted in a higher average daily gain compared to the 51.7 g/kg treatment. Additionally, more effective energy was used for growth compared to treatments with 40 g/kg and 51.7 g/kg non-protein nitrogen. Dietary treatment left most of the carcass characteristics unaffected. In conclusion, a low non-protein nitrogen content of low-fibre lamb finishing diets with similar degradable protein content affected animal performance favourably, despite having an opposite effect on nutrient digestibility.

Keywords: carcass, growth, prime gluten meal, soybean meal, urea

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Introduction

Body tissues and cells require protein for growth, development, and maintenance. Nitrogen has a crucial role in the synthesis of hormones, enzymes, and other essential molecules (Zhao et al., 2023). There are more than 700 amino acids in nature, but only 20 of them are building blocks for proteins in animal cells, some of which cannot be synthesized by some animal species (Wu et al., 2014). Essential amino acids are defined as either those whose carbon skeletons cannot be synthesized or insufficiently synthesized de novo in animal cells relative to its needs for maintenance, growth, development, and health. These amino acids must then be provided in the diet to meet requirements (Hou et al., 2015). Optimizing the efficiency of dietary crude protein requires selection of complementary feed protein supplements to provide the types and amounts of protein, which will meet, but not exceed, the nitrogen needs of rumen microorganisms and that of the animal (NRC, 2001). Over a century of ruminant nutrition research has been devoted to understanding nitrogen metabolism, processes, and practices to increase utilization efficiency (Hristov et al., 2019).

Proteins provided to ruminants are composed of several sources, each with different properties that resist microbial degradation or are fermented at different rates (Polan, 1992). Protein quality and the requirement of the animal per se, are either metabolized in the rumen and expressed as degradable ingested protein, or not available for rumen microbial fermentation and referred to as undegradable ingested protein (NRC, 2007). The degradable ingested protein includes non-protein nitrogen (NPN) compounds (Kellems & Church, 2010). Ultimately, metabolizable protein is the most appropriate method to define animal protein requirements (NRC, 2007).

Urea is normally added to ruminant diets as an NPN source, which benefits animal production and saves on nitrogen costs (Hailemariam et al., 2021). Urea is hydrolysed to ammonia in the rumen by urease and used for microbial protein synthesis when enough carbohydrate is available (Mahmoudi-Abyane et al., 2020). Hence, microbes supply their own amino acid requirements (Ali et al., 2009), which become available to the host animal after they pass from the rumen to the small intestine (Cheeke, 2005). Sniffen & Robinson (1987), as well as Nichols et al. (2022), acknowledged that the protein requirements of ruminants can be met with the provision of urea as sole NPN source to the diet. Irrespective of protein quality, total nitrogen is generally considered to be the first limiting dietary component for the utilization of low-quality forages by microbes (Nolte et al., 2000). For example, milk production was maintained in cows when microbial protein was synthesised in the rumen using a purified carbohydrate diet with urea and ammonium salts as the only nitrogen sources, compared to cows on a protein-rich silage diet (Nichols et al., 2022). A greater by-pass protein source increases animal growth and muscle mass accretion compared to less by-pass protein. The inclusion of protein sources with the appropriate amino acid profiles catering to the needs of a growing animal results in greater growth performance and nitrogen utilization (El-Nomeary et al., 2021). Hoover (1986) noted that urea as sole nitrogen source is not sufficient since microorganisms lack specific amino acids. However, if a diet contains too much protein, the animal will waste energy by producing more ammonia than needed, which is then converted to urea and excreted (Milis & Liamadis, 2008). Hence, the quantity of protein provided is just as important as the source thereof (Sniffen & Robinson, 1987).

Even though there are inconsistencies in the literature, protein source and the quality thereof affects ruminant performance. Recently the optimum degradable versus undegradable protein ratio in ruminant diets was evaluated (Putri et al., 2021; Valizadeh et al., 2021). Even though protein research on ruminant nutrition was evaluated extensively, no literature could be found on the quantity of NPN within a finishing diet fed to lambs with similar degradable protein and possible effects on performance. Where the NPN content in a finishing diet is manipulated, the total degradable protein content thereof is also altered. Therefore, the aim of this study was to determine the effect of increasing dietary NPN content within a standard, low-fibre finishing diet with similar degradable protein content on nutrient digestibility and performance of lambs.

 

Materials and Methods

All procedures conducted during this study were approved by the Interfaculty Animal Ethics Committee for Animal Experimentation at the University of the Free State (Animal Experiment No. UFS-AED2016/0038).

The production and digestibility studies consisted of 60 and 32 South African Mutton Merino (SAMM) wether lambs, respectively; the SAMM is a dual-purpose (meat and wool) sheep breed (Neser et al., 2000). This breed was developed through selection for improved wool quality as well as carcass conformation, with high growth rate as the main characteristic (Skele et al., 2024).

In the production study of 71 d, lambs with a mean (SD) weight of 25.5 ± 2.6 kg were fasted overnight, weighed, stratified according to descending mass, and randomly allocated to four dietary treatments. This culminated in a randomised trial design with n = 15 lambs per treatment (n = 1 lamb per experimental unit). For the digestibility study of 7 d, 32 wether lambs with a mean (SD) weight of 43.5 ± 4.1 kg were randomly allocated to the same dietary treatments (n = 8 lambs per treatment, n = 1 lamb per experimental unit) representing the same trial design.

All animals were subjected to a standard health and vaccination program prior to the study as commonly practiced in the commercial feedlot sector of South Africa. The animals were injected with an antiparasitic remedy, dosed for tapeworm and inoculated against pulpy kidney, malignant oedema, blackquarter, tetanus, pasteurellosis, as well as pneumonic and septicaemic pasteurellosis. All animals were housed in pens (n = 1 lamb/pen; 1.404 m²) on elevated, wooden slatted floors within a naturally-ventilated building. The metabolic building was properly washed and disinfected before the onset of the study. Each pen was cleaned twice a week.

Four finishing diets were formulated to facilitate an incremental increase in the NPN content, but also to contain a similar nutrient composition (Table 1).

 

 

Feed grade urea was the protein source used to formulate the incremental NPN content of the diets. Soybean meal and prime gluten meal were included to ensure similar degradable, ingested protein content between treatments. The bypass protein potential of soybean meal is low (Cheeke, 2005) and is an example of a protein source with a high degradable, ingested protein content (NRC, 2007) with a high percentage of nitrogen present as true protein (±90%) (Kellems & Church, 2010). Urea is 100% degradable within the rumen (Soto-Navarro et al., 2003). Urea, prime gluten meal, and soybean meal contain on average 288%, 2.01% and 5.94% NPN, and 288%, 27.47% and 35.10% degradable ingested protein, respectively (NRC, 1996).

Treatments were described according to NPN content in ascending order, henceforth control (CON: 16.6 g/kg NPN), NPN1 (28.3 g/kg NPN), NPN2 (40 g/kg NPN), and NPN3 (51.7 g/kg NPN). All other chemical parameters were formulated to be similar. Treatment diets were fed to the animals in the processed mash form. No feed additives nor rumen modifiers, which may have affected the rumen environment, were included in the diets.

At the onset of the production study (day 0), lambs were subjected to a standard adaptation period. Lucerne hay was provided on an ad libitum basis where each respective treatment diet was fed to the lambs and increased incrementally 100 g/day/animal for 10 d. The animals were fed twice daily, at 08h00 and 16h00. Feed intake was recorded on a weekly basis, whereas total intake was used for statistical evaluation only.

The digestibility study of 7 d was conducted after adaption (10 d) to the treatment diets and faecal bags were fitted to the animals. The lambs were offered the same experimental diets as in the production study. To avoid variation in assessing the voluntary feed intake, a sequential method of feed allocation was followed by providing each animal a 15% refusal level of intake. Calculations were done daily by using a preceding three-day moving average of feed intake. The lambs were treated the same as during the production study and fed twice daily (08h00 and 16h00). Feed refusals (orts) were collected every morning just before the 08h00 feeding period. Faeces were collected twice daily for 7 d before feeding at 08h00 and 16h00. Each day's faecal output was amalgamated. Fresh, clean water was freely available to all the animals.

All samples were analysed for dry matter according to the Association of Official Analytical Chemists (AOAC) method 934.01 for chemical procedures (AOAC, 1990). The crude protein (method 990.03), ash (method 942.05), and organic matter were also analysed according to AOAC (1990), whereas non-structural carbohydrate content was determined as described by Van Soest et al. (1991). Neutral detergent fibre (aNDFom: Procedure A) analytical fraction was determined according to the method of Van Soest et al. (1991) using an ANCOM^200/220 Fibre Analyser (ANCOM Technology Corp., Fairport, NY, USA). Total acid detergent fibre (ADFom) was analysed according to AOAC method 973.18 (AOAC, 1990). Total lipid (ether extract) was extracted according to AOAC method 920.39 (AOAC, 1990). Gross energy (GE) was determined using a Leco® AC500 Isoperibol Calorimeter (Leco Corp., St. Joseph, MI) following ASTM (ASTM, 2009) standard D5865 (Cantrell et al., 2010). Metabolizable energy was calculated by multiplying the digestible energy content of the diet by a factor of 0.81 to account for energy losses via urine and fermentation gasses (McDonald et al., 2011). Non-protein nitrogen and degradable, ingested protein content of feeds were determined using published methods (NRC, 1996).

At the end of the production study, all lambs with a mean (SD) live weight of 49.1 ± 4.4 were slaughtered at a commercial abattoir after fasting overnight. Cold carcass weight was recorded 24 h after refrigeration at 2 °C to 4 °C according to the methods described by Fisher & De Boer (1994). The cold carcass weight was used to determine the dressing percentage (cold carcass weight divided by the empty stomach final live weight). The external length, shoulder circumference, and buttock circumference of each carcass was also recorded.

Meat evaluation was performed on the left side of each carcass. All carcasses were split between the 12^th and 13^th rib (thoracic vertebra) and fat depth was measured with a calliper (electronic digital calliper; Omni-Tech) 45 mm and 110 mm from the mid dorsal line (Carson et al., 1999). The area of the longissimus muscle (Muscuíus íongissimus dorsi) between the 12^th and 13^th rib was measured by tracing it directly onto transparent film (Edwards et aí., 1989). The traced outline was scanned with a scale bar and the eye muscle area measured using a video image analysis system (Soft Imaging System: Analysis® 3.0). The video image analyses system was calibrated with the scale bar.

Data were analysed as a completely randomized design using the General Linear Model (GLM) procedures in the Statistical Analysis System (SAS) program (SAS, 1999). Means were compared using the LSMEANS/DIFF with treatment as fixed effects. For post hoc analysis, Tukey's honestly significant difference (HSD) test was used to identify significant differences between treatment means at the 5% probability level. The description of the model used for ANOVA analysis was (Equation 1):

where Yij is the individual observations (dependent variable) of the i-th treatment (independent variable) and the j-th random error, μ is the general effect, ti is the effect of the i-th treatment, and εij is the random variation or experimental error. The i-th treatment effect (dietary NPN content) was defined and presented on a dry matter basis (Equation 2):

 

Results and Discussion

The effect of increasing NPN content in low-fibre finishing diets of wether lambs on apparent nutrient digestibility and metabolizable energy content are presented in Table 2. Treatment had no effect on the dry matter intake of the wethers in the digestibility study. The average dry matter intake of lambs fed the control diet (CON) was higher than in the other treatments. Even though the intake of treatments NPN1 to NPN3 was lower, it was not statistically lower than the control treatment. Voluntary intake therefore was not seen to influence apparent nutrient digestibility within the current study. A high intake decreases apparent total tract nutrient digestibility within ruminants due to a faster passage rate (shorter retention time) of digesta, and vice versa (McDonald et al., 2011).

There was a marked and increased effect of dietary treatment on organic matter (NPN2 and NPN3, compared to CON). This could, however, have been a result of NPN containing increased proportions of urea effecting faster ruminal nitrogen release. Generally, degradable true protein nitrogen is more slowly degraded in the rumen compared to an immediately-degraded protein source like urea. The nitrogen resulting from urea is incorporated into microbial protein less efficiently due to its speedy hydrolysis (McDonald et al., 2011). The NPN content of low-fibre finishing diets had no effect (P = 0.0958) on total dry matter digestibility (Table 2).

Crude protein digestibility increased (P = 0.0001) with NPN content in the similar degradable, ingested protein diets. Urea within the rumen is quickly released (Niazifar et al., 2024) and enhances fermentation (de Carvalho et al., 2020). Therefore, nitrogen digestibility increases when NPN is included in the diet (Kropp et al., 1977). Non-protein nitrogen compounds are highly soluble, rapidly converted to ammonia in the rumen and absorbed, thus yielding high digestibility values (Kellems & Church, 2010). This could be the case in the current study after increasing dietary urea content (NPN1, 2, and 3). The extent to which a feed protein is degraded depends on its inherent solubility in rumen liquid (Chalupa, 1975) and the time that it remains in the rumen (Dryden, 2008). Soto-Navarro et al. (2003) concluded that even with unlimited rumen ammonia from a concentrated diet, there are less benefits in microbial growth or digestion when using a true protein source (soybean meal) compared to an NPN source (urea).

Neutral detergent fibre digestibility was affected by NPN content (Table 2). An NPN content of 40 g/kg (NPN2) increased neutral detergent fibre digestibility compared to the CON (16.6 g/kg NPN). Similarly, acid detergent fibre digestibility increased substantially in treatments NPN1 (28.3 g/kg NPN) and NPN3 (51.7 g/kg NPN) compared to the CON. It is well documented that high levels of degradable protein enhance fibre fermentation considerably (Khandaker et al., 2012) and thus, increase fibre digestibility (Allen, 2000). The substantial increases in neutral and acid detergent fibre digestibility in the current study could therefore have been related to protein degradability. Hence, NPN as the degradable, ingested protein was comparable between diets. Belasco (1954) explained that at equivalent nitrogen levels, urea supported better cellulose breakdown than did any other protein meal (maize gluten, cottonseed, linseed, or soybean meal) of varying degradability, and was utilised more completely in an artificial rumen. The author showed that increasing urea from 1.3% to a maximum of 35% equivalent protein levels resulted in linear increases in cellulose digestion. Thus, within limits, the relationship between nitrogen supply, nitrogen utilisation, and cellulose breakdown was linear (Belasco, 1954). Polan (1992), however, stated that some rumen microorganisms were stimulated by more complex forms of nitrogen (amino acids or short peptides) than ammonia. Fibrobacter succinogenes (Matsui et aí., 1998) and Butyrivibrio fibrisoívens (French et aí., 2000) are cellulolytic organisms that prefer different nitrogen sources. Additionally, microorganisms acting on the non-structural (amylolytic) fraction derive ~65% of their nitrogen from amino acids or peptides, and the remainder from ammonia (NRC, 2007).

The digestibility of the ether extract fraction was affected by dietary treatment and increased with NPN1 (28.3 g/kg) and NPN2 (40 g/kg) compared to the CON (16.6 g/kg). There was no effect on ether extract digestibility with an NPN of 51.7 g/kg (NPN3). This effect of diet NPN content on ether extract digestibility is difficult to explain and requires more research.

Dietary treatment affected the metabolizable energy of the finishing diets in the current study. Comparable to organic matter digestibility, the metabolizable energy of treatment NPN2 was higher than the CON. This represented a 0.94% dietary urea inclusion (40 g/kg NPN). Nitrogen-free extract digestibility was not affected by the nature of the protein source (Milis & Liamadis, 2008) and this was reflected in the current study with no differences (P >0.05) in non-structural carbohydrate digestibility.

The effect of NPN in low-fibre finishing diets with similar degradable protein content on voluntary intake and performance of wether lambs are presented in Table 3.

Treatment had no effect on the dry matter intake (P = 0.2948) or metabolizable energy intake (P =0.7920) of finishing wether lambs (Table 3). The lack of treatment effect on metabolizable energy intake was not expected due to the significant effect of NPN on the metabolizable energy (Table 2). The dry matter intake corresponded with the lack of treatment effect (P = 0.3211) in the digestibility study (Table 2). The intake in the production study was, however, measured over a longer period (71 d). The lack of treatment effect on voluntary dry matter intake in the production study was not expected due to the possible influence of urea acceptability on voluntary intake of small stock (Dixon et al., 2003). Urea supplementation either has no effect or decreases dry matter intake in sheep (Wahyono et al., 2022). A lower intake of barley supplemented with urea presented to Merino wethers on low quality roughage was associated with an adverse flavour or a conditioned feed aversion to the urea supplement (Dixon et al., 2003). For finishing lambs, it is proposed that urea should be restricted to not more than 1% of the total diet because more than this could be unpalatable and decrease feed intake (Kellems & Church, 2010). Supplementing urea and molasses, however, increased dry matter intake of ram lambs, which was ascribed to higher dry matter and organic matter digestibility with the consequent reduction in the retention time of solid digesta in the reticulo-rumen (Can et al., 2004). This was not the case in the present study. The influence of urea inclusion on the dry matter intake of ruminants presented in literature is inconsistent and variable. In addition, dry matter intake in lambs were reduced following an increased inclusion of a true and degradable protein source (casein) in their diets (Swanson et al., 2004).

As in the current study, feed intake was similar with both NPN and soybean protein sources in purified diets fed to lambs, contrary to the slight decrease often found when urea is used in natural diets (Oltjen, 1969). Similar results were recorded by Shain et al. (1998) by feeding yearling cattle finishing diets containing either supplementary urea or other natural protein sources, as well as by Walker et al. (2006) who supplemented heifers' diets with urea or solvent-extracted soybean meal.

Increased neutral detergent fibre fermentation may reduce physical fill, stimulate flow from the rumen, and again allow for greater voluntary feed intake (Oba & Allen, 1999). An increased intake increases the liquid outflow of the rumen and feed particles in the early stages of digestion. This in turn results in more attached microorganisms (and thus also bacterial crude protein) flowing to the lower digestive system leading to an increase in microbial yield (Sniffen & Robinson, 1987). Even though both neutral and acid detergent fibre digestibility was affected by dietary NPN content, its low and similar content between diets (Table 1) was probably too low to affect the voluntary intake of the wether lambs.

A high NPN content (51.7 g/kg; NPN3) decreased (P <0.05) the growth efficiency (average daily gain) of wether lambs fed low-fibre finishing diets. The efficiency of gain presented as metabolizable energy utilized for the accretion of live weight gain (MJ metabolizable energy intake/kg live weight gain) of NPN2 and NPN3 decreased (P = 0.0176) compared to CON and NPN1 (higher values indicated less efficient growth). The positive effect of increased dietary NPN on organic matter digestibility and metabolizable energy (Table 2) did not, however, correspond to lamb growth efficiency. The lack of a treatment effect on the feed conversion ratio was not expected due to the strong influence of a high NPN on average daily gain. According to the NRC (2007), NPN sources should not provide more than one-third of the total nitrogen (crude protein) supplied for finishing ruminants. The recommendation for urea inclusion is no more than 15 to 25% of total crude protein in cattle and sheep fattening diets (Briggs, 1967). When urea inclusion provided more than 17.46% (NPN2) of the total crude protein in the current study, the average daily gain of the lambs declined (NPN3 compared to treatments CON and NPN1).

The amount and fermentability of organic matter in the diet complements the utilization of rumen ammonia concentration, i.e., fermentable energy in the rumen is required for efficient microbial use of ammonia-N for growth (Nichols et al., 2022). It is possible that additional free nitrogen in the rumen is excreted as urea in urine (diets higher in crude protein digestibility), which negatively affects energy metabolism and decreases growth. This could have related to decreasing average daily gain and an increase in metabolizable energy utilized for growth purposes, as occurs on a higher NPN content. Productive ruminants require a higher percentage of true and bypass protein in their diets to meet the amino acid requirements of the post-ruminal stage (Putri et al., 2021). Dietary protein sources that differ in the extent and rate of breakdown to ammonium and amino acids (NPN or true protein sources), affect animal performance (Kand & Dickhoefer, 2021). The degradability of crude protein in the rumen is related to the ruminal ammonia nitrogen concentration thereof (Mi et al., 2022). The rapid decomposition of urea in the rumen to ammonia can occur at a faster rate than the utilization of ammonia by microbes, leading to the accumulation and escape of ammonia from the rumen (Mahmoudi-Abyane et al., 2020). Energy required to process and excrete nitrogen in the animal's body includes energy lost through metabolic transformations, synthesis of urea, and excretion by the kidneys (Reed et al., 2017).

The amount of various amino acids arriving in the duodenum plays a key role in the performance of growing and lactating ruminants (Shan et al., 2007). Microbial synthesis supplies a decreasing proportion of the required protein, and substantial amounts must escape ruminal degradation to meet protein requirements of high yielding cows (Das et al., 2014). Any process that increases the quantity of amino acids absorbed, may raise the efficiency of metabolizable energy for growth purposes (Minson, 1990). From a quantitative perspective, true degradable protein appears to have a higher stimulatory effect on microbial essential amino acid flow to the duodenum compared to NPN (casein versus urea). This may affect the animal response and explains why natural protein is generally accepted as superior to NPN in terms of animal production (Nolte & Ferreira, 2005). A lower NPN content (more natural degradable proteins) seems to favour lamb growth (average daily gain) and efficiency within the current study (Table 3).

In contrast, Minson (1990) states it would be wrong to assume that a high proportion of bypass protein is always beneficial. Lambs were able to maintain growth with diets where virtually all the nitrogen was supplied by urea (Belasco, 1954). The addition of undegradable ingested protein (a feather and blood meal blend) to a diet also did not necessarily improve performance of finishing steers (McCoy et aí., 1998) nor lactation performance of goats (Soto-Navarro et aí., 2003). The same lack of response in feed efficiency was obtained in yearling cattle fed finishing diets containing either supplemental urea or other natural protein sources (Shain et aí., 1998). Walker et aí. (2006) proposed, in accordance with the previous statements, that diets containing urea as the predominant protein were sufficient to optimize gain and efficiency of feedlot cattle. McCoy et aí. (1998) mentioned that escape protein supplementation of finishing calf diets gave inconsistent results. Comparable to dry matter intake, the effect of protein quality on the production performance of ruminants presented in literature is also inconsistent and variable. None of these studies, however, formulated a comparable degradable, ingested protein content.

The effects of increasing NPN content in low-fibre finishing diets on the carcass characteristics of wether lambs are presented in Table 4. It was evident that dietary NPN content had a limited effect on the carcass characteristics.

Even though there was a dietary treatment effect on lamb growth efficiency (average daily gain) and metabolizable energy (weight gain) (Table 3), the lack of treatment effect on lamb dry matter intake, metabolizable energy intake, and final live weight corresponded to the lack of treatment effect on general carcass characteristics. However, increasing the NPN content of diets affected the area of the muscuíus íongissimus dorsi (P = 0.0457). An NPN content of 40 g/kg diet dry matter (NPN2) negatively affected íongissimus dorsi muscle area. In comparison to the current study, protein source did not alter carcass characteristics (dressing percentage, longissimus muscle area, 12^th-rib fat thickness, and marbling score) of feedlot heifers on a soybean diet compared to those that were fed urea (Walker et al., 2006). There seems to be inadequate literature regarding protein quality and its effect on lamb carcass composition. More research is required in this regard.

 

Conclusion

Increasing the NPN content of low-fibre finishing diets of South African Mutton Merino lambs positively affected nutrient digestibility and metabolizable energy. In contrast, the growth performance and efficiency of metabolizable energy utilization for growth decreased. It was thus apparent that there was an optimum NPN content in low-fibre finishing diets with comparable degradable, ingested protein content for small stock. A low dietary NPN content was beneficial for South African Mutton Merino production. It is accepted that contradictory literature exists on small stock growth response related to protein solubility. There appears to be inadequate recent literature regarding ruminant NPN research, let alone quality aspects thereof and its effect on small stock performance.

Acknowledgements

The support staff at the Department of Animal Science (University of the Free State) and Ms Liesl van der Westhuizen (Science Writer, University of the Free State) are gratefully acknowledged.

Funding

This study was funded by the Directorate Research and Development of the University of the Free State, Bloemfontein, South Africa.

Authors' contributions

OBE, AVF, and AH designed the experiment and OBE carried out the research trial. MDF completed the statistical analyses. OBE and AH structured the scientific content and OBE drafted the manuscript. AVF and AH assisted with the experimental design and research trial, while all authors provided editorial suggestions and approved the final manuscript. AH conducted the meat quality analysis.

Conflict of interest

The authors declare that there is no conflict of interest.

 

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Submitted 29 August 2024
Accepted 26 September 2024
Published 9 December 2024

 

 

# Corresponding author: einkamererob@ufs.ac.za