Effect of metabolizable energy levels in the diets on feed intake, nutrient digestibility and nitrogen retention of crossbred Boer goats

Doan Tri Dung, Vo Thi Phuong Tien, Truong Thanh Trung and Lam Phuoc Thanh

Faculty of Animal Sciences, Can Tho University, Can Tho 94000, Vietnam
phuocthanh@ctu.edu.vn

Abstract

This study aimed to evaluate the effects of different metabolizable energy (ME) levels in diets on feed intake, nutrient digestibility, ruminal fermentation and nitrogen retention of growing goats. Eight male crossbred Boer goats (♂ Boer × ♀ Bach Thao), 3 months old, weighing 14.9 ± 0.82 kg, were used in a double 4×4 Latin square design. The experiment consisted of 4 periods, each lasting 21 days including 15 days for adaptation and 6 days for sample collection. Four experimental diets were ME8.82, ME9.80, ME10.8 and ME11.8, corresponding to dietary ME levels of 8.82, 9.80, 10.8 and 11.8 MJ/kg DM. All diets were formulated to have 48% NDF and 14% CP content. The results showed that DM intake was highest in ME10.8 and lowest in ME11.8 (471 and 409 g/d; p<0,05). The digestibility of all nutrients was linearly increased (p<0.05) when increasing ME levels in the diets. At 3 h post-feeding, concentrations of NH₃-N and total volatile fatty acids linearly increased (p<0.001) and the highest values were detected in ME11.8, compared with other diets. The proportion of ruminal acetate linearly decreased (p<0.001) from 78.1 to 57.2% while ruminal propionate linearly increased (p<0.001) from 15.7 to 39.0% when increasing the ME level in the diet from 8.82 to 11.8 MJ/kg DM. ME11.8 showed lower (p<0.05) nitrogen excretion via feces and urine, but no effect was detected on nitrogen retention. Combined data suggest that dietary ME of 11.8 MJ/kg DM is an ideal diet for crossbred Boer goats 3-6 months of age.

Keywords: crossbred Boer goats, metabolizable energy, nitrogen retention, nutrient digestibility, ruminal fermentation

Introduction

Goat production plays an important role in the economic outcome of many developing countries, including Vietnam. It contributes to the supply of meat and milk products. Goat farming can be a part of the sustainable agricultural system when managed effectively, including the application of scientific and technological advancements in animal production, enhancing the efficiency of livestock breed management and development and ensuring a proactive supply of animal feed. Utilizing local agricultural and industrial by-products as potential feeds for goat production is a meaningful method for both environmental and economic contexts. Appropriate treatment of agricultural by-products such as straw, corn stalk, soybean meal, coconut meal and rice bran for ruminants, especially goats not only reduces feed costs but is also involved in environmental protection. In addition, goat manure is recycled as fertilizer to enhance soil fertility and provide nutrients for cultivated plants. This is in line with the trend of green and circular economic development, contributing to the creation of sustainable economic value.

Dietary metabolizable energy (ME) represents the proportion of energy in the feed that can be used for maintenance, growth, production and reproduction. Feeding Kamori kids with different ME levels in the diet showed significantly improved total feed intake, hematocrit value and average daily gain in a high-ME diet (Abbasi et al 2012). According to Rashid et al (2016), Black Bengal goats fed a dietary ME level of 11.3 MJ/kg DM showed the highest digestibility of almost all nutrients, compared with that fed 9.25 and 10.3 MJ/kg DM.

Goats are well known for their high adaptability to various feed resources. Previous researches were performed in goats to investigate the potential by-products as alternative feed, as well as the influence of dietary nutrient value on digestibility, ruminal fermentation and animal performance. An experiment by Thu (2017) was conducted to evaluate the effects of crude protein (CP) content on feed intake, nutrient digestibility, ruminal parameters and nitrogen synthesis in Bach Thao goats. A previous study by Thanh et al (2021) evaluated the positive effects of jackfruit leaves when replacing Para grass on feed intake, digestibility, nitrogen balance and ruminal fermentation in growing crossbred goats. However, the effects of dietary ME levels in crossbred Boer goats raised in the Mekong Delta of Vietnam were not investigated.

Thus, the objective of this study was to evaluate the effects of dietary ME levels on feed intake and nutrient digestibility of crossbred Boer goats, thereby determining and recommending the appropriate dietary ME levels for crossbred Boer goats.

Materials and methods

Ethics approval

The study received ethical approval for animal care, housing and sample collection procedures under the Animal Welfare Assessment (AE2022-05/TNN).

Study site

The study was conducted at an experimental goat farm located in An Binh Ward, Ninh Kieu District, Can Tho City, Vietnam. The chemical composition analysis of the experimental samples was performed at the Laboratory of Ruminant Production Techniques, Faculty of Animal Sciences, College of Agriculture, Can Tho University, Vietnam.

Animals, experimental design and diet

Eight male crossbred Boer goats (♂ Boer × ♀ Bach Thao) at 3 months of age, 14.9 ± 0.82 kg of body weight were used to conduct the experiment. Goats were kept in individual metabolic cages (1.2 m × 1.0 m × 1.2 m, L × W × H) and had free access to fresh water. The animals were arranged in a double 4 × 4 Latin square design, each experimental period lasted for 21 days including 15 days for adaptation and 6 days for sampling as recommended by McDonald et al (2010).

Diet (treatment) was developed by increasing levels of ME in the diets at 8.82, 9.80, 10.8 and 11.8 MJ/kg DM, corresponding to the ME8.82, ME9.80, ME10.8 and ME11.8. All diets were formulated to have 48% NDF and 14% CP content. These concentrations were obtained from our previous studies (in press). During the study, goats were fed a total mixed ration including concentrate and fresh elephant grass (EG). Diets were offered in equal amounts twice daily at 08:00 and 16:00. The concentrate was mixed at the experimental farm with the ratio of ingredients and chemical composition as shown in Table 1 and Table 2. Elephant grass was grown and harvested at 30-40 days intervals.

Sampling and measurements

All the feeds were weighed before feeding and supplied separately to the experimental goats. In detail, the sweet potato tuber by-product was fed at dry matter consumption depending on treatments. The tofu waste, soybean meal and urea were mixed with premix supplements before feeding. Forage sources were fed ad libitum and drinking water was always available. Refused feeds were weighed each morning. From d16 to d19, data on feed offered and refused, total feces and total urine were daily recorded for each goat. Feces were collected in wire-screen baskets placed under the floor of each cage, whereas urine was collected through a funnel into plastic buckets containing 50 mL of 10% (v/v) H₂SO₄. The acidification keeping the final pH of urine below 3 was necessary to prevent the loss of volatile ammonium and microbial degradation. After recording the weight, 10% of feces and urine during the whole day were collected and the sampling was conducted over 4 continuous days. The samples were stored at −20°C and then pooled for chemical analysis. On d20, ruminal fluid samples were collected at 0 h and 3 h post-morning feeding using a 100-mL syringe and the pH value was immediately determined using a digital pH meter. A portion of the samples was then filtrated through a clean double layer of cotton cloth. The liquid fraction was acidified with H₂SO ₄ 1M (10:1 v/v), centrifuged at 10,000×g for 15 minutes and then stored at −20°C for the analyses of volatile fatty acids (VFA) and NH₃-N concentrations.

Chemical analysis

Feeds and fecal samples were dried in a forced-air oven at 60°C for 48 h and then ground to pass a 1-mm screen (Cutting Mill SM 100, Retsch, Haan, Germany) before analysis. Chemical composition in feeds and fecal samples were determined according to standard methods of AOAC (1990) for dry matter (DM, method 934.01), total mineral (Ash, method 942.05), crude protein (CP, method 988.05) and ether extract (EE, method 920.39). The content of organic matter (OM) was calculated by 100 – Ash content (% of DM). Neutral detergent fiber (NDF) was analyzed following the methods of Van Soest et al (1991). Nitrogen-free extract (NFE) was calculated by following the equation: NFE = 100 − CP − CF − EE – Ash. The metabolizable energy (ME) of feed ingredients in the diet was calculated according to the equations of Abate and Mayer (1997).

Ruminal pH was measured by a pH meter (HI5222, Hana Instruments, US). Ruminal NH₃-N concentration was determined using the Kjeldahl method (AOAC, 1990). Concentrations of individual VFA were analyzed using a Thermo Trace 1310 GC system (Thermo Scientific, Waltham, MA, USA) equipped with a flame ionization detector. The inlet and detector temperature were maintained at 220°C. Aliquots (1 μL) were injected with a split ratio of 10:1 into a 30 m × 0.25 mm × 0.25 μm Nukol fused-silica capillary column (Cat. No: 24107, Supelco, Sigma-Aldrich, St. Louis, MO, USA) with helium carrier gas set to a flow rate of 1 mL/min and initial oven temperature of 80°C. The oven temperature was held constant at the initial temperature for 1 min and thereafter increased at 20°C/min to a temperature of 180°C and held for 1 min and increased at 10°C/min to a final temperature of 200°C, and a final run time of 14 min (Bharanidharan et al., 2018). Individual VFA peaks were identified based on their retention times, compared with external standards including acetic, propionic, butyric, valeric, iso-butyric and iso-valeric acids (Sigma-Aldrich, Louis, MO).

Statistical analysis

Data were statistically analyzed using the General Linear Model procedure of SAS OnDemand for Academics 2021 (SAS Institute Inc., Cary, NC, USA). The statistical model was

Y _ijkl = µ + S _i + A_j+ D_k+ P_l+ ε_ijkl

where, Y_ijk= the dependent variable, S_i= the effect of square (i = 1-2), A_j = the effect of animal (j = 1-4), D _k = the effect of diet (l = 1-4), P _k = the effect of period (k = 1-4) and ε _ijk = the residual effect. Significant differences among diet means were compared using the Tukey test. Differences were declared at p<0.05, and the tendency was declared at 0.05≤p<0.1.

Results and discussion

Feed and nutrient intake

DM intake increased gradually (p<0.05; Table 3), reached its peak at ME10.8, then decreased at ME11.8. CP intake of goats quadratically decreased (p<0.01) when dietary ME increased from 8.82 to 11.8 MJ/kg DM, the highest values were detected in ME9.80 and ME 10.8. CF intake of experimental goats was linearly decreased (p<0.01) as increasing ME level in the diets. Nevertheless, the intake of EE was increased (p<0.01) up to 3.07 times in ME11.8, compared with ME9.80. As a result of the ME level increase in the diet, ME intake was higher (p<0.001) by 33.2% and 27.8% in ME10.8 and ME11.8, compared with ME8.82. However, no effect on NDF intake wasdetected in this study.

Dry matter intake (DMI) is an important factor determining feed utilization in ruminants which is simply an important determinant of energy intake (Devendra, 1997). The DMI of goats in the current study was nearly equal to that reported by Wahyuni et al (2012) with an average DMI of 434 g/goat/day (422-442 g/goat/day). The DMI of goats in the ME11.8 diet in this study was much lower than that of Brand et al (2020) in the 11.3 MJ/kg DM diet (1,279 g DMI/day). The DMI of current goats was similar to that of Hossain et al (2003), which was 362- 406 g/day. However, CP intake in this study was higher than that reported by Hossain et al (2003), ranging from 40.4-47.7 g/day, when increasing the ME level in the diet from 10.02 to 11.98 MJ/kg DM. Differences between studies were due to the discrepancy in ration ingredients and chemical composition of feed ingredients. However, the goats in the current study consumed only 24.8-28.3 g/kg LW. For this reason, Pal et al (2010) used 5-month-old goats (fattening stage) in the study, whereas 3-month-old goats (growing stage) were used in this study.

Digestibility

The digestibility of DM, OM, CP, CF, EE and NDF linearly increased (p<0.001; Table 4) with increasing levels of ME in the diets of goats. Digested DM also linearly increased (p<0.01) when the ME level was increased in the diets. Digested EE in ME11.8 was 4.86-fold higher (p<0.01) than that in ME9.80. Digested NDF in ME11.8 was 32.8% greater (p<0.01) than that in ME9.80.

The nutrient digestibility of ruminants mainly depends on the chemical composition of the diets. The digestibility of DM, OM and CP in the current study was higher than that in the study of Wahyuni et al (2012) which revealed the digestibility of DM (46.5-50.8%), OM (49.8-53.5%) and CP (52.1-57.0%) in Boer × Thai Native crossbred goats. Rashid et al. (2016) suggested that the digestibility of DM, OM, CP, CF and NDF increased as the energy in the goat diet increased, mainly due to the higher amount of energy available in high-energy pellets is beneficial for bacteria to increase nutrient digestibility. Similarly, increasing in DM and OM digestibility was also noted in Yunnan sheep in diets containing ME levels at 9.8 and 10.4 MJ/kg DM (Wang et al 2024). In line with previous studies, the digestibility of DM, OM, CP, CF and NDF in this study also gradually increased with the increase in dietary ME level from 8.82 to 11.8 MJ/kg DM. The elevation in nutrient digestibility in the current study could result from higher energy and lower fiber content in experimental diets. The improvement in nutrient digestibility of goats in the current study showed the digestion efficiency and absorption capacity of goats. This could enhance the productivity and health of growing goats while reducing environmental impact by decreasing the amount of waste from undigested feed.

Ruminal fermentation patterns

Ruminal pH value was not different among treatments before feeding. However, at 3 h post-feeding, it linearly decreased (p<0.001; Table 5) from 6.53 to 6.25 corresponding to the increase in dietary ME level from 8.82 to 11.8 MJ/kg DM. Ruminal NH₃-N concentration linearly increased (p<0.001) from 35.7 to 40.1 mg/dL at 0 h and from 41.3 to 58.6 mg/dL at 3 h. Total VFA concentration at 0 h was not different (p>0.05) among treatments, but at 3 h, the highest value (p<0.001) was detected in ME11.8. Compared with concentrations at 0 h, total VFA content at 3 h was increased by 14.3, 17.3, 24.4, and 32.8 mM, corresponding to ME8.82, ME9.80, ME10.8 and ME11.8, respectively. Noteworthy, the VFA profile was remarkably altered after 3 h post-feeding. The proportion of acetate linearly decreased (p<0.001) from 78.1 to 57.2% while propionate linearly increased (p<0.001) from 15.7 to 39.0% when increasing the ME level in the diet from 8.82 to 11.8 MJ/kg DM. Consequently, C2/C3 ratio reduced (p<0.01) by 3.30 times in ME11.8, compared with ME8.82. The proportion of ruminal C3 showed a positive linear trend as increased level of ME in the diets (y = 7.7171x - 53.349; R² = 0.97; Figure 1). Increasing the dietary ME level to 9.8 MJ/kg DM resulted in an acceleration (p<0.01) in the proportions of iso-C4 and iso-C5, then gradually decreased when the ME level increased to 10.8 and 11.8 MJ/kg DM.

The present study showed that dietary ME affected ruminal pH, NH₃-N and total VFA after 3 h feeding. Although there was a difference in ruminal pH among treatments, the variation in ruminal pH values among treatments was tiny (only 0.33) and the ruminal pH values of these four ME diets were within the normal range for ruminants, implying normal conditions for ruminal fermentation function. As shown in Table 5, ruminal pH values ranged from 6.25 to 6.58 at 3 h after feeding, which were within the optimal range for cellulolytic bacterial activity and protein digestion (Van Soest, 1994). Higher pH is beneficial for bacterial adhesion and this is an important prerequisite for fiber digestion while a pH below 6.0 may inhibit cellulolytic bacteria. The optimal pH value of a healthy rumen ranges from 5.9 to 7.2, where fibrinolytic microorganisms are most active (Farghaly et al 2019). Ruminal NH₃-N is known to be the main nitrogen source for bacterial growth and bacterial protein synthesis (Erdman, 1986). Ruminal NH₃-N concentration reflects the dynamics between protein degradation and protein synthesis by bacteria in the rumen (Reynolds and Kristensen 2008). Increasing the ME levels in the diets increased ruminal NH₃-N concentration, which may stimulate the growth of feed-degrading bacteria in the rumen. The determination of an effective dietary ME level for goats with beneficial effects on ruminal fermentation requires experimental evidence.

Figure 1. Positive linear trend between dietary ME and ruminal C3 proportion

Volatile fatty acids are an important energy source for ruminants. The production and proportion of VFA in the rumen fluid are important indicators of the ability and mode of rumen fermentation (Zhang et al 2020). A dramatic increase in VFA content observed at 3 h post-feeding suggests a high fermentability of feed ingredients in diets with elevated ME. In addition, the increase in propionic acid proportion and decrease in acetic and butyric acid proportions in the rumen are believed to be beneficial for the efficiency of energy utilization (Knapp et al 2014). This is consistent with the present study, in which goats fed on a diet with an ME level of 11.8 MJ/kg DM showed reduced ruminal acetate and butyrate, while their ruminal propionate increased compared to those fed a diet with an ME level of 8.82 MJ/kg DM. Acetate and butyrate are used for the synthesis of lipid precursors, acetate concentrations may also be associated with improved digestion of structural carbohydrates and propionate is the major VFA used for gluconeogenesis in ruminants (Yu et al 2020).

Nitrogen balances

The utilization and accumulation of N in goats were largely influenced by ME level in the diet; these data are shown in Table 6. N intake quadratically increased (p<0.01) when dietary ME was 9.80 MJ/kg DM, then reduced when ME level was increased up to 11.8 MJ/kg DM. Nitrogen excreted via feces and urine linearly reduced (p<0.05) when increasing rates of ME in the diet. N excreted via feces and urine in ME11.8 was 21.8 and 39.6% lower (p<0.05) than those in ME9.80, respectively. N retention was not different (p>0.05) among the diets, but the greater (p<0.05) N retention / N intake was detected in ME11.8 (72.5%) compared with that in ME9.80 (64.7%). Figures 2 shows a close relationship between level of ME in the diet and nitrogen retention (y = -0.1486x² + 3.0862x - 8.3027; R² = 0.88).

Given the importance of N utilization in goats, especially under the influence of different dietary ME levels, it is of interest to examine this aspect in detail. The N intake and N retention in this study were lower than those of Hamchara et al (2018) by 30.3-34.2 g/day and 13.7-20 g/day, respectively. However, % N retention/N intake was higher, when the previous study reported % N retention/N intake ranging only from 45.3-63.4%. Fecal nitrogen includes both dietary indigestible nitrogen as well as transformed fecal nitrogen. Therefore, depending on microbial N production, apparent N digestibility may be altered without any actual change in available amino acids at the tissue level (McDonald et al 1995). The lower urinary N in ME11.8, compared to ME9.80, may be a reflection of the nitrogen in the rumen. This can depend on the quality and solubility of the diet. The nitrogen may have been lost from the rumen as ammonia, then converted to urea before being excreted in the urine.

Figure 2. Curvilinear trend in dietary ME and nitrogen retention

According to the study of Ali et al (2022), where goats fed 4 rations of Guinea grass had nitrogen consumption ranging from 6.0-14.0 g/day. This result is equivalent to the current study. However, nitrogen accumulation in goats is referenced from 1.5-4.6 g/day, lower than that of the current study. In a study of Pal et al (2010), nitrogen consumption in goats fed a ration comprising 50% grass meal and 50% mixed feed was 8.61 g/day and the accumulated N rate was 71.7%, equivalent to the current study.

Conclusions

The present study showed that increasing ME in the diet of growing goats from 8.82 to 11.8 MJ/kg DM resulted in a linear increase in all nutrient digestibility. Ruminal NH₃-N and total VFA concentrations at 3 h feeding linearly increased with the highest values detected in ME11.8. ME11.8 showed lower nitrogen excretion via feces and urine, but no different was detected for N retention. Combined data suggests that 11.8 MJ of ME/kg DM is an ideal diet for crossbred Boer goats aged 3-6 months.

Conflict of interest

We have no conflict of interest.

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