PASTURE SCIENCE

Productivity, chemical composition and in situ ruminal degradation kinetics of Morus alba at diferent regrowth ages

E. Torres-Navarrete^{1  *}  , A. Sánchez-Laiño¹  , Í. Espinoza-Guerra¹  , A. Barrera-Alvarez¹  , D.M. Verdecia²  , J.L. Ramírez²  , R.S. Herrera³ 

¹Facultad Ciencias Pecuarias, Universidad Técnica Estatal de Quevedo (UTEQ), Quevedo, Los Ríos, Ecuador

²Facultad de Ciencias Agropecuarias, Universidad de Granma, Apartado Postal 21, Bayamo, C.P. 85 100, Granma, Cuba

³Instituto de Ciencia Animal, Apartado Postal 24, San José de las Lajas, Mayabeque, Cuba

ABSTRACT

Using a random block design and four replications with six treatments consisting on regrowth ages (30, 45, 60, 75, 90 and 105 days), their biomass production, dry matter, chemical composition and in situ ruminal degradation kinetics of Morus alba, was evaluated in la María, Los Ríos, Ecuador. It was sampled in plots of 1,200 m², in which a uniformity cut was applied at 0.5 m high over the soil level, without using irrigation or fertilization. Edible biomass, DMY, DM, CP, NDF, ADF and OM were determined, as well as in situ ruminal degradation kinetics (DM, NDF and OM). There were increases of dry matter and edible biomass production of 1.77 and 0.45 t/ha^-1, respectively, as well as 7.5, 11.21 and 18.28 percent units for DM, NDF and ADF, while there was a decrease for CP and OM of 4.99 and 8.44 percent units, respectively up to 105 days. For degradation kinetics of dry matter, the best results were obtained at 30 days with 81.24%, 7.08%, 0.41%/h, 88.32%, 66.06%, 63.78%, and 61.51% for soluble fraction (a), potentially degradable (b), degradation rate (c), potential degradability (a+b) and effective degradability at ruminal passage rate of 2, 5 and 8 %, respectively. This same performance was maintained for OM and NDF. It can be concluded that regression equations were established, which explain the close relation among age, edible biomass production and dry matter. Dry matter and crude protein values were lower than those reported for this species in the international literature for production systems in tropical areas. These differences could be attributed to the zero use of irrigation and fertilization during the research. It is important to highlight that soluble fraction, potential and effective degradation of DM, OM and NDF were maintained with values superior to 50%. Further studies are suggested in other periods of the year, cut height, phytochemical composition, intake tests and animal response.

Key words: shrubs; biomass; effective digestion; age; regrowth; mulberry

INTRODUCTION

The search for species with potential for animal production in Latin American countries has brought about the study of native or introduced trees, not conventionally used in the production systems in tropical areas (^{Naranjo and Cuartas 2011}). One of these resources is mulberry (Morus alba), which is a forage tree adapted to tropical conditions and it has shown wide possibilities for feeding ruminants and non-ruminants. Variations in their bromatological composition are a result of regrowth age, position of leaves and stem, fertilization level and some other factors (^{Borges et al. 2014}).

This a tree with multiple uses and its foliage can be advantageous for animal production because of its high forage potential. In this sense, many studies have been performed for demonstrating the versatility of this plant in tropical and subtropical areas, highlighting its use as protein biomass bank in paddocks with low nutritional quality grasses. This tree can be compared with multipurpose legume shrubs, recommended to be implemented by small and medium farmers (^{Zach et al. 2017})

Mulberry stands out by its excellent capacity for biomass production, chemical composition, adaptability to several soil and climate conditions and availability. Its foliage shows protein concentrations between 15 and 28%, with 90 % of in vivo digestibility and its amino acid composition is similar to that of soybean meal, being considered as a good source of amino acids, out of which almost half are those considered as essential (^{Alpizar et al. 2014}). This is one of the forage specie that shows excellent characteristics of palatability and intake for goats and bovine cattle, and demonstrates adaptability to a wide range of ecosystems (^{Franzel et al. 2014}).

Although there are many results on the scientific literature that explain the effect of action mechanisms of ruminal microbial population in different nutrients within forage shrubs and trees, their degradation speed, digestion of cell wall components and nitrogen compounds, as well as the proportion of bypass protein of promising species (^{Gutiérrez 2015} and ^{Olafadehan and Okunade 2018}).

In Los Ríos province, Ecuador, some of the species used as protein plants in many regions, like mulberry, are not so popular and many times farmers do not know them or resist to know its use as supplement for ruminants and non-ruminants due to their lack of knowledge about its nutritional contribution.

Therefore, it is important to study the effect of regrowth age on chemical composition and in situ ruminal degradation kinetics of Morus alba in the current edaphoclimatic conditions of Los Ríos province, Ecuador.

MATERIALS AND METHODS

Research area, climate and soil. This study was developed in areas and Laboratorio de Rumiología y Metabolismo Nutricional (RUMEN) of the Universidad Técnica Estatal de Quevedo, located in Campus Experimental "La María”, km 7 1/2 de la vía Quevedo-Mocache, Los Ríos, Ecuador, which geographical location is 01o 6' South and 79o 29' West and at 73 mosl during the time between January and May, 2018 (winter, rainy season).

The climate of this area is classified as humid subtropical (^{García 2004}), with rains of 2,020.6 mm during the experimental period. Mean, maximum and minimum temperatures were 25.87, 33.25 and 23.5°C, and relative humidity was 90%. These indicators are within the range of historical mean up to 2014 (2,000 mm; 25.4, 33.2 and 23 °C for mean, maximum and minimum temperature, respectively, and 89.5% of relative humidity). The soil of this area is inceptisol (^{Soil Survey Staff 2003}) and its chemical composition is presented in table 1.

Table 1 Soil characteristics 

Indicator	Value	SD
pH	5.36	0.03
N, cmolc kg^-1	1.48	0.05
P, cmolc kg^-1	5.30	0.20
K, cmolc kg^-1	0.52	0.01
Ca, cmolc kg^-1	1.59	0.05
Mg, cmolc kg^-1	0.82	0.05
Sand, %	24.00	2.65
Lime, %	56.00	2.65
Clay, %	20.00	3.46

Treatment and experimental design. A random block design and four replications were used and the treatments were regrowth ages of 30, 45, 60, 75, 90 and 105 days.

Procedures. Sowing of experimental plots (40x30=1,200m²) was performed in January, 2017, with a distance between furrows of 1 m and 1 m between plants. There was an establishment period of one year, up to January 2018, and then, a uniformity cut at 0.50m over the soil level was performed. From that moment, samplings were conducted according to treatments (30, 45, 60, 75, 90 and 105 days). The soil was not irrigated nor fertilized during the experiment.

For yield components, 10 plants were taken at random for each ages and later the cut of the rest of the plot was performed, removing 50 cm of border effect. Afterwards, leaves, petioles and stems lower than 2.0 cm of edible biomass (EB) were individually separated. Later, plant material was homogenized and samples were taken, which were dried in an air circulation oven for 72 hours at 65 ºC. For this, 200 g of each sample were used.

Chemical composition determination. Samples were dried at room temperature in a dark and ventilated room for 12 days, then, they were grinded up to 1mm particle size, and stored in amber flasks at room temperature. DM, CP and OM were determined according to ^{AOAC (2016)}, NDF and ADF regarding ^{Goering and Van Soest (1970)}. All the analyses were performed by duplicate for each replication.

In situ ruminal degradation kinetics. Four Brahman bulls were used with 450.3±35.2 kg of weight, provided with a ruminal cannula (four inches of internal diameter, Bar Diamond, Parma, Idaho, USA). The animals were kept in individual pens and fed with a diet based on Saboya grass (Megathyrsus maximus), King grass (Cenchrus purpureus) and mulberry (Morus alba), and water was offered ad libitum.

In situ ruminal degradation was determined through the nylon bag technique in the rumen, described by ^{Orskov et al. (1980)}, and bags (15 cm x 10 cm) were used for incubations with a 45 µm pore size. Two bags were located in each animal, containing 10g of DM of each treatment per replication, and they were incubated for 0, 6, 12, 24, 48, 72 y 96 h. Two empty bags (control) were included in each time for determining correction factor for cleaning effect.

At the end of incubation periods, bags were removed from the rumen and cleaned with current water and dried at 65 ºC, up to obtaining a constant weight. Later, weights were recorded for their analysis and interpretation. DM disappearance was fit to the equation p=a+b x (1-e^-ct) (^{Orskov et al. 1980}),

where:

p	is DM disappearance in time (t)
a	is the soluble fraction per cleaning of bags at h 0 (%)
b	is the insoluble but potentially degradable fraction (%)
c	is the degradation rate of b (h^-1)

Effective degradability (DMED, OMED, NDFED and ADFED) was calculated for three ruminal passage rates (k): 0.02, 0.05 and 0.08 %, according to the equation

ED = a + [(b x c) / (c+k)], where a, b, c and k have been previously described.

Statistical analysis and calculations. All the statistical analyses were carried out with ^{SAS 9.3 (2011)}. Data was analyzed with GLM procedure and minimum square means were compared with ^{Tukey (1949)} test (P<0.05). For normal distribution of data, Kolmogorov-Smirnov (^{Massey 1951}) test was used and ^{Bartlett (1937)} test was used for variances. Degradation kinetics parameters were calculated with GRG NONLINEAR resolution mode of SOLVER function of Microsoft EXCEL®.

In order to establish functional relationship among edible biomass, dry matter yield and age, linear, square, cubic and Gompertz equations were analyzed. For selecting the equation with the best fit, indicators like high R² value, high significance, low standard error of estimation and of terms, significant contribution of terms and equation according to ^{Guerra et al. (2003)} and ^{Rodríguez et al. (2013)}, were taken into consideration. The SPSS (^{Visauta 1998}) program was used for all the previous analyses.

RESULTS

Edible biomass and dry matter yield increased (P<0.05) with plant age and polynomic of third order and linear equations were fitted with the EB, DMY and age. With the best results (1.77 and 0.45 t/ha at 120 days respectively (figure 1).

Figure 1 Morus alba production at different regrowth ages 

For chemical composition of Morus alba (table 2), there were significant differences (P<0.05) among regrowth ages for all the evaluated indicators. When comparing 30 with 105 days, there are increases of 7.5, 11.21 and 18.28 percent units for DM, NDF and ADF, respectively, while CP and OM decreased in 4.99 and 8.44%, respectively.

Table 2 Chemical composition of mulberry (Morus alba) at different regrowth ages 

Regrowth age, days	Variables
	DM	CP	NDF	ADF	OM
30	15.62^f	17.92^a	39.17^d	12.20^e	85.46^a
45	16.69^e	17.30^a	39.97^d	12.72^e	84.90^ab
60	17.31^d	16.64^ab	43.21^c	16.21^d	84.87^ab
75	20.26^c	15.96^b	43.27^c	22.53^c	83.58^b
90	23.12^b	14.28^c	45.23^b	26.93^b	80.48^c
105	27.56^a	12.93^d	50.38^a	30.48^a	77.02^d
SE±	0.49	0.24	0.28	0.89	1.24
P	0.0001	0.001	0.001	0.001	0.001

^abcdef Values with different letters differ according to ^{Tukey (1949)} (P<0.05)

Indicators of the kinetics of in situ degradation of dry matter (table 3) showed significant differences (P<0.05), which decreased with the increase of regrowth ages. The best results were obtained at 30 days with 81.24, 7,08%, 0.41%/h, 88.32, 66.06, 63.78 and 61.51% for soluble fraction (a), potentially degradable (b), degradation rate (c), potential degradability (a+b) and effective rate of ruminal passage (2, 5 and 8%), respectively.

Tabla 3 Kinetics of in situ ruminal degradation of dry matter of mulberry (Morus alba) at different regrowth ages 

Indicators kinetics	Regrowth ages, days						SE±	P
	30	45	60	75	90	105
a	81.24^a	80.41^a	80.35^a	78.93^b	78.92^b	77.56^c	1.82	0.003
b	7.08^a	6.98^ab	6.67^b	6.45^bc	6.28^c	5.01^d	0.56	0.001
c	0.41^a	0.39^a	0.38^ab	0.32^b	0.26^c	0.25^c	0.43	0.002
PD	88.32^a	87.39^a	87.02^b	85.38^c	85.20^d	82.57^e	0.79	0.001
ED (2%)	66.06^a	65.73^a	64.56^b	64.48^b	62.16^c	60.32^d	0.70	0.001
ED (5%)	63.78^a	63.35^a	62.27^b	62.92^ab	61.84^c	59.08^d	0.82	0.002
ED (8%)	61.51^a	60.99^a	59.08^b	58.45^c	56.54^d	56.36^d	1.05	0.001

^abcdef Values with different letters differ according to ^{Tukey (1949)} (P<0.05)

a: soluble fraction; b: potentially degradable fraction; c: degradation rate of b;

PD: potential degradability (a+b); ED: effective degradability of ruminal passage rates (2, 5 and 8 %)

For organic matter, indicators of kinetics (table 4) showed similar performance to that of DM, with values of 74.23, 3.74%, 1.44%/h, 77.97, 61.08, 60.78 and 60.04%, para a, b, c, PD, ED (2, 5 and 8%), respectively. The best results (P<0.05) were obtained at 30 days of regrowth.

Table 4 Kinetics of in situ ruminal degradation of organic matter of mulberry (Morus alba) at different regrowth ages 

Indicators kinetics	Regrowth ages, days						SE±	P
	30	45	60	75	90	105
a	74.23a	72.70b	70.61c	65.04d	64.48d	58.15e	1.76	0.0002
b	3.74a	3.57a	2.56b	2.47b	1.59c	1.27c	1.01	0.003
c	1.44a	1.17a	1.12a	0.99b	0.96b	0.48c	0.05	0.004
PD	77.97a	76.270a	73.17b	67.51c	66.07c	59.42d	1.72	0.0001
ED (2%)	61.08a	57.80b	56.33bc	55.60c	53.43d	52,08d	1.64	0.0001
ED (5%)	60.78a	56.54b	55.39bc	54.52cd	53.36d	51.95e	1.64	0.0001
ED (8%)	60.04a	57.48b	55.28c	54.04c	52.26d	51.50d	1.69	0.0001

abcdef Values with different letters differ according to ^{Tukey (1949)} (P<0.05)

a: soluble fraction; b: potentially degradable fraction; c: degradation rate of b;

PD: potential degradability (a+b); ED: effective degradability of ruminal passage rates (2, 5 and 8 %)

Indicators of kinetics of NDF degradation (table 5) had a marked effect of regrowth age with a gradual decrease during maturity of the plant with values of 34.19, 4.86 %, 0.03%/h, 38.34, 13.46, 12.21 and 16.18% para a, b, c, PD y ED (2, 5 and 8%), respectively. It is important to highlight that degradability rate, (c) showed no significant differences.

Table 5 Kinetics of in situ ruminal degradation of neutral detergent fiber (NDF) of mulberry (Morus alba) at different regrowth ages 

Indicators kinetics	Regrowth age, days						SE±	P
	30	45	60	75	90	105
a	74.28^a	53.61^b	52.73^b	45.31^c	42.09^d	40.80^d	1.83	0.0001
b	7.41^a	4.60^b	3.32^b	2.97^c	2.93^c	2.55^d	0.08	0.0019
c	0.10	0.09	0.08	0.08	0.07	0.07	0.02	0.6954
PD	81.69^a	58.21^b	56.05^c	48.28^d	44.99^d	43.35^e	3.90	0.0012
ED(2%)	64.47^a	61.44^b	60.67^b	60.28^b	55.61^c	51.01^d	1.83	0.0001
ED (5%)	62.96^a	59.67^b	57.62^c	54.16^d	52.74^e	50.75^f	1.09	0.0001
ED (8%)	61.85^a	60.86^b	55.35^c	50.34^d	48.57^e	45.67^f	0.85	0.0001

^abcdef Values with different letters differ according to ^{Tukey (1949)} (P<0.05)

a: soluble fraction; b: potentially degradable fraction; c: degradation rate of b;

PD: potential degradability (a+b); ED: effective degradability of ruminal passage rates (2, 5 and 8 %)

DISCUSSION

The importance of climatic factors in the social and economic life of a country is known. Plant species are able to exist, reproduce and maintain only in certain climatic and edaphic contexts, which can be considered as the tolerance of the species to these conditions (^{Herrera 2015}).

With the development of science, it has been demonstrated that not only climate factors influence on plant productivity. Others such as soil characteristics, fertilization, water availability, management and sowing season, and some others, have an important function in the production of plant systems (^{Herrera et al. 2018}).

Alternative livestock production systems are based on the use of perennial crops capable of generating large amounts of biomass and nutrients. In this area, trees and shrubs play a fundamental role, mainly mulberry (Morus sp.), which is widely distributed in tropical and subtropical regions, Japan, India, China, Korea, Colombia, Cuba, Venezuela, North America and Africa. More than 14 countries report the sowing of this species by farmers, indicating its adaptability to a wide range of ecosystems (^{Hussain et al. 2017}). It is known and used for its excellent nutritional value and high palatability. This forage becomes a real option as a supplement to low quality pastures in ruminants, as well as for the substitution of concentrated foods based on cereals in rations for growing dairy cattle and the production of meals for non-ruminants.

Although its benefits and importance in livestock production systems are known, it is not widely used by farmers in Los Ríos province, Ecuador. Hence the importance of conducting studies on the performance of this species in this region, since there is not enough information about its productive performance and nutritional contribution.

^{Hernández-Sánchez et al. (2015)} studied the effect of the regrowth age on the production of edible biomass of Morus alba and Hibiscus rosa-sinensis and found, in both species, a performance similar to that found in the current research with increases up to 60 days, then stable performance until 90-105 days with 1.58 t / ha, associating this response to the fact that total biomass increases with age due to the increase of the photosynthetic process and synthesis of metabolites necessary for the growth and development of plants, but the edible biomass tends to be stable due to lignification and volume of stems.

^{Martín et al. (2014)}, when evaluating three varieties of Morus alba, found that Acorazonada variety had the best performance in the rainy season, with 60 days (6.35 t DM/ha), for Indonesia in the same period, but with 90 days of regrowth (6.45 t DM/ha). Tigreada variety had the lowest results at 60 (6.02 t DM/ha) and 90 days (6.19 t DM/ha). These values ​​were above those obtained in the present study, which is determined by management differences (organic fertilization with poultry manure) and edaphoclimatic conditions prevailing in the research. It has been shown that yield and edible biomass were mainly influenced by fertilization, cutting age and season.

The performance found in the chemical composition (table 2) is due, among other aspects, to increase of the cell wall and to physiological and anatomical changes that occur as the plant ages, which causes a decrease of the proportion of cytoplasmic content, the cell lumen is reduced with its soluble components and the fibrous compounds increase. This performance coincides with those reported by ^{Rodríguez-Zamora and Elizondo-Salazar (2012)} and ^{Olafadehan and Okunade (2018)}, who state that with forage maturity, their quality is affected, affecting intake, although they emphasize that Morus alba is used as supplement during food scarcity period due to its capacity as well as other tree and shrub species to maintain high amount of green biomass and with high protein percentages.

^{Hernández-Sánchez et al. (2015)}, when assessing the effect of regrowth age (30, 60, 90 and 120 days) on the chemical composition of mulberry (Morus alba), reported results superior to those found in the present study with 28.96, 21.87 and 46.19% for DM, CP and NDF, respectively, with the application of high levels of nitrogen fertilization (450 kg/ha/year), although the primary effect of N for the accumulation of DM, CP levels and increasing biomass quality is known, this aspect, together with the interaction with climatic factors, directly influenced on the differences found in nutrient contributions.

^{García-Soldevilla et al. (2007)}, when evaluating mulberry foliage as a supplement for calves in rotational grazing with guinea Likoni (Megathyrsus maximus cv. Likoni) under conditions of the western region of Cuba with precipitations superior to 1,000 mm and very fertile soils, reported percentages of CP (21.43 and 19.54%) and NDF (33.28 and 24.05%), for rainy and dry periods, respectively. On the other hand, ^{López et al. (2014)}, under conditions of the eastern area of Cuba with rains below 1,000 mm, low fertility soils and 90 days of regrowth, obtained 20 and 34% of CP and NDF, respectively. While in the region where the present research was conducted, precipitations with over 2,000 mm and soils with higher fertility, values ​​obtained by the authors were 17.37 and 50.38 % of CP and NDF, respectively. Differences found are due to the performance of climate variables, soil and plant maturity.

While, ^{Rodríguez et al. (2014)}, reported for Moringa oleifera, Leucaena leucocephala, Morus alba and Trichanthera gigantea values ​​of OM (82-91%), PB (20-27%) and FDN (30-50%), although the OM and NDF of the present study are within the range of the values ​​reported in this research, variations of the values ​​reported by the literature are a result of the effect of the parts of the collected plants, their phenological state, season in which they are obtained, cut frequency and environmental and management conditions in which the collected material was developed (^{Luna-Murillo et al. 2016}).

The performance of in situ degradability kinetics of the DM, OM and NDF (tables 3, 4 and 5) could be attributed to the characteristics of the material of origin, age, phenology and period of the year, as these factors have a determining influence on the contents of fiber components (^{Zach et al. 2017}). In addition, forage mass degradability will depend on the relative proportion of each chemical component and its individual digestibility. On the other hand, its reduction with the increase of maturity is also influenced by the increase in structural components, as well as the Si and the monomeric components of lignin (^{Trabi et al. 2017}).

In forages from grasses, creeping legumes, trees and shrubs, the cell wall is thin in the early growth stages, with little fiber, which allows its easy rupture by ruminal microorganisms and short times of digestion. When age increases, vascular structures of leaves become thicker, as do the vascular tissue and the sclerenchyma. Leaves and stems are lignified and become physically stronger and difficult to reduce in size. Another aspect that limits digestibility is the presence of secondary metabolites (condensed tannins), which bind to the fiber and proteins limiting the action of microorganisms, as well as their defaunant properties (saponins), inhibits ruminal fermentation, the number of protozoa and cellulolytic bacteria and decreases protein degradability (^{Franzel et al. 2014}, ^{Hussain et al. 2017} and ^{Olafadehan and Okunade 2018}).

CONCLUSIONS

Regression equations were established, which explain the close relationship among age, edible biomass production and dry matter.

Values ​​of dry matter and crude protein are below those reported for the species, in the international literature, for production systems in the tropics. These differences could be attributed to the non-use of irrigation and fertilization during research. It is worth noting that the soluble fraction, potential and effective degradation of DM, OM and NDF remained with values ​​superior to 50%.

Future research is needed with the evaluation of other cut heights, periods of the year, fertilizer application, cut density and determination of secondary compounds, as well as the relationship of the latter with ruminal degradation and the optimum allowable levels in the diets from which the animal metabolism begins to be affected.

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Received: March 20, 2019; Accepted: May 20, 2019

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