ANIMAL SCIENCE

Changes in the in vitro ruminal fermentation dynamics of diets for cattle, based on corn grains and different levels of sweet potato (Ipomoea batatas, L.) integral silage

C. Solís^{1  *} 

R. Rodríguez² 

Y. Marrero² 

O. Moreira² 

L. Sarduy² 

M.H. Ruiloba³ 

¹Facultad de Ciencias Agropecuarias, Universidad de Panamá, Convenio SENACYT-IFARHU, Panamá

²Instituto de Ciencia Animal, Apartado Postal 24, San José de las Lajas, Mayabeque, Cuba.

³Grupo de Ciencia y Tecnología para el Desarrollo (GRUCITED), Cuba

Abstract

In vitro ruminal fermentation dynamics of isoenergetic (10.9 MJ ME kg^-1) and isoproteic (12.1% CP) diets for cattle were determined, with gradual replacement levels of 0.0 (T₀, control), 32.4 (T₃₂), 77.6 (T₇₈) and 100.0% (T₁₀₀) of ground corn grain, on a dry basis, mainly by integral sweet potato silage. Accumulated gas production during the 96 h of in vitro fermentation was analyzed with a mixed generalized linear model. The increases in gas production during the initial stage were 73.0, 80.0, 98.0 and 103.8, in the intermediate stage were 82.0, 75.5, 73.2 and 70.5, and in the final were 50.0, 51.8, 45.2 and 44.8 mL g^-1 of incubated organic matter (incOM) for T₀, T₃₂, T₇₈ and T₁₀₀, respectively. In the initial stage, the highest increases of T₇₈ and T₁₀₀ were attributed to the greater availability of soluble carbohydrates and sugars. In the intermediate period, the increase in T₀ was explained by the degradation of protein matrix that surrounds corn starch. In the final stage, the lower increases were related to the limited availability of fermentable substrates and microbial recycling. On average, T₇₈ and T₁₀₀ showed greater fermentative potential (259.2 mL.g^-1 incOM), microbial efficiency rate (0.13), maximum speed (12.4 mL.g^-1 incOM h^-1) and less time to reach maximum speed (9.90 h). It is concluded that, in in vitro conditions, diets with sweet potato integral silage provided greater nutrient availability for ruminal microorganisms, which favors energy supply for the different metabolic processes in the rumen.

Key words: fermentation; diets; starch; maximum speed; microbial efficiency

Introduction

Concentrates for cattle and other commercial species of zootechnical interest include corn and other cereals as energy sources, which availability and prices compromise their use in many countries, since they compete with human food and, generally, their import is required (^{Knowles 2012}). Starch is the main component of these energy sources in conventional ruminant systems, with levels between 55 and 75% (^{Egaña 2000} and ^{Huntington et al. 2006}).

The use of animals with high productive potential raises feeding and supplementation levels, which requires greater use of energy sources (^{Meléndez 2003}). This situation explains the interest for searching non-traditional sources, with high production potential and lower cost. Sweet potato (Ipomoea batatas L.) is one of the most cultivated horticultural species, with world production of 176 million t year^-1 (^{FAO 2016}).

Sweet potato, as an alternative resource for animal feeding, is emerging as a multipurpose crop, since, in addition to being used for human consumption, it is useful for animal feed (foliage and roots) (^{Ojeda et al. 2010}). It is a versatile crop, since its foliage can be used fresh, silage and hay, while the tuber can be used fresh, silage and dehydrated, in the form of meal (^{Tique et al. 2009}).

Nutritional studies report that ruminal degradability of sweet potato integral silage (tuber and foliage) is from 29.7 to 66.8%, on a dry basis, when incubated between 6 and 72 h, very similar to that of corn silage (^{Rendón et al. 2013}). In addition, sweet potato integral silage has good organoleptic and fermentative characteristics (^{Sánchez 1996}, ^{Quezada 2001} and ^{Solís and Ruiloba 2017}), but more information about its use in ruminants is required.

The objective of this study was to determine the dynamics of ruminal in vitro fermentation with isoenergetic and isoproteic diets, mainly composed by corn grains and different inclusion levels of sweet potato integral silage.

Materials and Methods

Silage production and chemical composition of diets. Four isoproteic and isoenergetic diets or treatments were evaluated (table 1), with gradual replacement levels of 0.0 (T₀, control), 32.4 (T₃₂), 77.6 (T₇₈) and 100.0% (T₁₀₀) of ground corn grains, under dry basis, mainly by sweet potato integral silage. For silage preparation, CIP-14 variety of sweet potato was used. It was harvested at 120 d after sowing and it was previously dried to produce an integral mixture with 66.7% of the tuber and 33.3% of dry foliage.

Out of this integral mixture, five plastic microsilos were prepared, approximately 1.0 kg of capacity each, tightly sealed for 45 d. Chemical characterization of each microsilo, as of the other diet ingredients, included dry matter (DM), organic matter (OM), crude protein (CP) (^{ADAC 2016}) and true protein (TP) (^{Bernstein 1983}), neutral detergent fiber (NDF) and acid detergent fiber (ADF) (^{Goering and van Soest 1970}) and ammonia nitrogen (^{Chaney and Marbach 1962}), expressed as a percent of total nitrogen (N-NH₃,% TN).

After the analysis of each microsilo, a compound sample was prepared, which was chemically characterized for the evaluation analyzes of diets. Diets were formulated with ground corn grain (Zea mays), swazi (Digitaria swazilandensis) chopped hay, soy bean (Glycine max) meal, African oil palm (Elaeis guineensis) meal, urea and mineral salt (table 1). For that, a ME content of 13.4, 7.5 (^{NRC 1996}), 11.72 (^{NRC 1984}), 12.4 (^{Vargas and Zumbado 2003}), 0.0 and 0.0 MJ/kg DM was considered for ground corn grain, swazi (Digitaria swazilandensis) chopped hay, soy bean (Glycine max) meal, African oil palm (Elaeis guineensis) meal, urea and mineral salt, respectively. It was assumed that silage energy content was 10.5 DM, MJ kg^-1, similar to that reported by ^{Solís (2011)} for this type of material.

Table 1 Chemical composition of evaluated experimental diets  

Diet components	Substitution level of corn grains in the diet, % dry basis
	T0	T32	T78	T100
Ground corn	55.12	37.27	12.36	0.00
Swazi hay	33.52	26.82	9.53	3.40
Soy bean cake	10.05	7.87	10.05	9.97
Sweet potato integral silage	0.00	19.49	59.78	73.21
Oil palm cake	0.00	7.43	7.02	12.26
Mineral salt	0.50	0.50	0.50	0.50
Urea	0.81	0.65	0.77	0.74
TOTAL	100.00	100.00	100.00	100.00
Chemical composition of the diet
CP (%)	12.08	11.92	12.30	12.20
ME, MJ kg^-1	11.00	10.90	10.70	10.70
NDF (%)	39.54	38.60	37.34	33.34
Starch (%)*	37.18	34.34	36.53	34.55

Swasi: Digitaría swazilandensis, Oil palm cake: Elaeis guineensis, *Estimated starch

To study fermentation dynamics, in vitro technique of producing gas in glass bottles, described by ^{Theodorou et al. (1994)}, was used. An amount of 1.0 g of dry matter (DM) of each treatment was incubated in 100 mL bottles in a culture medium (^{Menke and Steingass 1988}) and inoculum of ruminal microorganisms, in a proportion of 20% of the total incubation volume (80 mL).

Ruminal content of two cannulated cows in rumen, fed ad libitum with grass forage and 2.0 kg of concentrated feed with free access to water and mineral salts, was used as inoculum. Ruminal liquor was collected before offering food in the morning.

The methodology described by ^{Rodríguez et al. (2019)} was used to determine gas production. It was measured at 2, 4, 6, 8, 10, 12, 16, 20, 24, 36, 48, 72 and 96 h by means of an HD8804 manometer, coupled to a TP804 pressure gauge (DELTA OHM, Italy). After each measurement, the gas was released until the external and internal pressures of the bottles were equal (pressure measured in Pascal (Pa)). Gas volume was estimated from pressure data, using the pre-established linear regression equation (^{Rodríguez et al. 2013}):

$\mathrm{G}\mathrm{a}\mathrm{s}\mathrm{ }\left(\mathrm{m}\mathrm{L}\right)\mathrm{ }=\mathrm{ }\left(\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{ }\right[103\mathrm{ }\mathrm{P}\mathrm{a}]+4.95)/2.5858),\mathrm{ }{R}^2=\mathrm{ }0.981;\mathrm{ }(\mathrm{n}=132)$

Gas volume (mL) was expressed per gram of incubated organic matter (incOM). To estimate gas producton kinetics, single-phase model of Gompertz was used:

$\mathrm{Y}\mathrm{ }=\mathrm{ }\mathrm{A}\mathrm{*}\mathrm{E}\mathrm{x}\mathrm{p}\mathrm{ }(-\mathrm{B}\mathrm{*}\mathrm{E}\mathrm{x}\mathrm{p}(-\mathrm{C}\mathrm{*}\mathrm{t}\left)\mathrm{ }\right)$

where:

Y	- gas production at time t (mL g^-1 incOM)
A	- potential of gas production (asymptote, when t = ∞; mL g^-1 incOM)
B	- gas production relative rate (mL h^-1)
C	- constant factor of microbial efficiency (h^-1)
t	- incubation time (h)

In addition, the incubation time at which maximum speed (TVmax) of gas production was reached, based on the second derivative of Gompertz model, evaluated at zero (inflection point of this type of sigmoidal model). Maximum gas production rate (Vmax; mL g^-1 incOM h^-1) was also estimated, by replacing TVmax in the first derivative of the model.

Statistical analysis. To determine accumulated gas production, being a measure repeated in the same experimental unit, the methodology proposed by ^{Gómez et al. (2019)} was used. First, it was checked if data met normality assumption for Shapiro-Wilk (^{Oliveira-Borgatti et al. 2015}) test. By not fulfilling this requirement, the generalized linear mixed model was used using Glimmix procedure (^{SAS 2010}). Repetitions (bottles) were considered as random effect. Different distributions were tested on data: Poisson, Gamma, Normal Log and Igauss. The best fit was achieved with Gamma distribution and its link function was Log. For comparison of means, Tukey-Kramer fixed range test (^{Kramer 1956}) was used for P <0.05. In the model, treatments (T₀, T₃₂, T₇₈ and T₁₀₀) were considered as effects with four bottles per treatment and four replicates, as well as incubation times and interaction (treatments x times). Four control bottles (bottles without substrate, to know the contribution of ruminal inoculum gas) were included. To simplify the analysis, incubation time (96 h) was divided into three phases. Initial stage included times 2, 4, 6, 8 and 10, intermediate stage was composed 12, 16, 20 and 24, and the final stage had 36, 48, 72 and 96 h.

Results and Discussion

Chemical composition of silage. Sweet potato integral silage showed good organoleptic characteristics: pH (3.5), DM (30.1%), CP (6.0), NDF (20.0,) ADF (17.0) and ammonia nitrogen /% total nitrogen (N-NH₃ /% TN) (2.2%). The organoleptic characteristics and acidity were similar to those reported by ^{Solís (2011)}, who ensiled integral sweet potato, in equal proportion tuber / foliage, using silo press technique.

In vitro ruminal fermentation of diets. In vitro gas production (mL g^-1 incOM) showed interaction between treatments and incubation times (P <0.0001) in the three stages in which the fermentation was divided (table 2). Accumulated gas production increased as incubation time increased, but this increase depended on the treatment. Diets based on sweet potato integral silage showed higher accumulated gas volumes compared to the treatment without silage (T₀), mainly the levels T₇₈ and T₁₀₀. There were no differences among these last during the incubation period (P <0.05).

Tabla 2 Accumulated in vitro gas production with the substitution of corn for sweet potato integral silage (mL g^-1 incOM)  

Initial stage
	Incubation times (h)
Treatments	2	4	6	8	10	SE± and probability
T0	0.72^j (2.05)	2.31^g (10.04)	3.25^e (25.91)	3.90^c (49.37)	4.32^b (75.03)	0.04 P<0.0001
T32	0.80^j (2.21)	2.44^g (11.43)	3.39^e (29.61)	4.01^c (55.20)	4.41^b (82.09)
T78	1.30^i (3.67)	2.74^f (15.48)	3.63^d (37.57)	4.24^b (69.11)	4.62^a (101.98)
T100	1.51^h (4.51)	2.83^f (16.95)	3.69^d (39.89)	4.29^b (73.17)	4.68^a (108.28)
Intermediate stage
	Incubation times (h)
Treatments	12	16	20	24	SE± and probability
T0	4.61^f (100.07)	4.88^d (131.97)	5.07^c (159.50)	5.20^b (181.91)	0.02 P<0.0001
T32	4.67^e (106.86)	4.91^d (136.29)	5.08^c (161.36)	5.21^b (182.41)
T78	4.88^d (131.93)	5.09^c (162.29)	5.23^b (186.26)	5.32^a (205.11)
T100	4.92^d (137.24)	5.11^c (166.42)	5.24^b (189.04)	5.33^a (207.29)
Final stage
	Incubation times (h)
Treatments	36	48	72	96	SE± and probability
T0	5.35^f (210.74)	5.45^e (231.63)	5.52^d (248.51)	5.56^bcd (260.74)	0.01 P<0.0001
T32	5.36^f (213.24)	5.46^e (234.88)	5.53^cd (252.79)	5.58^abc (265.04)
T78	5.45^e (233.07)	5.53^cd (252.70)	5.59^ab (268.28)	5.63^a (278.93)
T100	5.45^e (232.52)	5.53^d (251.50)	5.59^ab (266.93)	5.63^a (277.30)

^{a, b, c, d, e, f, g, h, i, j}: Transformed means with different letters differ at p<0.05; ( ): Original means, mL; p: probability value; SE±: standard error of transformed means

From the beginning of fermentation process (initial stage), treatments with sweet potato integral silage showed greater accumulated gas production, performance that remained until the end of this stage. However, there were no differences (P> 0.05) between T₀ and T33, as well as between treatments with high levels of silage (T₇₈ and T₁₀₀). When comparing the accumulated gas production between hour 2 and 10, an increase of 73.0, 80.0, 98.0 and 103.8 mL g^-1 incOM was observed for T₀, T₃₂, T₇₈ and T₁₀₀, respectively.

In diets with silage, sugars and carbohydrate complexes of sweet potato that did not fermented during silage process were responsible for this increased fermentative performance. Sweet potato tuber, main component of this silage, contains high sugar levels, between 8.26 and 31.65% (^{Aliaga and Nieto 2009}), and 71.4% of starches (^{DeBlas et al. 2010}). However, in the silage process, microorganisms use approximately 4.0% of sugars (^{Fernández 1999}). This is not the case with starches, which remain as such in the silage (^{Martínez-Fernández et al. 2014}).

In studies including potatoes (Solanum tuberosum), ^{Noguera et al. (2006)} also observed that increasing the availability of non-structural carbohydrates with rapid degradation in the diet, increased the final volume of gas production.

During the initial stage of this study, treatments with high corn levels (T₀ and T₃₂) had the lowest gas productions, which could be related to their low sugar content (1.0 to 2.0), according to ^{Martínez-Guardia et al. (2016}). However, starches were high (72.0%), as described by ^{DeBlas et al. (2016)}. Being protected by a protein matrix, starch degradation is slower than sugars. ^{Depetris (2013)} reported that this protein matrix, that covers corn starch granules, limits starch digestion, prevents microbial colonization and retards the action of amylolytic enzymes.

In this research, estimated starch levels (table 1) of diets were very similar, 36.2% (± 1.27) on average. This supports the approach that lower gas production in treatments with high levels of corn, in the initial fermentation stage, is related to the lower degradability of this carbohydrate.

In the intermediate stage, accumulated gas production increased as silage level and incubation times increased, with higher levels of gas production in high silage treatments. However, increase in gas production between 12 and 24 h decreased as silage level increased in the diet, with values of 82.0, 75.5, 73.2 and 70.5 mL g^-1 incOM for T₀, T₃₂, T₇₈ and T₁₀₀ respectively. In relation to the initial stage, gas production increased in the control diet with corn (T₀), but decreased in diets with silage. This decrease could be related to the appreciable reduction of sugars and fermentable starches. The highest gas productions in T₀ can be attributed to the penetration of amylases in the protein matrix that surrounds and protects corn grain starch, which favored its fermentation, as described by ^{Mendoza and Ricalde (2016)} in studies of cattle feeding with diets rich in grains.

The variables under study, in the final fermentation stage, had a similar effect on the accumulated gas production. Gas production, between 36 and 96 h, decreased significantly, with values of 50.0, 51.8, 45.2 and 44.8 mL g^-1 incOM for T₀, T₃₂, T₇₈ and T₁₀₀, respectively. This is attributed to the limited availability of fermentable substrates, but also to microbial recycling, especially at the end of fermentation (^{García 2009}). Regarding diets with silage, criteria of ^{LI Jian-nan et al. (2014)} should be considered, who stated that silage process decreased in situ ruminal degradability of DM and sweet potato and potato tuber starch. These authors found that, at 24 and 72 h of incubation, 45.5 and 15.0% of sweet potato starch did not experience ruminal degradation.

The sweet potato integral silage treatments contained oil palm cake, cataloged by ^{Rivadeneira (2018)} as a medium quality food. In this study, results of in vitro degradation of this ingredient, fermented in parallel, indicated low contribution in gas production (3.75, 3.46, 3.20, 3.13, 2.60 and 2.20 mL h^-1 at 12, 24, 36, 48, 72 and 96 h, respectively). This indicates that it had no significant effects on the performance of treatments, regarding gas production. All diets contained urea for CP balance. However, authors such as ^{Carnevali et al. (2002)} and ^{Velásquez et al. (2013)} indicated that urea does not contribute directly to gas production, it only helps to achieve the nitrogen level required for adequate microbial activity.

Table 3 shows kinetic parameters of accumulated gas production, estimated with Gompertz model. Treatments with sweet potato integral silage had a greater potential for accumulated gas production (parameter A), which is related to a greater nutrient availability, which favor microbial growth and, consequently, gas production. This performance is related to the content of available soluble carbohydrates, as well as with the quantity and quality of fiber from diets with silage. Relative rate or mean gas production rate (parameter B) showed very similar values among silage treatments, but with lower values than the treatment with the highest corn level (T₀). This can be explained by the highest fermentative activity in the intermediate and final stages of incubation, due to the best availability of fermentable substrates. Microbial synthesis efficiency (parameter C) slightly increased in diets with silage, which indicated higher microbial growth per unit of time, greater substrate degradability and gas production (^{Rodríguez et al. 2017}).

Table 3 Kinetic parameters of in vitro gas accumulated production, according to Gompertz model  

Treatment	Parameter A (±SE)^(1)	Parameter B (±SE)	Parameter C(±SE)	SE^(2)	R2	Vmáx	TVmáx
T0	244.16 (±1.85)	3.83 (±0.14)	0.10 (±0.003)	12.98	0.98	8.98	13.43
T32	248.01 (±2.11)	3.57 (±0.14)	0.11 (±0.003)	14.67	0.97	10.04	11.57
T78	260.34 (±2.13)	3.63 (±0.15)	0.13 (±0.004)	15.78	0.97	12.45	9.92
T100	258.00 (±2.21)	3.62 (±0.16)	0.13 (±0.004)	16.61	0.97	12.34	9.90

⁽¹⁾Standard error of the parameter; ⁽²⁾ Standard error of the curve; Parameter A: mL g^-1 incOM; Parameter B: mL h^-1; Parameter C: h^-1; Vmax: mL g^-1 incOMh^-1; TVmax: mL h^-1

The curvilinear performance of gas production speeds is presented in figure 1. Regardless of the treatment, in the first 18 h of incubation, the highest gas production speeds were obtained, mainly in treatments with high silage levels. These performances depended on the availability of fermentable substrates, especially sugars, and easily fermented starches. Practically after 48 h of incubation, the fermentative activity was minimal, which can be related to substrate depletion, but also with microbial recycling (^{Cone et al. 1997}), as a result of the accumulation of intermediate and final products of its metabolism. They act, physically or chemically in the medium, and affect microbial activity.

Figure 1 Performance of gas production speed (mL g^-1 incOM h^-1) in the time 

It is concluded that diets with the highest levels of sweet potato integral silage achieved higher accumulated gas productions. In the first 18 h of incubation, silage diets showed greater fermentative activity, while in high corn levels, this performance occurred after this time. Diets with this type of silage showed greater nutrient availability for ruminal microorganisms, which favors energy contribution for different metabolic processes in the rumen.

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Received: June 26, 2019; Accepted: December 19, 2019

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