Animal Science

Evaluation of the substitution of granule corn by integral sweet potato silage (Ipomoea batata, L.) in a diet for the fattening of male cattle

0000-0001-5195-4566M. H. Ruiloba¹  , 0000-0003-0118-6743J. Maure²  , 0000-0003-2472-556XC. Solís^{3  *} 

¹Grupo de Ciencia y Tecnología para el Desarrollo (GRUCITED), República de Panamá

²Instituto de Investigación Agropecuaria de Panamá (IDIAP), República de Panamá

³Universidad de Panamá, Facultad de Ciencias Agropecuarias, República de Panamá

ABSTRACT

In order to evaluate the substitution of ground granule corn by integral sweet potato silage in a diet for male cattle fattening, a completely randomized design with factorial arrangement 2x2 was used. A total of two substitution levels of the granule corn by integral sweet potato silage, dry basis were studied: 0.0 and 100.0 % and two evaluation periods (28 days each), with previous adaptation of 21 days. The diets were iso-energetic (10.88 MJ ME kg^-1 DM) and iso-protein (12.0 % CP). The silage had good organoleptic characteristics. There was not effect of the silage (p > 0.05) on dry matter intake, crude protein and metabolizable energy (2.66 kg, 0.321 and 27.97 MJ 100 kg^-1 LWday¹). The daily weight gain (p > 0.05), the dry matter conversion and metabolizable energy (p > 0.05), with average of 1.84 kg animal^-1 day^-1, 6.86 kg and 72.00 MJ kg^-1 weight gain , respectively was neither affected. The substitution of 0.0 % showed better crude protein conversion than those of the 100.0 % (0.796 y 0.849 kg kg^-1of weight gain) (p < 0.01). The weight in hot carcass was not affected by the silage, with average of 269.4 kg animal^-1 (p > 0.05).The feeding cost of the treatment with the silage was 21.4 % lower than those with the ground granule corn. It is concluded that the integral sweet potato silage have viability as competitive alternative for the substitution of the granule corn in fattening diets of male cattle.

Key words: silage; sweet potato; substitution level; weight gain; feeding cost

The sweet potato (Ipomoea batata) is a crop of great biomass production and adaptation to a variety of tropical edaphoclimatic conditions (^{Ruíz et al. 2012}). Other important characteristic of this crop is that the tuber and the foliage can be use in cattle feeding (^{Solís et al. 2019}). On dry basis, the tuber has high content of sugars (8.3 - 31.6 %) and starch (60.0 - 70.0 %), low crude protein content (CP, 3.5 - 5.0 %) (^{Aliaga and Nieto 2009} and ^{DeBlas et al. 2010}), while the foliage show more content of CP(8.3 - 19.7 %), crude fiber (8.7 - 32.9 %) and ash (8.6 - 20.2 %) (^{Sologuren 2008} and ^{Vélez 2019}).The integral sweet potato silage( tuber + silage) have good fermentative and nutritional characteristics (^{Alvarado 2015} and ^{Solís et al. 2019}), with CP levels from 4.2 to 9.2 % and in vitro digestibility of dry matter (IVDDM) from 64.9 to 90.2 %, depending on the tuber/foliage relation (^{Alvarado 2015} and ^{Solís and Ruiloba 2017}). In dairy cattle with high production, ^{Quezada (2001)} introduced the integral sweet potato silage (ISPS) in the feeding, without prove adverse effects on live weight, food intake and milk production. In growing male cattle, ^{Solís (2011)} achieved to substitute more than 90% of the metabolizable energy (ME) provided by the ground granule corn (GKC), without affecting the live weight gain (LWG). When considering the nutritional and productive characteristics of the integral sweet potato silage, the objective of this study was to evaluate the substitution of the ground granule corn by ISPS in an finishing diet for male cattle.

Materials and Methods

The study was carried out at the Centro Experimental El Ejido, from Instituto de Investigación Agropecuaria de Panamá (IDIAP), at 25 m.o.s.l, anual average temperature of 27.4 °C, rainfall of 1203 mm year^-1, relative humidity of 75 % and annual average solar radiation of 1188.5 watt m^-2. The Tainung 66 variety was used, due to its favorable yield of fresh biomass (86 t ha^-1, 53.0 % tuber and 47.0 % foliage), according to ^{Batista (2006)} reports. This variety was cultivated according to the methodology described by ^{Ruíz et al. (2012)} and it was harvested at 125d after sowing The harvested material was left in the field for 48 h to facilitate the pre-drying (^{Solís 2011}). The tuber and the foliage were simultaneously introduce in a mechanic chopper for achieving an homogeneous integral material and chop size between 0.5 and 2.5 cm for the foliage and 0.5 and 1.5 cm for the tuber. The material was ensiled in a trench type silo without additives and it began to use at 30 days after its manufacture.

Through a completely random design and factorial arrangement 2 x 2 two substitution levels of the GKC by ISPS (CS), dry basis, in a fattening diet of male cattle: 0.0 and 100.0% (CS₀ and CS₁₀₀) and two evaluation periods (EP₁ and EP₂) were studied. The experimental diets were iso-protein and iso-energetic (table 1).

Table 1 Ingredients and chemical composition of the experimental diets (dry basis) 

Ingredients	Chemical composition			Diet composition, %
	CP, %	ME*	NDF, %	CS0	CS100
GKC^1	9.5	13.0	9.8	50.2	0.0
Swazi^1 hay	4.5	7.5	72.0	29.0	18.2
Soybean^1 cake	44.0	11.7	10.3	10.5	10.5
Coquito^2 cake	15.0	12.1	66.8	9.5	19.7
ISPS^3	5.5	10.9	22.0	0.0	50.2
Urea	270.0	0.0	0.0	0.0	0.6
Mineral salt	0.0	0.0		0.8	0.8
Total				100.0	100.0
CP, %				12.0	12.0
ME*				10.9	10.9
NDF, %				33.20	38.40

ME: metabolizable energy, MJ kg DM¹; NDF: neutral detergent fiber

^{1NRC (1996)}, ^{2Vargas and Zumbado (2003)}, ^{3Solís and Ruiloba (2017)}

A mealy corn, ground at 2.0 - 3.0 mm particle size was used. In order to balance the CP and ME levels in the diets, coquito cake and urea was included. The coquito cake is a byproduct from the mechanic extraction of the fruit oil of the African palm (Elaeis guineensis).

According to each treatment, the dry supplement base on GKC, soybean cake, coquito cake, urea and mineral salt was prepared. In CS_0, the supplement was only offered in the feeder, while in the CS₁₀₀ it was mixed with ISPS. In both treatments, the hay was offered whole in hanging feeders. It was included 5.0 % additional to the quantity corresponding to the ration to compensate the waste on the floor. The complete ration was offered at a rate of 3.0 and 6.0 kg fresh (2.70 kg DM) 100 kg LW d^-1 for CS₀ and CS₁₀₀ in equal parts, twice a day (8:00 a.m. and 2:00 p.m.) and water ad libitum.

A total of five male crossing animals Brahman-Brown Swiss per treatment were used, with weigh and average age at the start of the adaptation period of 386.8 (± 24.0) kg and17.0 months of age. They were management in their growing and developing stages under adequate feeding conditions in grazing. Initially, the animals received treatments against endoparasite and ectoparasite. Each group of animals was allocated in a 30 m² covered pen. The study lasted 77d and included an adaptation period of 21 d and two evaluation periods of 28 d each (EP₁ and EP₂). At the beginning of the study and at the end of each period the animals were weighed in fasting, at 8:00 a.m. In the adaptation period, the amount of offered food was periodically increased until get to the established maximum level and in the evaluation periods was fitted every 14 d in function of the LW and the established intake level. In the first evaluation period, the LW was defined from the LWG estimate in accordance with the ^{NRC (1996)}, and in the second one based on the weight gain obtained in the first. The food intake was daily recorded per group, according to the amount offered and rejected. Every 14d, the average intake per animal was calculated. In addition, samples from the diets ingredients for the DM and CP determination were taken.

When finish the second evaluation period (EP₂) and immediately after determine the final weight, the animals were taken to the slaughterhouse, where they were individually weighed when enter. Here they stay in resting and without food during 8h until their slaughter, moment in which the individual weight of the hot carcass was taken. The evaluation indicators included LWG, intake of DM, (DMI), CP, (CPI) and ME (MEI), DM conversion (DMC), CP (CPC) and ME (MEC), yield in hot carcass (YHC) and intake ration cost (IRC). The intake ration cost only included the ingredients cost. For the ground granule corn and integral sweet potato silage, was of $0.338 and 0.166/kg MS (American dollar, $).

The statistical analysis included the normality test (^{Shapiro and Wilk 1965}) and analysis of variance. In the case of the LWG, the initial live weight in the evaluation period as covariate was used. The analysis of variance of LWG included the effect of the independent variables CS and EP, the effect of the randomized variable nested animal in CS and the interaction CS*EP. For the DMI, CPI and MEI, the analysis only included CS and. The data were analyzed with the statistical program SAS (^{SAS 2010}).

Results and Discussion

The sweet potato silage showed light brown color and typical smell of a lactic fermentation, characteristics similar to those obtained by ^{Solís (2011)} when ensiling integral sweet potato with the silo-press technique. These characteristics showed enough availability of carbohydrates easily fermentative for an adequate fermentation activity in the ensilage process.

The variables of DM, CPAMD ME intake showed normal distribution (Pr > 0.0500). The DMI was not affected by the interaction CS*EP (p = 0.7472), CS (p = 0.6998) and EP (p = 0.5678).Table 2 show the DMI values in function of the CS and EP, with general average of 2.66 kg DM 100 kg LW d^-1, very similar to the established feeding level, practically without rejection. The average intake of GKC and ISPS was similar between treatments, with values of 1.31 and 1.36 kg DM 100 kg LW d^-1 for CS₀ and CS₁₀₀.With a control diet, based on granule corn, ^{Solís and Ruiloba (2017)} informed that the inclusion of ISPS up to 60 %, dry basis, did not affected the DMI of growing male cattle (2.64 kg DM 100 kg^-1 LW d^-1, as average), while with 73 % of ISPS, the DMI decreased 15.6 %, due to the rejection of silage, constituted in a large extent by its forage fraction (stems). When considering an average starch content for the GKC and ISPS (^{Corcuera et al. 2016} and ^{DeBlas et al. 2019}) and 44.3 % (^{Solís 2020}), it was estimated a starch intake of 0.87 and 0.60 kg 100 kg^-1 LW d^-1, which corresponded to 33.0 and 22.4 % of intake DM for CS₀ and CS₁₀₀,respectively.

Table 2 Intake of dry matter, crude protein and metabolizable energy per treatment 

	DM intake, kg 100 kg^-1 LW d^-1			CP intake, kg 100 kg^-1 LW d^-1			ME intake, MJ 100 kg^-1 LW d^-1
Treatment	Evaluation period		Average	Evaluation period		Average	Evaluation period		Average
	1	2		1	2		1	2
CS0	2.63	2.65	2.64	0.313	0.316	0.315	28.18	28.89	28.55
CS100	2.64	2.71	2.68	0.324	0.330	0.327	27.30	27.88	27.59
Average	2.64	2.68		0.318	0.323		27.72	28.39
p	0.5678		0.6998	0.6020		0.2072	0.3081		0.1605

p: probability

SE: standard error 0.003, 0.004 and 0.006 for DMI, CPI and MEI respectively

The CPI and MEI were not affected by the interaction and the independent variables studied (p ≥ 0.1607). The average values of CPI and MEI per treatments are show in table 2, with general average of 0.321 kg and 28.10 MJ 100 kg^-1 LW d^-1, respectively. Based on these intakes, the intake DM showed CP content of 11.93 and 12.20 % and the ME of 10.80 and 10.30 MJ kg^-1 DM for CS₀ and CS₁₀₀, respectively. According to ^{NRC (1996)} recommendations for animals with weight and LWG similar to this study, the CPI was higher in 16.4 %, performance that also depends on the quality of the protein sources used. The excessive CP intake led to an additional energetic expense, which can affect the animal response (^{Di Marco 2006}). Whereas, the MEI was equal to that indicate by the ^{NRC (1996)}. In terms of the MEI, the GKC and ISPS contribute 16.75 and 14.65 MJ 100 kg LW d^-1, which corresponded to 59.0 and 52.2 % of the ME total intake that showed CS₀ and CS₁₀₀, respectively, although the LWG did not affected. The ISPS only replace 88.5 % of the ME provided by the GKC. The difference was covered with the coquito cake, with contribution of 10.2 and 23.5 % of MEI of CS₀ and CS₁₀₀, respectively. On the fat content of coquito cake of 13.2% , dry basis (^{Vargas and Zumbado 2003}) in the diet CS₀ and CS₁₀₀, this one represented 1.25% and 2.60 of the intake DM. However, when considering the true contribution of the GKC, the fat level in the diet CS₀ increased approximately 2.75%. In both diets, these levels did not has negative effects on the digestibility of DM and the fiber, due to there were lower to the 4.0% showed by ^{Martínez et al. (2011)} as maximum level for do not affect this indicators.

In the adaptation period, the average live weight gain (LWG) was similar between treatments: 1.364 kg animal^-1 d^-1. In the evaluation periods, this variable was normal (Pr > 0.0500) and was not affected by the covariable LW (p = 0.4290), and neither by the interaction CS*EP (p = 0.2580) and variables nested animal in CS (p=0.4192), CS (p=0.2295) and EP (p = 0.4600), with general average of 1.836 kg animal d^-1. With diets similar to those of this study ^{Solís and Ruiloba (2017)} did not obtained differences when substituting the ME of GKC by the one of the sweet potato silage in growing male cattle, although the average LWG was lower (1.400 kg animal d^-1). The LWG obtained with the diet based on GKC (table 3) was similar to that informed with diets rich in granules for cattle finishing (^{Buckner et al. 2007}, ^{Arelovich et al. 2012} and ^{Vittone et al. 2015}).

Table 3 Live weight and live weight gain per treatment 

Substitution level of corn	LW, kg animal^-1		LWG, kg animal^-1 d^-1
	Initial	Final	Evaluation period		CS
	LWi^1	LWf^2	EP1	EP2	Average
CS0 (control)	414.7	519.6	1.860	1.884	1.876
CS100 (diet with ISPS)	415.3	515.3	1.850	1.740	1.795
Average	415.0	517.4	1.855	1.812
p	0.9700	0.7502	0.4600	0.2295

¹Live weight at starting the EP₁, ²Live weight at finishing the EP₂; SE: standard error, 6.900, 6.540 and 0.050 for LWi, LWf and LWG, respectively.

The interaction CS*EP was not significant for the DM conversions (p = 0.7328), CP (p = 0.4143) and ME (p = 0.1600). The conversion of DM (DMCO) and metabolizable energy (MECO) (table 4) were not affected by silage level and experimental period (table 4), with average of 6.85 kg and 72.01 MJ kg^-1 of LW increase, respectively. In contrast, the CP has better conversion in the corn treatment (table 4), difference that could be related with the urea, although this one only contributed approximately 13.5 % of the ration CP, a very lower level to the maximum of 30.0 % recommended. With similar diets, ^{Solís (2011)} did not obtained effect of the ISPS level on these conversions, with average values of 6.18, 0.790 and 16.48 for DMCO, CPCO and MECO, respectively.

Table 4 Conversion of the dry matter, crude protein and metabolizable energy of the experimental diets 

	DM conversion (DMCO), kg 100 kg^-1 LW d^-1			CP conversion (CPCO), kg 100 kg^-1 LW d^-1			ME conversion (MECO), MJ 100 kg^-1 LW d^-1
Treatment	Evaluation period		Average	Evaluation period		Average	Evaluation period		Average
	1	2		1	2		1	2
CS0	5.98	7.34	6.66	0.709	0.883	0.796	63.89	80.80	72.34
CS100	6.28	7.81	7.04	0.771	0.927	0.849	64.94	78.50	71.72
Average	6.13	7.58		0.740	0.905		64.39	79.63
p	0.0016		0.1214	0.0001		0.0066	0.0001		0.5313

p: probability; SE: standard error, 0.009, 0.005 and 0.125 for DMI, CPI and MEI, respectively

The weight of hot carcass was not affected for the CS (p = 0.6921) with average of 272.4 and 266.5 kg animal^-1 for zero level and 100% of substitution, respectively, result that validate the effect recorded when using ISPS in the weight and gain of LW at finishing.

In energetic terms, ISPS contribute much of the ME of the diet, mainly starch, although sugars too, despite in the silage process part of the sugars are fermented (^{Fondevila 2015}). However, the sweet potato was ensiled in relatively big pieces (0.5 - 1.5 cm), which could limit the fermentative action of the bacteria on these compounds, favorable effect to the energetic contribution of ISPS of the diet. In the silage, sugars and starches were mainly providing by the tuber.

Under in vitro conditions, 1.0 mm particle size and 24h of incubation, ^{Solís (2020)} obtained average degradabilities for the starch of the integral sweet potato mixture, pre-drying and ensiled of 86.4 and 82.9 %, respectively, difference attributed to the silage process. ^{LI Jian-nan et al., (2014)} reported that the silage process decreased the in situ ruminal degradability of the starch from the silage sweet potato tuber of 83.8 to 54.5% at 24h of incubation. This performance could be attibuted to that during the silage process the fermentated starch mainly corresponded to the degradable fraction, which proporcionally increased the resistant fraction. ^{Englyst et al. (1999)} showed that the most of starches have a fraction quickly digestible, slowly digestible and other resistant. ^{Solís (2020)} also refers that at 24h of in vitro incubation the starch of GKC and ISPS was degraded 75.0 and 67.0 %, while at 24 h of incubation 90.1 and 82.9 %, with degradability rate of 2.48 and 2.01 %/h, respectively. At a rate of passage of 0.06 %/h was reported for the starch of the granule corn that the average effective ruminal degradability was of 61.9 % (^{Offner et al. 2003}, ^{Jiang 2005} and ^{Calsamiglia 2016}) and for the sweet potato tuber of 54.9 % (^{Jiang 2005}).These results not only show low ruminal fermentation level of the ISPS starch, also make possible high bypass level to the low parts of the gastrointestinal tract. Other authors (^{Dreher et al. 1984}, ^{Offner et al 2003}, ^{Tecson 2007} and ^{Calsamiglia 2016}) informed ruminal degradability values for the corn starch higher to that indicate for the sweet potato tuber.

The starch degradation depends on its type or nature, but also of own physiological aspects and of external components which surround the granule (^{Giuberti et al. 2014} y ^{Faccio et al. 2020}). If it is compare with the corn starch, the granule of the sweet potato starch have high content of CP and ash, low fiber content and equal content of lipids and N free extract (^{Hernández et al. 2008} and ^{Alvani et al. 2011}). Starches can form insoluble complex between phospholipids and amylose or the amylopectin ramified chain, with reduction effect of the enzymatic hydrolysis of this carbohydrates, as has been informed with several cereals (^{Crowe et al. 2000} and ^{Singh et al. 2010}).During the silage process, the starches has enzymatic hydrolysis, effect that in the sweet potato tuber can mainly occur on the amylopectin (^{Knowles et al. 2012}),with increase in the amylose/pectin proportion, which decreased the starch degradability (^{Bednar et al. 2001} and ^{Brewer et al. 2012}). ^{Solís (2020}) results showed that the silage process produce 11.0 % of starch loss in the sweet potato integral mixture, via enzymatic hydrolysis and effluents. It is also point out that the amylopectin have higher molecular weight and superficial area than the amylose (^{Singh et al. 2010} and ^{Tetlow and Bertoft 2020}), characteristic that make it more sensitive to amylolitic attack. The hydration is a characteristic of the starch granule, which increase with temperature (^{Hernández et al. 2008}).In the silage, this process can occur with the granule breaking, situation that will favor the starch hydrolysis and sugars release for the fermentation .

The morphological aspect of the granule can also affect the starch hydrolysis. The granule of the starch corn has polygonal shape and those of the sweet potato, spherical shape (^{Hernández et al. 2008}). This allow higher superficial area and possibility of amylolitic attack to the corn starch. Regardless the size, the granule of corn starch and sweet potato has dimensions of 15.0 and 12.4 microns, respectively (^{Hernández et al. 2008}). As higher size, lower specific area per occupied volume, which showed that the sweet potato granule has higher superficial area and sensitive to amilolitic attack. The characteristics of the granule surface can also affect the enzymatic hydrolysis level of the starch. The corn granule has small holes or pores that allow the amylose inlet, facilitating the starch hydrolysis (^{Dreher et al. 1984}). Whereas, in plain surfaces, like the granule of the potato (^{Singh et al. 2010}), the enzyme has to break it or eroded for penetrate its inside, situation which can be similar in the sweet potato and other tubers. This can cause great resistance to the enzymatic hydrolysis.

It is known that in the corn, the starch is surrounded by the protein matrix which affects or retard their enzymatic hydrolysis (^{Bednar et al. 2001} and ^{Gómez et al. 2016}), obstacle that the sweet potato did not has. It has been showed a reverse relation between the amylose-amylopectin proportion and starch degradability (^{Bednar et al. 2001} and ^{Brewer et al. 2012}), product of high amount of hydrogen bonds that the amylose make, situation which made it more compact with less surface per molecular area (^{Brewer et al. 2012}) and more resistance to enzymatic action. As average, the amylose-amylopectin relation of the corn is higher than those of the sweet potato, with values of 27.4:72.6 and 22.1:77.9 %, respectively (^{Hernández et al. 2008}, ^{Gómez et al. 2016} and ^{Manzanillas 2018}). However, there is not information of this proportion for the starch of the sweet potato silage, process that can increase it due to the starch loss, possibly as consequence of high enzymatic hydrolysis of the amylopectin. Another aspect to be considered is the possibly presence of amylose inhibitors, as has been informed in beans, rye, wheat and oat (^{Singh et al. 2010}), although there is not information for the sweet potato.

There are many factors which can affect the starch hydrolysis, ones in favor of corn, but others in favor of sweet potato. However, results available in vitro and in situ show low ruminal degradability for sweet potato starch, situation that can represent low energetic availability for the microbial activity, but high availability at intestinal level as glucose. Other aspect in favor of a diet with ISPS is that a lower energetic contribution of starch at ruminal level could be compensating by the sugars in the tuber. It is also to consider that the silage approximately contribute 30.0 % of NDF of the diet, which contribute to the ruminal energetic production. This is maintain in in vivo and in vitro studies (^{Solís 2011} and ^{Solís et al. 2021}), where there were obtained degradabilities for the NDF of 64.0 % for a diet high in ISPS, similar to the one of this study, and 33.0 % for ISPS only, respectively.

The lower ruminal degradability of the starch could represent high flow of this one to the low parts of the gastrointestinal tract, but in absolute terms depends on the amount intake, degradability rate and ruminal passage. With ISPS is possible that certain amount of sugars be bypass and reach the intestine. This carbohydrates are use more efficient intestinal way than ruminal (^{Owens et al. 1986}), performance that favors the use of intake energy in the diet with ISPS.

Some of the qualities or characteristics stated in terms of sugars, starch and NDF explain the capacity of diets based on integral sweet potato silage to generate LW gains equals or similar to those reached with ground granule corn, as took place in this study and was previously informed with similar diets in developing beef cattle (^{Solís and Ruiloba 2017}).

The intake ration cost (RC) was of 3.69 and 2.90 $ animal^-1 d^-1 for CS₀ and CS₁₀₀, respectively, which show decrease of 21.4 %, in favor of the diet with silage. This economic result and the productive indicators analyzed show the viability of the integral sweet potato silage, as competitive alternative for the substitution of the granule corn in finishing diets of male cattle.

Conclusions

The substitution of granule corn by integral sweet potato silage reduce the feeding cost, allow similar live weight gains, did not affect the intake and conversion of dry matter, metabolizable energy and of the weight in hot carcass.

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Received: June 10, 2022; Accepted: December 20, 2022

^*Email: [e-mail]

Conflict of interest: The authors declare that there is not conflict of interest among them. .

Authors contribution: M. H. Ruiloba: Conceptualization, data curation, formal analysis, funding acquisition, methodology, project administration, resources, validation, writing -original draft, writing and review and editing of the last version. J. Maure: Conceptualization, investigation, project administration, resources, supervision, writing, review and editing of the last version. C. Solís: Formal analysis, funding acquisition, methodology, validation, writing-original draft, writing, review and editing of the last version.