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Sweet potato (I. batatas L.) is a crop with agronomic, productive and nutritional characteristics of interest for animal feeding, due to its high biomass production in tubers (roots) and foliage (stems and leaves) (Solís 2011). The sweet potato tuber contains high levels of sugars (8.3 to 31.6 %) (Aliaga and Nieto 2009), mainly sucrose (Grüneberg et al. 2015) and starches (55.0 to 71.5 %) (Grüneberg et al. 2015, DeBlas et al. 2019 and Solís 2020). In contrast, its crude protein (CP) content is low (3.0 to 7.1 %) on dry basis (Almeydas 2016 and DeBlas et al. 2019). The constitution of starch is 75.0 to 83.0 % amylopectin and 17.0 to 25.0 % amylose (Agama et al. 2013 and Jiménez et al. 2019).
Few studies are reported in the literature on the use of fresh sweet potato tuber (FSPT) in ruminants feeding. Goyzueta (1963) substituted 59 % of the corn bran for FSPT in a diet for cattle, and achieved better live weight gain (LWG), food conversion and lower feeding cost. Ruggia et al. (2010) also obtained higher LWG, when supplementing grazing steers with FSPT. Its energy composition and high in vitro digestibility, fresh and ensiled, (Backer 1976 and Alvarado 2015) foresees good nutritional responses in terms of milk and meat production.
Based on the above, the objective of this study was to evaluate the productive response of male cattle, by substituting the corn grain by different levels of fresh sweet potato tuber in a fattening diet.
Materials and Methods
The study was carried out at the El Ejido Experimental Center, from the Instituto de Investigación Agropecuaria de Panamá (IDIAP), at 25 m o. s. l, temperature of 27.4 °C, rainfall of 1203 mm, relative humidity of 75 %, wind speed of 1.1 m/s and annual average solar radiation of 1188.5 watts/m2, with geographic location 7.9116-80.3686.
The sweet potato variety Tainung 66 was used, with a high tuber yield (45.4 t ha-1) (Ruíz et al. 2009). Harvesting began at 125 d after sowing, for which mechanical extraction and storage under natural shade were used. Initially, the harvesting process was carried out every 12 d, but it was reduced to 8 d, due to the physical deterioration of the tuber. Before harvesting, random samplings were carried out to determine the dry matter (DM) content and biomass yield of the tuber.
A completely random design was applied, in a 3 x 2 factorial arrangement, where three levels of FSPT were evaluated in a control diet based on milled corn grain (MCG) and two evaluation periods (EP: 1 and 2). The FSPT levels were 0.0, 28.0 and 56.0 % on a dry basis (TL: TL0, TL28 and TL56), which corresponded to reductions in the MCG level of 0.0, 50.0 and 100.0 %, respectively.
The diets were isoenergetic and isoprotein, made with swazi hay (Digitaria swazilandensis), MCG (Zea mayz, average particle 2 to 3 mm), soybean cake (Glicine max), nut cake (Elaeis guineensis), FSPT, urea and mineral salt (table 1), according to the recommendations of the NRC (1996). To estimate the metabolizable energy (ME, MJ kg-1 DM), values of 7.53, 12.53 and 11.72 were used for hay, MCG and soybean cake (NRC 1996), 2.90 for nut cake (Vargas and Zumbado 2003) and 11.13 for the FSPT (DeBlas et al. 2010, 2019).
Table 1 Composition and content of crude protein, metabolizable energy and neutral detergent fiber of the experimental diets, on a dry basis, %.
| Ingredients | Diet composition | ||
|---|---|---|---|
| TL0 | TL28 | TL56 | |
| Swazi hay | 28.7 | 23.0 | 17.3 |
| Corn grain | 55.3 | 28.0 | 0.0 |
| Soybean cake | 10.0 | 10.0 | 10.0 |
| Nut cake | 5.0 | 9.7 | 15.0 |
| Sweet potato tuber | 0.0 | 28.7 | 56.0 |
| Urea | 0.0 | 0.3 | 0.7 |
| Mineral salt | 1.00 | 1.00 | 1.00 |
| Total DM | 100.0 | 100.0 | 100.0 |
| Chemical composition of diet | |||
| Crude protein, % | 11.40 | 11.10 | 11.10 |
| ME, MJ/Kg DM (*) | 11.13 | 11.17 | 11.10 |
| NDF (%) | 30.30 | 29.40 | 28.50 |
(*): estimated values TL: FSPT level.
The complete ration was offered at a rate of 2.65 kg DM 100 kg of LW d-1, which corresponded to 3.0, 4.6 and 6.2 kg fresh 100 kg of LW d-1 for TL0, TL50 and TL100, respectively, in equal parts, twice a day (8:00 a.m. and 2:00 p.m.). Fresh water was also supplied ad libitum. The tuber was daily processed in an electric mincer, at an average size of 2 to 3 cm. It was mixed with the rest of the ration in the trough, except the hay. The latter was offered in hanging bags with an additional 5.0 % to compensate for trampling losses.
A total of twelve crossbreed male cattle (Brown Swiss by Brahman) were used, with an average initial weight of 358+23kg, average age of 16-18 months. Four animals per treatment were random distributed and two animals per pen were confined in covered pens (25m2). The study included an adaptation phase of 18 days and an evaluation phase with two periods (EP), of 30 days each. The animals were weighed individually, at the beginning and end of each period, fasting, at 8:00 a.m., on a digital scale. In each EP, samples of the ingredients from the ration were taken to determine DM and CP, according to the methodology described by AOAC (2016).
Productive and economic variables were evaluated: live weight gain (LWG), intakes of DM (DMI), CP (CPI), metabolizable energy (MEI), food conversion (FC), CP (CPC), ME (MEC) and feeding cost (FC). To determine the FC, the production cost of the chopped fresh tuber was estimated. Local market prices were used for the other ingredients in the ration. The production cost of the FSPT was obtained from the records of expenses, agronomic biomass yield and useful material offered to the animal.
Each evaluation variable was tested for normality (Shapiro and Wilk 1965). When this assumption was fulfilled, an analysis of variance was applied, which included the initial weight as covariance and the Tukey-Kramer (Kramer 1956) test to detect differences (p <0.05). For the statistical analyzes, the statistical package SAS, version 9.3 (2011) was used.
Results and Discussion
The FSPT showed average DM content and biomass yield of 32.0 (30.5-33.5 %) and 7.97 t DM ha-1 at 120 d after sowing. These yields are similar to those reported by Ospina et al. (2003) for different sweet potato clones. There were not changes during the tuber harvest period in its external appearance and texture. The sweet potato tuber is a perishable product, with a very active metabolism after harvest, a situation that was observed in this study. In a postharvest characterization of this tuber, under storage conditions at 30 ºC and 60 % relative humidity, García et al. (2014) reported daily fresh weight losses of 2.51 %, with appreciable variations in the texture and dehydration of the tissue, but with acceptable quality during the first 13 d.
The LWG (table 2) was not affected by the interaction TL * EP (P ≥ 0.58), TL (P ≥ 0.28) and EP (P ≥ 0.27), although the FSPT diets showed a tendency to greater response. The average LWG for EP1 and EP2 was 1.740 and 1.68 kg animal d-1, respectively. The control diet (TL0) showed LWG typical of high grain diets (Livas 2015 and Vittone et al. 2015). The LWG obtained in the diets with FSPT were attributed, to a great extent, to the contribution and energy quality of this product.
Table 2 Live weight gain, intake of dry matter, crude protein and metabolizable energy in the evaluation phase
| Indicator | Sweet potato tuber level in diet, % dry basis | ||||
|---|---|---|---|---|---|
| 0.0 (TL0) | 28.0 (TL28) | 56.0 (TL56) | SE± | ||
| Initial weight, kg (*) | 371.0 | 362.0 | 386.0 | 4.370 | 0.65 |
| Final weight, kg (*) | 468.0 | 462.0 | 491.0 | 10.900 | 0.58 |
| LWG, kg animal-1 d-1 (*) | 1.620 | 1.670 | 1.756 | 0.055 | 0.28 |
|
| |||||
| DM, kg | 2.66b | 2.67a | 2.76a | 0.010 | 0.01 |
| CP, kg | 0.303a | 0.293b | 0.300a | 0.001 | 0.01 |
| ME, MJ | 29.50 | 20.16 | 20.00 | 0.020 | 0.12 |
(*) in the evaluation period
abc: means with different letters differ to P < 0.05
TL: FSPT level
Backer et al. (1980), with a diet based on this tuber and urea, obtained LWG of 0.823 kg animal d-1 and food conversion of 6.63 kg DM kg of increase in LW-1, results that could imply a synchronization of energy-N and use of urea at an adequate ruminal level. Ruggia et al. (2010), when supplementing grazing steers (raygrass, Lolium multifloran) with FSPT (1.0 % with respect to live weight, fresh base), they also achieved LWG of 1.30 kg animal d-1, 62.5 % higher than that of the group, only in grass. Based on the results of different authors, Heuzé et al. (2015) showed that in diets for beef cattle, the substitution of 50 to 100 % of corn grain by dried sweet potato tuber resulted in very similar LWG, which corresponds to what was obtained in this study. The DMI was not affected by TL * EP (P> 0.10) and EP (P> 0.30), only by TL (P <0.01), with higher intake at the highest level of FSPT (table 2), 3.6 % higher than the intake with TL0 and TL28. This higher intake was attributed to the increase in experienced DM content. The average DMI per evaluation period was 2.70 and 2.68 kg 100kg live weight-1 d-1 for EP1 and EP2, respectively. With the TL56 diet, the tuber intake was 1.56 kg DM 100 kg of live weight-1 d-1.
There was no rejection of the rations and the animals did not show clinical symptoms of ruminal disorders. This allows inferring that higher intake levels can be obtained from this tuber. Backer et al. (1980) achieved an ad libitum intake of 2.41 kg DM (tuber-urea) 100 kg of live weight-1 d-1.
From an average starch content of 68.2 % for the corn grain and 64.2 % for the FSPT (DeBlas et al. 2010, Grüneberg et al. 2015, DeBlas et al. 2019 and Solís 2020), the average starch intake was estimated between 0.975 and 1.003 kg, 100 kg of live weight-1 d-1, which represented 37.8, 37.4 and 36.2 % of the total DM intake of TL0, TL28 and TL56, respectively.
The CPI (table 2) was not affected by TL * EP and EP (P> 0.05). On the other hand, the TL28 showed lower intake than the other two treatments (P <0.001). EP1 and EP2 had an average CPI of 0.301 and 0.296 kg, 100 kg of live weight-1 d-1, respectively. Of the total CP intake in TL28 and TL58, 6.3 and 19.3 was contributed by urea, levels that are low with respect to the recommended maximum (Barker 1970).
The TL*EP interaction and independent variables did not affect the MEI (P> 0.12) (table 2), with an average of 28.70 and 29.45 MJ 100 kg of live weight-1 d-1 for EP1 and EP2, respectively. The FSPT contribution to the total ME intake was 0.0, 29.0 and 59.0 % for TL0, TL28 and TL56, respectively. In the control treatment, the MCG contributed 64.5 % of the total ME intake. To achieve isoenergetic diets, the level of nut cake were increased, with a contribution to the total ME intake of 5.4, 10.8 and 17.3 % in TL0, TL28 and TL56.
Regarding the requirements of the NRC (1996) for beef cattle with similar weight and LWG conditions as those of this study, the food balance showed that the intake of DM, CP and ME supported the obtained LWG. However, they were 11.4, 1.6 and 7.7 % higher than those recommended by the NRC, respectively.
The food conversion, CP and ME were not affected by the interaction (TL*EP). Only the conversion of ME by TL was affected (table 3), with better conversion for diets with FSPT. This performance supports the nutritional quality of diets with FSPT, and makes possible a lower feeding cost. However, ME estimates were analyzed from values available in the literature. In the case of sweet potato tuber, this information was limited, which warrants some caution in the conversion results.
Table 3 Food conversion, crude protein and metabolizable energy in the evaluation period
| Indicator | Substitution level | Evaluation period | ||||||
|---|---|---|---|---|---|---|---|---|
| TL0 | TL28 | TL56 | EP1 | EP2 | SE± | |||
| FC, kg DM kg Increased LW-1 | 6.88 | 6.68 | 6.62 | 0.81 | 6.50 | 6.95 | 0.25 | 0.17 |
| CPC, kg kg increased LW-1 | 0.784 | 0.738 | 0.732 | 0.08 | 0.778 | 0.728 | 0.27 | 0.02 |
| MEC, MJ kg increased LW-1 | 76.44a | 72.38b | 76.96b | 0.01 | 73.82 | 73.30 | 0.29 | 0.06 |
LW: live weight.
The responses obtained in diets with FSPT are attributed, to a large extent, to the high sugar and starch content of this product, energetic components of great influence on the digestive and metabolic processes of the animal. At the ruminal level, these carbohydrates define the speed and extent of fermentation (Cone 1998). Sugars, like the soluble fraction of starches, are rapidly fermented, which increases cell mass, production of volatile fatty acids and synthesis of microbial protein (Gómez et al. 2016). LI Jian-nan et al. (2014) reported a starch disappearance rate from the dried and milled sweet potato tuber of 27.6 % h-1 in the first two hours of incubation in the rumen, which decreased as the incubation time increased. In theory, this pattern of energy availability may favor the use of rapidly fermentable N (Herrera et al. 1990) and bacterial growth, not only due to a greater synchronization of energy and protein, but also due to the constant availability of energy. However, the experimental information is contradictory, and does not validate this synchronization approach (Oba 2011 and Fondevila 2015).The tuber was supplied to the animal in small pieces (2.0 - 3.0 cm), an aspect that could reduce the degradation speed of its easily fermentable carbohydrates, due to its accessibility to microbial activity.
Another important ruminal parameter is the effective degradability rate. For tuber starch, for a passage rate of 0.060 h-1, Jiang (2005) reported an in situ value of 54.9 %, with a significant bypass fraction. It is also possible that in the liquid phase of the rumen a fraction of sugars pass to the lower parts of the gastrointestinal tract (Cone 1998). With respect to the ruminal way, this allows higher efficiency of energy use of sugars and starches (Zavaleta 2007). According to estimates by Oba (2011), 5.0 % of the available sugars escape from the rumen without being fermented, an effect that may be higher when supplying the fresh tuber, whole or chopped.
Another aspect that supports the responses obtained with FSPT is its high starch content, which, like MCG, allowed diets high in this nutrient, on average 37.1 %, dry basis, a level that produces a decrease in ruminal pH and an increase in the production of propionic acid, a glucogenic compound that favors the animal response (Zavaleta 2007). It has also been showed that this effect on pH decreases methanogenesis, a process that involves energy expenditure, favors bacterial growth, increases the passage rate and saves energy (Stern et al. 1994 and Gómez et al. 2016).
When considering some chemical and ruminal characteristics that affect the animal response, the sweet potato tuber have an average starch content slightly lower (64.7 vs 66.7 %) with respect to the corn grain, (Grüneberg et al. 2015, DeBlas et al. 2019 and Solís 2020). However, its sugar content is much higher (16.0 vs 1.6 %) (Aliaga and Nieto 2009 and Martínez et al. 2016). On average, both materials present very similar in situ degradabilities, 82.0 and 97.8 %, at 24 and 48 h of incubation, respectively (Batajoo and Shaver 1998, LI Jian-nan et al. 2014 and Gómez et al. 2016). The same situation is shown for the degradability rate of 0.058/h (LI Jian-nan et al. 2014 and Calsamiglia 2016), a value that implies a mean degradation time (t1/2) of 11.9 h. In terms of effective ruminal starch degradability, an average value of 61.9 % has been reported for the corn grain (Offner et al. 2003, Jiang 2005 and Calsamiglia 2016) and 54.9 % for the sweet potato tuber (Jiang 2005), values that show a higher proportion of bypassing starch for this tuber. The amylose content of starch inversely limits its digestion (Rooney and Rflugfelder 1986 and Bednar et al. 2001). Information from various authors refers to an average content of amylose and amylopectin for corn starch of 27.4 and 72.6 % (Hernández et al. 2008 and Gómez et al. 2016) and for sweet potato tuber starch of 22.1 and 77.9 % (Tecson 2007, Hernández et al. 2008 and Manzanillas 2018), respectively. These chemical and ruminal values make possible the competitive substitution of corn grain by sweet potato tuber in fattening diets.
The cost of the FSPT and MCG was 0.18 US dollars and 0.40 kg dry-1, which represents $ 0.06 and 0.13 MJ ME-1, respectively, values that enable the economic viability of this tuber in cattle feeding. The cost of diets was $0.38, 0.30 and 0.23 kg dry-1 for TL0, TL28 and TL56, respectively. With respect to TL0, the feeding cost of TL50 and TL100 decreased 18.0 and 28.3 %, respectively.
