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Introduction
Northern Mexico has been experiencing extreme temperatures and dry seasons that lead to a reduction in forage production and nutritional quality. Under these production conditions, the use of additives emerges as an alternative in livestock feeding (Herrera-Torres et al., 2014). Additives improve the animal feed intake and productive performance (Garcés et al., 2004); thus, livestock farmers use the silage process for the conservation of forages. The silage process is carried out by acidification and fermentation of carbohydrates soluble in lactic acid and volatile fatty acids by lactic acid producing microorganisms under anaerobic conditions; in addition, it inhibits the growth of pathogenic microorganisms and allows the nutritional characteristics of forage to be preserved for later use (Wilkins et al., 1999). Oats is an important crop used in Northern Mexico, it is commonly used for silage as it requires less water for growth and is very useful for late planting when growing conditions do not justify the use of corn crops (Sánchez et al., 2014). Furthermore, oat forage has been shown to be a good forage source for ensiling; however, its metabolizable energy content is relatively low (Condori-Quispe et al., 2019). On the other hand, ground corn is an important ingredient for silage due to its energy content; consequently it is commonly used as an ingredient when ensiling forages (Moscoso-Muñoz et al., 2020; Ortiz et al., 2017). Sunflower is a crop that tolerates soil moisture deficit; this characteristic allows to withstand the shortage of rains and prolonged droughts; in addition, sunflower grain (SG) is rich in crude protein and crude fat (mostly polyunsaturated free fatty acids), which confer the ability to SG of being used as additive (Basarab et al., 2008). Therefore, the objective of this study was to determine the effect of the addition of different proportions of ground corn and sunflower grains on fermentative and nutritional quality of oat silage.
Materials and methods
Study area
This study was carried out at Faculty of Veterinary Medicine and Zootechnics of the Juárez University of the state of Durango. The oat forage (Cuauhtémoc cv.) was randomly harvested from irrigated crops located nearby the Faculty area in Durango, Mexico. Sunflower grain (Madero 31 cv.) and ground corn (Caribu cv.) were purchased at a local store. The chemical composition of the ingredients is presented in Table 1.
Table 1 Chemical composition of oat forage, corn grain and sunflower grain used in the production of microsilos (g kg-1).
| Oat forage | Corn grain | Sunflower grain | |
| Dry matter | 205 | 855 | 900 |
| Crude protein | 123.5 | 93.1 | 193 |
| NDF | 710.6 | 77.2 | 700.5 |
| ADF | 590.9 | 20.5 | 450.1 |
| Ash | 124.1 | 14.3 | 380 |
| Lignin | 25 | 10.3 | 150 |
| IVDMD | 581.7 | 705.5 | 604.3 |
NDF: neutral detergent fiber, ADF: acid detergent fiber, IVDMD: in vitro digestibility dry matter
Preparation of micro-silos
Experimental micro-silos were prepared with different proportions of oat forage (OF), sunflower grain (SG) and ground corn (GC). Twenty-seven experimental micro-silos were prepared by mixing solely oat forage (T1), and oat forage with ground corn and sunflower grain (T2 to T9) as described in Table 2. Oat forage was harvested at late milk maturity stage (Rosser et al., 2016). Subsequently, forage was cut to a particle size of 2 t0 4 cm; afterwards, experimental micro-silos were hermetically sealed into plastic containers (19 L) for 30 d. Once the time had elapsed, the silages were opened for analysis.
Experimental design and experimental unit
A completely randomized design was used under a 3 × 3 factorial arrangement with three mixtures of oat forage and ground corn, and three levels of sunflower grain, resulting in nine treatments with three replications, The experimental units were the micro-silos.
Silage fermentation analysis
Once the silages were opened, the following variables were evaluated: pH was measured according to the method described by Tobía et al. (2004) using a potentiometer (Model HI 83142, Hanna Instruments, Mexico City); lactic acid was evaluated according to Borshchevskaya et al. (2016); ammonia-nitrogen (NH3-N) concentration was evaluated using the procedure proposed by Galyean (2010).
Chemical analyses
Samples of each experimental micro-silo were dried into a forced-air oven at 55 °C for 72 h, and ground to 1 mm particles using a Wiley mill (Arthur H. Thomas, Philadelphia, Pennsylvania, USA) and stored for further analyses. Dry matter and ash were determined by drying the samples according to procedures proposed by AOAC (2015). Crude fat (CF) was calculated by extracting fat using the soxhlet equipment as proposed by AOAC (1990). The CP concentration was calculated by determining the total nitrogen (N) content using the micro-Kjeldhal technique (Method 920.87; 5) and multiplied by a fixed conversion factor (6.25) according to AOAC (1990). Neutral detergent fiber (NDF), acid detergent fiber (ADF) and lignin concentrations were determined following methods proposed by Van Soest et al. (1991). Non-structural carbohydrates (NSC) were estimated as the difference resulted from the equation NSC = [100 - (CP + CF + Ash FDN)]. In vitro dry matter digestibility (IVDMD) at 48 h was estimated in triplicate by incubating samples of experimental micro-silos (DaisyII®, ANKOM Technology, Fairport, New York, USA) according to procedures described by the manufacturer.
Statistical analysis
The data were analyzed through analysis of variance using the GLM procedures of SAS version 6 (SAS Institute, 1989) using the model
where yij is the response, µ is the mean, τi is the treatment effect, βj level sunflower, (τβ)ij interaction effect and £ is the experimental error.
Means comparison was performed with the Tukey test declaring significant differences at P ≤ 0.05; highly significant differences were declared at P ≤ 0.01 and very highly significant differences at P ≤ 0.001 (Equation 1).
Results and discussion
Table 3 shows the chemical composition of experimental silages. No significant interaction between OF-GC mixtures and SG level was observed (P > 0.05). DM content was lower in the mixture 95-5-10 (P ≤ 0.05). However, contents of DM registered in this research are into acceptable values for silage of good quality (Mohd-Setapar, et al., 2012); these authors mentioned that silages should contain from 30 to 35 % of DM. Results of this study agree with those obtained by Apráez-Guerrero et al. (2012) who registered values of 28.78% in oat silage, whereas Ortiz et al. (2017) reported 20.4% in maralfalfa silage.
Table 3 Chemical composition of experimental silage (g kg DM-1).
| Ratio OF-GC × SG (%) | DM | CP | CF | NSC | IVDMD |
| 100-0-0 | 30.4 ab | 12.0 a | 3.31 d | 58.9 ab | 51.5 c |
| 100-0-5 | 32.97 a | 11.8a | 11.0 a | 55.2 bc | 59.8 c |
| 100-0-10 | 28.4 ab | 11.1a | 10.8 a | 54.5 bc | 68.9 ab |
| 95-5-0 | 29.8 ab | 10.9 a | 3.9 d | 63.0a | 64.8 b |
| 95-5-5 | 28.9 ab | 11.1 a | 11.6 a | 52.0 c | 65.81 ab |
| 95-5-10 | 23.7 c | 8.5 b | 9.7 ab | 54.5 bc | 70.87 ab |
| 90-10-0 | 26.6 bc | 11.5 a | 5.6 c | 56.4 bc | 71.5 a |
| 90-10-5 | 30.1 ab | 10.8 a | 5.8 c | 57.3 b | 69.6 ab |
| 90-10-10 | 28.3 ab | 11.9 a | 10.4 a | 54.2 bc | 70.6 ab |
| SEM | 0.40 | 0.19 | 0.21 | 0.32 | 0.55 |
| OF-GC × SG | NS | * | *** | ** | ** |
Different letters within a column indicates significant differences (Tukey, P ≤ 0.05). *: (P ≤ 0.05), **: (P ≤ 0.01), ***: (P ≤ 0.001), NS: no significant difference, DM: dry matter, CP: crude protein; CF: crude fat, NSC: non-structural carbohydrates, IVDMD: in vitro dry matter digestibility, SEM: standard error of the mean.
Interaction OF-GC × SG level was significant for protein content (P ≤ 0.05; Table 3). The lower protein concentration was observed in treatment 95-5-10 (P ≤ 0.05); however, the protein concentrations registered in this research are within the optimal range according to de Blas et al. (2010). Likewise, Van Soest (1994) mentioned that lower PC values (6-8 %) in the cattle diet can negatively affect ruminal nitrogen metabolism and feed intake. The protein content obtained in this study was higher than that registered by Abdelhadi and Santini (2006) in corn and sorghum silages (6.1 and 6.37 %, respectively), and Jensen et al. (2005); meanwhile, Castillo et al. (2009) observed a protein content of 10.4 % in corn-bean silage.
Interaction OF-GC × SG level was very highly significant for CF content (P ≤ 0.001; Table 3). The higher content of CF was observed in silages with sunflower grain. Thisr may be related to the high oil content in SG (42 %) (McGuire and McGuire, 2000). These results indicate that fat content in oat forage is poor and silages are enriched by the addition of GC and SG in all treatments.
Interaction OF-GC × SG level was highly significant for NSC content (P ≤ 0.01; Table 3). The inclusion of SG and GC increased the NSC concentration in silages, which improves energy content and fermentation rate (Amer et al., 2012). On the contrary, a dilution effect in metabolizable energy was observed when OF increased in experimental micro-silos due to the lower contents of NSC. Otherwise, the NSC values registered were lower than those obtained by Araiza-Rosales et al. (2013) in corn silages.
Interaction OF-GC × SG level was significant for IVDMD (P ≤ 0.01; Table 3). The IVDMD increased with SG due to the lower degree of lignification of oat forage. Moreover, micro-silos with CG provided soluble carbohydrates which improved digestibility (Aragadvay-Yungán et al., 2015; Ortiz et al., 2017). Additionally, the treatment with the lowest NDF and lignin content has the lowest IVDMD; on the contrary, the higher the content of NDF and lignin, the higher the IVDMD. The digestibility of forage also plays an important role in animal production (Li et al., 2014). It is worth mentioning that the IVDMD depends on physical characteristics of forage, especially the fiber content; low NDF and ADF contents lead to a rapid increase in digestibility of DM. This agrees with results obtained in this study and with those reported by Huhtanen et al., 2007 and by Zhang et al., 2016. Regarding to the fiber content in experimental micro-silos, no significant interaction OF-GC × SG level for NDF was observed (P > 0.05; Table 4); however, the values obtained for NDF are in an acceptable range for good quality forage (< 60 g kg DM-1). These results may be explained by the increased hydrolysis of hemicellulose that occurs during silage fermentation. At this stage, pentoses are released and may be fermented into lactic and acetic acids (McDonald et al., 2002). Conversely, higher concentrations of NDF were registered by Britos et al. (2007) in pasture silage enriched with buttermilk.
Table 4 Detergent fibers content of oat silages with different levels of ground corn and sunflower grain (g kgDM-1).
| Ratio OF-GC × SG (%) | NDF | ADF | LIG |
| 100-0-0 | 51.5 b | 36.3 b | 5.0 c |
| 100-0-5 | 54.7 ab | 42.9 a | 6.9 bc |
| 100-0-10 | 55.2 ab | 43.6 a | 10.7 ab |
| 95-5-0 | 59.5 a | 43.4 a | 12.6 a |
| 95-5-5 | 57.8 ab | 39.0 ab | 12.8 a |
| 95-5-10 | 52.4 b | 34.4 ab | 10.8 ab |
| 90-10-0 | 58.0 ab | 33.3 b | 14.8 a |
| 90-10-5 | 55.3 ab | 35.6 b | 12.2 a |
| 90-10-10 | 54.8 ab | 32.6 b | 13.36 a |
| SEM | 0.39 | 0.41 | 0.32 |
| OF-GC × SG | NS | ** | ** |
Means with different letters within a same column indicates significant differences (Tukey, P ≤ 0.05). *: P ≤ 0.05, **: P ≤ 0.01, ***: P ≤ 0.001, NS: non significant, SEM: standard error of the mean, NDF: neutral detergent fiber, ADF: acid detergent fiber, LIG: lignin.
Interaction OF-GC × SG level was highly significant (P ≤ 0.01; Table 4) for ADF content. The ADF concentration in all microsilos was higher than the optimum value (25 %) as reported by Phiri et al. (2007). The lignin content of SG was relatively high and it is reported to be within 20-25 % according to Taha et al. (2012); in addition, Kimiaeitalab et al. (2017) reported ADF contents of 70 %. Due to this, when SG is added an increase in ADF and lignin is observed in the experimental micro-silos; however, IVDMD was not affected by the addition of SG. This can be explained as a possible dilution effect that can be attributable to a reduction in oat forage and an increase in CG.
Interaction OF-GC × SG level was significant (P ≤ 0.01; Table 4) for lignin content. The lignin values registered in this study were higher than those reported by Castro et al. (2006) in silage of Tifton 85 (Cynodon spp.) pasture and star grass silage. Moreover, contents of ADF and NDF are 40 and 70 %, respectively.
pH and N-NH3
Interaction between OF-GC × SG level for pH, N-NH3 and lactic acid was not significant (P > 0.05; Table 5); however, the values determined in this study are within the optimal range recommended by Van Soest et al. (1991). Additionally, Evangelista et al. (2000) prepared silages with Cyonodon that presented pH values of 4.5 to 5.3 which are similar to those obtained in this study; low pH values avoid deterioration. The obtained values can be explained by the low content of soluble carbohydrates (Vu et al., 2019) which promotes the production of lactic acid. These results agree with those reported by Aragadvay-Yungán et al. (2015) in sunflower silage.
Table 5 Fermentative parameters of oat silage with different levels of ground corn and sunflower grain (g kgDM-1).
| Ratio OF-GC × SG (%) | pH | NH3-N/total N | Lactic acid |
| 100-0-0 | 4.2 a | 7.7 c | 0.8 d |
| 100-0-5 | 4.3 a | 9.3 bc | 1.0 d |
| 100-0-10 | 4.3 a | 12.9 a | 1.2 d |
| 95-5-0 | 4.2 a | 11.9 ab | 1.1 d |
| 95-5-5 | 4.3 a | 11.3 ab | 1.5 b |
| 95-5-10 | 4.3 a | 7.4 c | 1.6 b |
| 90-10-0 | 4.2 a | 7.3 c | 1.3 b |
| 90-10-5 | 4.2 a | 9.9 bc | 1.3 b |
| 90-10-10 | 4.2 a | 9.7 bc | 1.8 a |
| SEM | 0.03 | 0.24 | 0.1 |
| OF-GC x SG | NS | NS | NS |
Differents letters within a column indicates significant differences (Tukey, P ≤ 0.05). *: P ≤ 0.05, **: P ≤ 0.01, ***: P ≤ 0.001, NS: non significant, SEM: standard error of the mean.
On the other hand, the highest concentration of N-NH3 was registered in silage with 10 % SG (100-0-10); this increase can be attributed to the presence of microorganisms capable of improving proteolysis when they adhere to the substrate due to a reduction in the fiber fractions (Berumen et al., 2015). Moreover, this parameter is an indicator of the catabolism of proteins and aminoacids (Junior et al., 2017). The results obtained in this study are similar to those reported by Zanine et al. (2010) in corn silage (14.6 %), but higher than those mentioned by Ortiz et al. (2017) in maralfafa silages.
The highest concentration of lactic acid was registered in the treatments whit 10% of ground corn as well as in treatments where SG was added. The values of lactic acid in this study are considered as adequate (Kung and Shaver, 2001) and may guarantee a good fermentation of forage (Schroeder, 2004). Moreover, the concentrations registered in this study were higher than those reported by Apráez-Guerrero et al. (2012) in oat forage silages.
