Livestock Research for Rural Development 30 (4) 2018 Guide for preparation of papers LRRD Newsletter

Citation of this paper

Milk yield of crossbred cattle grazing sorghum sudangrass hybrid with concentrate supplementation during dry season

F F Simili, M L P Lima1, M I M Medeiros2, L C Roma Júnior1, C C P Paz1 and R A Reis3

Instituto de Zootecnia/APTA/SAA - Rodovia Carlos Tonani, km 94, Sertãozinho, SP, Brasil
flaviasimili@iz.sp.gov.br
1 Instituto de Zootecnia/APTA/SAA Sertãozinho, SP, Brasil
2 APTA/SAA Centro Oeste - Bauru, SP, Brasil
3 Departamento de Zootecnia, UNESP, Jaboticabal, SP, Brasil

Abstract

The Sorghum Sudangrass Hybrid (SSH) can be used as alternative pasture to be grazed during dry season due to its high tolerance to soil water shortage, lower temperatures and short photoperiod, compared to others perennial grasses. However, there are few reports that used this method as a grazing alternative. The experiment was carried out during three years in order to evaluate the SSH dry pre-grazing forage mass (DFM), stem and leaf proportion and leaf/stem rate (L/S) as well as further analyzing crude protein (CP), neutral detergent fiber (NDF) and milk production of cows receiving or not a supply of 3 Kg of concentrate. There was effect of year and grazing cycle (P < 0.0001) on DFM. The averages were 3041, 1926 and 3482 kg/ha on the 1st, 2 nd and 3rd year, respectively. The CP leaves was higher in 1st and 2nd year with averages of 18.2 and 16.9 % respectively, however, the lowest value was obtained in 3 rd year (P < 0.0001) with average of 12.12%. Milk production of cows that received concentrate supplement was higher (P = 0.0110) than cows did not receive concentrate, milk yields were 17.44 and 15.07 kg/day, respectively. The use of SSH as a pasture during dry season and concentrate supplementation provide an increase of milk production in crossbred cows.

Key words: annual grass, milk composition concentrate, tropical grass, Sorghum bicolor x Sorghum sudanense


Introduction

Milk in Brazil is produced mainly in pasture, with low use of concentrated supplements, which makes it extremely competitive in the international market, but there is little research evaluating the productive responses of milk cows to concentrate supplementation. Pasture based diets are lower in energy than confinement diets. High producing or high-net merit cows may not meet their energy requirements from grazing alone (Peyraud and Delagarde 2013), and some farmers may try to overcome this by supplementing energy through concentrates to cows on pasture.

In a tropical climate situation, there is seasonality in forage production because of the marked period of dry season and rainy season, which makes production systems mostly dependent on the planning for use of conserved forage or forage with high drought tolerance (Aguilar et al 2014). Sorghum [Sorghum bicolor (L.) Moench.] is an important forage crop for dairy cows in many milk producer regions of the world as frequent drought with high summer temperatures, reducing forage production from pastures (Astigarraga et al 2014). Forage sorghum can be sowed later summer than corn because sorghum use water much more efficiently and, when exposed to drought situation, with low rainy period, still produces acceptable yields (Pedersen 1996).

Sorghum Sudangrass Hybrid (SSH) (Sorghum bicolor x Sorghum sudanense) has been increasing in animal feed, due to its is ease grow crop and because its fast establishment and growth and, mainly. It has ease handling for cutting or grazing, besides good nutritional value and high forage production, which makes it a viable alternative to offer green forage of high nutritional value (Lima et al 2017). Also sorghum and pearl millet contribute to grow plants year-round systems forage with good quality, an important food for lactating cows or fast growing animals (Fribourg 1995).

Astigarraga et al (2014) evaluated the milk yield of cows kept under SSH grazing system and obtained average of 19.91 kg/daily, milk fat content of 3.5% and milk protein content of 3.1%. There is a lack of results available in literature about dairy milk grazing SSH. This grass can be used as alternative of annual grazing in order to maintain forage production stability, during dry season due to the tolerance of water shortage and lower environment temperatures like those on the autumn and winter in Central and Southeast Brazil.

The aim of the study was to evaluate the SSH forage mass, in a rotational grazing system as well as the milk production and quality of crossbred cows supplemented, or not, with concentrate as an alternative of pasture during dry season.


Material and methods

The experiment was conducted for three consecutive years, at the APTA experimental livestock farm (Agência Paulista de Tecnologia dos Agronegócios), Ribeirão Preto, SP, Brazil (located at 21º42’S, 47º24’W with 535 m of altitude). The soil type was Oxisol, classified according Embrapa (1999). The result of soil fertility analyses were : pH 5.1; organic matter 34 g/dm3; 41 mg P/ dm3; sum of bases of 50.6 mmol SB/dm3; base saturation of 57%. The climate was tropical, with dry winters and rainy summer. Maximum and minimum air temperature and rainfall rate are shown in Figure 1.

During the spring-summer prior to the experiment, the area was used to grow just soybeans. After soybean’s harvest, there was used glyphosate herbicide to desiccate any remaining weeds and then it was sowed the SSH 1P400 (Dow Agroscience) during the off-season. The fertilizer was 8-28-16+Zn applied by a vacuum planter, suitable for no tillage (95% efficiency). The row spacing were 0.8 m and the seeds quantity was 12 kg/ha.

After SSH growth stage completed, the total area (10 ha) was subdivided into 16 paddocks (6.250 m2 each) to use as a rotational grazing cycles surrounded by portable electric fence. A common area was used as resting area. Water was provided using water drinker. Shadow was provided from big trees and mineral salt supplement was also used. The occupation period was two days per paddock.

On the first year (Year 1) the sowing occurred on March 1st, and the grazing cycles took place from April 17th to May 2nd. On the second year (Year 2) the sowing started on March 17 th followed by the grazing cycles on May 19th and July 4th. Sowing on the third year (Year 3) was March 10 th but due to severe low rainfall (Figure 1) registered on the period, only one grazing cycle took place on May 3rd. To avoid the prussic acid poisoning risk the grazing started when no plant at canopy have height lower than 50 cm (Similli et al 2013) or after minimum of 2 weeks regrowth (Ruh 2017). The first grazing started when the average of canopy SSH height reached 1.2 m (measured on the average of inflexion point of leaf).

Figure 1. Rainfall (mm) and maximum and minimum air temperature (0C) for the three years of study.

The two treatments studied were the use or not of supplementation of 3 kg/day of concentrate supplement per dairy cows grazing SSH. The concentrate composition (with 19% of crude protein - CP) was 55% of corn, 17% of soybean meal, 25% of cottonseed meal (38% CP) and 3% of mineral salt.

Fourteen Holstein × Gyr crossbred dairy cows (milk yield 18 kg/d; days in milk 100; body weight 500 kg) were grouped according to milk yield, days in milk, and body weight. Cows were randomly assigned to two treatments and were milked twice a day at 6am and 3pm in order to measure milk production with the calf presence during milking. Consumed milk by the calves was not quantified. During the samplings, milk production was measured seven days on each grazing cycle, consecutively. Milk was sampled by collection of 10% of milk produced per cow, automatically using a device in the milking machine. The milk quality was quantified measuring fat protein, lactose and total solids content.

The variables studied in each grazing cycles were pre-grazing forage mass (DFM), plant height stem/leaf ratio, as well as chemical composition including crude protein (CP) and neutral detergent fiber (NDF) analysed according AOAC (1995).

The DFM was analyzed prior to grazing period. The samples were made cutting on ground level, all the plants, two lines of 2 meters each. Two sub-samples were made: one to dry the entire plant (leaf, stem, dead material and inflorescence) and the other one for leaf separation (leaf blade), stems (sheath) and dead material. Subsequently, these subsamples were dried in a circulation oven forced air at 60 oC during 72 hours to achieve constant weight.

The plant height was measured on the average of inflexion leaf point, before grazing, using a graduated ruler on ten randomized points on each experimental paddock. The stem and leaf proportion was calculated. The leaf /stem ratio was obtained by dividing the dry mass of the leaves by the dry mass of the stem.

Statistical analysis was performed using completely randomized blocks design for milk production as well as forage yield and chemical composition of plant. Grazing cycles were considered subplots and they were analyzed as a nestled effect within a year. The effects of year and cycle are not independent, because in years 1 and 2 were performed two cycles/year and in year 3 only one cycle was performed. Statistical results were made by PROC GLM (SAS 2003) and means were adjusted.


Results and discussion

To start the grazing cycle, the height plant target was 1.2 m, but the elongation of plant was so fast, the target was obtained just for initial paddocks of first grazing cycle. The height plant means was higher for Year 3 (Table 1) and had significant variation also for year (Table 1) and effect of cycle nested into year. The unfolded results are shown in Figure 2. The mean plant height variation from 0.9 m to 1.9 m showed the difficulty of SSH plant management, but the severe grazing in the 1 st cycle of 1st year induced the regrowth of leaves and changed the leaves/stem proportion (Figure 2b) at 2nd grazing cycle of 1st year. That regrow was helped by the climate, achieving the best leaves/stem proportion obtained in this study (Table 1- Figure 2b).

On that occasion the rain was abundant during April and May of Year 1 (Figure 1) differently from other years that low rain quantities influenced the regrowth of plant. Such effect contributed to increased leaves DFM and leaves/stem proportion in Year 1 (Table 1). Its effect was significantly of cycle (year) for all variables studied for structural and productive characteristics of SHH (Table 1).

Table 1. Structural and productive characteristics of SSH, per experimental years of evaluation.

Year 1

SEM

Year 2

SEM

Year 3

SEM

P value
year

P value
cycle (year)

MEAN

MEAN

MEAN

Plant Height (m)

1.40 b

0.03

1.37b

0.03

1.76a

0.04

<0.0001

<0.0001

Pre-grazing DFM (Kg DM/ha)

3041a

140.5

1926b

179.1

3482a

226.5

<0.0001

<0.0001

Leaves DFM (Kg DM/ha)

944a

39.8

450b

53.2

854a

64.1

<0.0001

<0.0001

Stem (Kg DM/ha)

2044a

112.5

1231b

150.4

2203a

181.4

<0.0001

<0.0001

Leaves (%)

37.84a

1.2

24.67b

1.69

25.00b

2.0

<0.0001

<0.0001

Stem (%)

58.17b

1.2

60.84a

1.7

62.70a

2.0

<0.0001

<0.0001

Relation leaves/stem

0.78a

0.03

0.42b

0.04

0.40b

0.05

<0.0001

<0.0001

ab Means in the same row without common letter are different (P<0.05).

According to Fonseca et al (2012), to maximize the herbage intake rate for beef heifers, management targets should permit animals to graze swards of Sorghum at canopy height of approximately 50 cm, on one hand. On the other hand, all young sorghum plants were associated with prussic acid poisoning risk and it could be avoided with adequate grazing management. After a frost event or extreme drought SSH should not be grazed before rest of 2 weeks (Ruh 2017) and also to minimize risk, the minimum height of plant should be 45 cm (Griggs et al 2008) or 50 cm (Similli et al 2013) and those recommendations were used in present study.

The fast elongation of SSH plant made height management difficult and induced worse leaf/stem proportion (Figure 2 b) showing small plants presented best leaves/stem relation, depending of the rain events. Similar results were measured when tall plants presented worse leaves/stem relation (Fonseca et al 2012, Simili et al 2013). On annual grass basis it is essential to control the SSH height in order to provide higher leaf proportion. The fast stem elongation can be a problem to maintain ideal height of plant. According to McCartor and Rouquette (1977) who related SSH rapid growth rates, to maintain good pasture quality under soil water stressed situations makes grazing management very difficult.

Figure 2. Plant height (a), relation leaves/stem (b), pre-grazing DFM (c) and leaf and
stem proportion (d) of SSH studied three consecutive years.

The DFM evaluated was between 1.4 and 4.7 ton/ha for the entire plant, showing a good potential of production of SSH (Figure 2c). There was an important effect of the years and grazing cycle on pre-grazing DFM.

The Year 1 had tall plants, measured 1.93 m on 1st grazing cycle with relation leaf/stem of 0.39 and the pre-grazing DFM was 4762 kg DM/ha. The following grazing cycle showed lower plants measuring 0.88 m, on pre-grazing DFM was 1319 kg DM/ha with 48% (610 kg DM/ha) of leaves resulting in a leaf/stem relation of 1.18. Differently the results of present study, Fonseca et al (2012) proved to reach higher results of DFM 4204 kg/ha and 2135 kg/ha for leaves with similar tall plants.

Similar to present study, Stobbs (1973) demonstrated that long grazing intervals are associated with high biomass density, generally with low-density leaf. Stem elongation intensified forage accumulation compromises the canopy structure, therefore reducing leaf/stem rate.

The CP of entire plant, leaves and stem were affected by year. It showed higher CP value for leaves in Year 1 and for stem in Year 3 (Table 2). CP also was affected by cycle (nested in year) and results for CP of entire plant, leaves and stem was unfolded for each grazing cycle and are shown in Figure 3(a).

The leaves CP were higher for 2nd cycle of Year 1 (20.1%) and Year 2 (19.5%) as result of regrowth. It was also observed lower CP associated with higher DFM and taller plants. Those findings corroborate with Gelley et al (2017), who proved CP negatively correlated to DFM of switchgrass, sorghum × sudangrass, and bermudagrass. The presented results also are close the McCuistion et al (2011) results, with values varying from 3450 and 3080 kg DFM/ha, CP between 19.9 and 22.0% when the two SSH types were evaluated.

NDF was affected by year just for leaves and stem (Table 2). NDF of stem was affected by cycle (nested in year) and the results are shown in Figure 3(b). Means were higher the 67% and no relation was detected with DFM. The values showed on the present work are according to the literature (Astigarraga et al 2014, Aguilar et al 2014, Lima et al 2017).

According to some authors forage nutritive value declines with maturity and the forage digestibility decreases following the first forage harvest (Burns et al., 1997; Difante et al., 2008; Nave et al., 2013; Richner et al., 2014; Temu et al., 2014). Similarly to those authors, on the present study it was observed an increase of NDF of stem in second grazing cycles of Year 1 and 2 (Figure 3b), probably because the maturation of stem once the leaves increased with regrow after grazing.

Table 2. Crude protein (CP) and neutral detergent fiber (NDF) of entire plant, leaves and stem of SSH in 3 years of evaluation.

Year 1

SEM

Year 2

SEM

Year 3

SEM

P value
year

P value
cycle (year)

MEAN

MEAN

MEAN

Crude protein

Entire plant (%)

11.39ab

0.94

10.04b

0.49

13.73a

0.77

<0.0001

<0.0001

Leaves (%)

18.22a

1.28

16.94a

0.67

12.12b

0.95

<0.0001

0.0007

Stem (%)

7.13b

0.80

5.73b

0.44

8.31a

0.73

0.0001

0.0047

NDF

Entire plant (%)

68.94ª

1.16

69.19ª

0.61

70.46ª

0.87

0.4820

0.8136

Leaves (%)

67.05b

1.10

72.13a

0.58

69.08b

0.82

<0.0001

0.2259

Stem (%)

68.95ª

1.02

67.80ª

0.57

66.73b

0.76

0.0242

0.0015

ab Means in the same row without common letter are different (P<0.05).



Figure 3. Crude protein (a) of leaves, stem and entire plant of SSH, and neutral detergent fibre (NDF) of stem (b) plant of SSH.

The Year 3 presented lower rainfall prior to the experiment, between January and February. Only one grazing cycle was evaluated and the climatic variations affected the growth of plant and the quality, since CP of leaves was lower than Year 1 and 2 (Table 2).

Milk yield was significantly higher when the cows were supplemented by 3kg of concentrate, including results for 3.5% fat-corrected milk yield (Table 3). The low rainfall recorded on Year 3 decreased milk yield, probably in function of low CP leaves content on this year (12.12% - Table 2).

The interactions amongst the grazing cycles and years were not significantly different for milk yield, 3.5% fat-corrected milk, milk fat, protein, lactose and total solids of milk (Table 3). However, the interaction between year and treatment was significant for milk fat (%), and the results show that the supplementation of concentrate was important to keep steady milk quality over the Year (3 Table 3), as CP of leaves decreased (Table 2).

The concentrate supplementation did not affect the milk fat in Year 1 and 2 (Table 3) proving it is important to offer to milk cows a pasture with good quality and the leaves CP is essential to promote higher milk production with good quality for cows grazing SSH.

Similar to the results presented here, Astigarraga et al (2014) worked with cows grazing SSH and receiving 3.1 kg of concentrate/day. They found milk yield of 19.9 kg/day, milk fat content of 3.5%, milk protein of 3.09% and Fat corrected milk (kg/day) 18.5. Those authors observed milk protein content had a tendency to be slightly higher for cows grazing BMR sorghum, reflecting the response in milk production, greater energy intake, and a positive energy balance.

According to McEvoy et al (2009) the quantity of forage mass allowanced for cows was responsible for milk production, being higher quantities forage mass allowanced improved cow milk production. It was not observed in present work. Only high forage mass in grazing system alone do not induced higher milk production in Year 3, because the poor environmental conditions for plant grow affected the plant CP while decreasing cow milk production on one hand. On the other hand, lower forage mass produced in Year 2 (1926 kg MS/ha - Table 1) provided enough feed to maintain the milk production, similar to Year 1 (Table 3), including good CP presented in leaves (Table 2).

Table 3. Crossbred cows milk yield and composition, grazing SSH during 3 years, receiving 3 kg of concentrate supplement or not.

SSH Without
supplement

SSH With
supplement

MEAN

SEM

P value
trat

P value
Year

P value
trat*year

Milk yield (kg/day)

Year 1

16.44

18.06

17.25a

0.59

Year 2

16.71

17.63

17.17a

0.71

0.0110

0.0189

0.2406

Year 3

12.07

16.64

14.35b

0.85

MEAN

15.07B

17.44A

3.5% fat-corrected milk (kg/day)

Year 1

16.30

16.70

16.50a

0.63

Year 2

15.49

16.60

16.05a

0.76

0.0273

0.0033

0.0886

Year 3

9.93

15.23

12.58b

0.90

MEAN

13.90B

16.18A

Milk fat (%)

Year 1

3.45aA

3.01bB

3.23a

0.127

Year 2

3.02bA

3.11aA

3.07a

0.154

0.7052

0.0926

0.0490

Year 3

2.37cB

3.08bA

2.73a

0.184

MEAN

2.95A

3.07A

Protein (%)

Year 1

-

-

-

-

Year 2

3.00

2.98

2.99b

0.055

0.2811

0.0084

0.1461

Year 3

3.12

3.37

3.25a

0.066

MEAN

3.06A

3.18A

Lactose (%)

Year 1

-

-

-

-

Year 2

4.46

4.43

4.45a

0.040

0.6640

0.2853

0.9209

Year 3

4.53

4.50

4.52a

0.048

MEAN

4.49A

4.47A

Total solids (%)

Year 1

-

-

-

-

Year 2

11.37

11.46

11.42a

0.243

0.2432

0.9422

0.2605

Year 3

10.97

11.93

11.45a

0.291

MEAN

11.18A

11.69A

Means followed by a same lowercase letter in a column and uppercase letter in a row are statistically equal (P<0.05).


Conclusion


Acknowledgements

This study was financed by FAPESP (grant number 2004/13427-3)


References

Aguilar P B, Pires D A A, Frota B C B, Rodrigues J A S, Reis S T and Rocha Júnior V R 2014 Nutritional characteristics of BMR mutant and normal sorghum genotypes used for cutting and grazing. Acta Scientiarum. 36: 259-264. http://www.scielo.br/pdf/asas/v36n3/v36n3a04.pdf

AOAC 1995 Association of Official Analytical Chemists. Official methods of analysis of the Association of the Analytical Chemists. 16. ed. Washington.

Astigarraga L, Bianco A, Mello R and Montedónico D 2014 Comparison of Brown Midrib Sorghum with Conventional Sorghum Forage for Grazing Dairy Cows. American Journal of Plant Sciences. 5: 955-962. http://www.scirp.org/journal/ajps http://dx.doi.org/10.4236/ajps.2014.57108

Burns J C, Pond K R, Fisher D S and Luginbuhl J M 1997 Changes in forage quality, ingestive mastication, and digesta kinetics resulting from switchgrass maturity. Journal Animal Science. 75:1368–1379. doi:10.2527/1997.7551368x

Difante G S, Nascimento Jr D, Silva S C, Euclides V P B, Zanine A M and Adese B 2008 Tillering dynamics of marandu palisadegrass submitted to two cutting heights and three cutting intervals. Revista Brasileira de Zootecnia. 37:189–196. doi:10.1590/S1516-35982008000200003

Embrapa 1999 Centro Nacional de Pesquisa de solos (Rio de Janeiro, RJ). Sistema Brasileiro de Classificação de solos. Embrapa Produção de Informação. Brasília: Brazil. 412p.

Fonseca L, Mezzalira J C, Bremm C, Filho R S A, Gonda H L and Carvalho P C F 2012 Management targets for maximising the short-term herbage intake rate of cattle grazing in Sorghum bicolor. Livestock Science. 145: 205–211. https://doi.org/10.1016/j.livsci.2012.02.003

Fribourg H A 1995 Summer annual grasses. 463–472. In R F Moomaw, R.S., and T L Mader. 1991. Double cropping seed and for-Barnes et al. (ed.) Forages. Vol. 1. An introduction to grassland age crops with small grains in the Upper Midwest. Journal Production Agriculture 5th ed. Iowa State Univ. Press, Ames.

Gelley C H, Nave R L C and Bates G E 2017 Influence of Height-Based Management on Forage Nutritive Value of Four Warm-Season Forage Grasses. Crop, forage & turfgrass management. 3: 1-9. doi:10.2134/cftm2016.10.0067

Griggs T C, Drake D R and Parkinson SC 2008 Production and Quality of warm season annual forage grasses. Proceedings, Idaho Alfalfa and Forage Conference, 26-27 February.

Lima M H M, Pires D A A, Moura M M A , Costa R F, Rodrigues J A S and Alves K A 2017 Nutritional characteristics of Sorghum hybrids hay (Sorghum sudanense vs. Sorghum bicolor). Acta Scientiarum. 39: 229-234. doi: http://dx.doi.org/10.4025/actascianimsci.v39i3.32524

McCartor M M and Rouquette Jr. F M 1977 Grazing pressure and animal performance from pearl millet. Agronomy Journal. 69: 983–987. doi:10.2134/agronj1977.00021962006900060020x

McCuistion K C, McCollum F T, Greene L W, MacDonald J and Bean B 2011 Performance of stocker cattle grazing 2 sorghum-sudangrass hybrids under various stocking rates. The Professional Animal Scientist 27: 92–100. https://doi.org/10.15232/S1080-7446(15)30454-X

McEvoy M, O’Donovan M, Kennedy E, Murphy JP, Delaby L and Boland T M 2009 Effect of pre-grazing herbage mass and pasture allowance on the lactation performance of Holstein–Friesian dairy cows. Journal of Dairy Science. 92: 414–422. doi: https://doi.org/10.3168/jds.2008-1313

Nave R L G, Sulc M and Barker D J 2013 Relationships of forage nutritive value to cool-season grass canopy characteristics. Crop Science. 53:341–348. doi:10.2135/cropsci2012.04.0236

Pedersen J F 1996 Annual Forages: New Approaches for C-4 Forages. In: Janick, J., Ed., Progress in New Crops, ASHS Press, Alexandria, 246-251.

Peyraud J L and Delagarde R 2013 Managing variations in dairy cow nutrient supply under grazing. Animal: An International Journal of Animal Bioscience. 1:57–67. http://doi.org/10.1017/S1751731111002394

Richner J M, Kallenbach R L and Roberts C A 2014 Dual use switchgrass: Managing switchgrass for biomass production and summer forage. Agronomy Journal. 106:1438–1444. doi:10.2134/agronj13.0415

Ruh K and 2017 Comparison of two different grazing systems incorporating cool and warm season forages for organic dairy cattle. Doctoral thesis, University of Minnesota, Minneapolis, MN, United States of America. http://hdl.handle.net/11299/185550

Simili F F, Lima M L P, Medeiros, M I M, Paz C C P, Ruggieri A C and Reis R A 2013 Hydrocyanic acid content and growth rate of sorghum x sudangrass hybrid during fall. Ciência e Agrotecnologia 37: 299-305. http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1413-70542013000400002&lng=en&tlng=en

Sas 2003 Statistical Analysis System -. Institute Inc. SAS/STAT User´s guide. SAS Institute Inc.,Cary, NC

Stobbs T H 1973 The effect of plant structure on the intake of tropical pastures. I. Variation in the bite size of grazing cattle. Australian Journal of Agricultural Research 24: 809-819.

Temu V, Rude B and Baldwin B 2014 Nutritive value response of native warm-season forage grasses to harvest intervals and durations in mixed stands. Plants. 3:266–283. doi:10.3390/plants3020266


Received 15 December 2017; Accepted 9 March 2018; Published 1 April 2018

Go to top