It is better to have purified DNA for GBS. You can try QIAGEN PCR purification Kit to get purified DNA or few ethanol washes. 260/280 should be around 1.8.  

You can follow up on this link below.

Dear Gerardo Sánchez

are your sample and Your kits fresh??

What is your organ?

Dear Gerado

• this can be a problem linked to the use of CTAB itself in order to efficiently solubilise plant based materials for DNA extraction
• In essence, you can end up with residual poly saccharides and salt  culminating in low 260/280 and low 260/230 ratios respectively
• I would suggest performing a trizol chloroform extraction following CTAB and then as suggested by Fanna take the upper aqueous phase from the chloroform extraction and apply to a Qiagen column: In general with a high volume of starting material and  moreover complex plant based material a column alone doesn't always remove all impurities:
• By solubilising with CTAB and then subjecting to an initial Trizol chloform extraction what you then apply to the column will be devoid of complex impurities (Polysaccharide & lipid derivatives; mostly the latter for plant based materials) and column purification will then be much more effective

Hola Gerardo,

Yes, it's advisable that you extract highly purified DNA before you move on to GBS (mainly to eliminate the DNA quality as the source of trouble if something doesn't work and you need to troubleshoot).

Can you please provide more details on what is the source of your DNA such as what tissue is used, is it fresh or frozen or dried, what's your homogenising method, etc?

I've been optimising our CTAB DNA extraction method after facing similar issues to what you describe (extracting fungi DNA) and I can recommend the following:

1. Don't use too much starting material, 50-100mg should be sufficient.

2. Freeze it in liquid Nitrogen and pulverise using mortar & pestle or beads homogeniser.

3. After you incubate the tissue in CTAB for 1 hour (with RNase, 1% SDS, 1% PVP), centrifuge your lysate at 12,000g for 10 minutes, transfer the supernatant to a new tube and only then perform the Phenol-Chloroform extraction.

4. After the Phenol-Chloroform extraction, collect the supernatant and transfer to a new tube (leave some in the tube, so you make sure you don't disturb the middle layer with all the cell debris), and perform another centrifuge at 12,000g for 15 minutes, collect the supernatant and transfer to a new tube (don't touch the deposit at bottom of the tube).

5. Add ice-cold Isopropanol (1:1 vol as your collected supernatant), incubate on ice for 30-60 minutes, centrifuge at 5,000g for 5 minutes, discard the supernatant and continue with x2 70% ethanol washes (700 uL, short vortex, incubate for 2 minutes and centrifuge at 5,000g for 5 minute in each wash).

6. Discard the supernatant, spin-down, remove ethanol residue with pipette, air-dry for not more than 5 minutes and resuspend in 50-100 uL DNase-free TE or pure water.

Buena suerte, Ido