The end of March can be a very good time in South Texas. It should be warm in the days (80s-90s) down south and cool off into the 60s at night. There is still the occasional cold front at that time of year which can drop day time highs into the 60s, but AC can still produce in that case. It needs to rain early in the year to be really productive.
As for how to herp, AC can be good before it gets above 80° or so. Morning and nightime roadhunting can be productive if you run a quiet road and the temperatures cooperate. It may cool off too quickly at night. It just depends on the weather that week. So if it is cool, find trash to flip. If it is hot, roadhunt. And pray for rain.
I woudn't head up towards Del Rio. Even late March is too early for much activity up that way. You will see a lot more herps if you work up towards the coastal side of SoTex. The west side is good, but once I got much north of Laredo, I would head over to the east side in March.
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Chris Harrison
San Antonio, Texas