If you observe yellowing along veins, it is likely that something toxic has been taken up by the vascular tissue. This is particularly true if the yellowing is followed by necrosis of tissues. Many herbicides cause similar symptoms of toxicity.
Typically, herbicide injury symptoms appear in regular patterns in the field, e.g. at the ends of rows where the sprayer turned, every 5th row where the sprayer overlapped. Cultivars can have greater or lesser resistance to herbicide injury, so you may notice symptoms on some varieties but not on others.
