And why did Yakov put the sticks for the Mekusharos, but not for the Atufim?
Ramban: Had he put sticks for all of them, Lavan would have ended up with nothing. 1
Redak (also cited in Ramban): Even though he put sticks only for his own animals, if Lavan saw so, he would suspect that he did so also for Lavan's. Yakov did so only for the minority 2 that conceive early; Lavan would come to check only for the majority, which conceive late. 3
Rashbam (41): He did so for the Mekusharos, i.e. the majority 4 that become pregnant early. He was not concerned for the Atufim, which become pregnant later.
Moshav Zekenim (37): Yakov used sticks to tie his animals (make them Mekusharos), lest they mate with Lavan's. There was no concern for Atufim (sheep totally covered to protect their wool), which cannot mate. Refer to 30:37:1:7.
Tosfos ha'Shalem (40:4): Atufim would give birth by themselves. He needed to use sticks in order that the Mekusharos would give birth. 5
He preferred to put for the Mekusharos, for they are the majority (according to Rashbam 41), or they are stronger (PF).
This is unlike Rashbam (41), who says that most are Mekusharos.
Perhaps also Seforno (Pasuk 40) explains like this. (He connotes that concern for suspicion applied only to the Atufim.)
This is unlike Redak, who says that most are Atufim.
This shows that he did not intend to make them give birth spotted. If so, he would have done so for all of them!