It is by no means sure that Iranian Farmers mixed with steppe populations. Lazaridis modeled it only to show a three way model fitted better than a two way model and explained in the comment section of Davidski that his is not the final word.
http://eurogenes.blogspot.nl/2016/0...howComment=1466599307814#c7517378404372112535
I would think it nearly impossible that Iranian farmers migrated north of the Caucasus. First, Yamnaya and Iranian farmers have different uniparental markers. Secondly, Yamnaya was a pastoral culture whereas Iranian farmers depended greatly on cereals. Considering the fact that Ukraine mostly has Löss soils and is one the most fertile area's in the world - it still is one of the worlds bread baskets - it would be hugely strange if migrating farmers didn't use these for cereal growing on a vast scale. However, archaeological evidence points to minor cereal growing in Yamnaya culture.
It would make sense if not the fact that Yamnayas changed genetically from what they represented as pure hunter gatherers. The only way to explain the change is if they had mixed with Iranian Neolithic in fairly large proportions.
Here is an example based on HarappaWorld admixtures. This is a best model for Yamnaya I've found from all the available sources. Mind that we might not have the perfect/true source yet. First 3 samples are the source, above are the proportions. 27% of IN, 68% EHG and 5% WHG (for some reason). 4th is the model made out of the sources, and 4th a real Yamnayan to compare to.
[TABLE="width: 697"]
[TR]
[TD][/TD]
[TD="align: right"]0.27[/TD]
[TD][/TD]
[TD][/TD]
[TD="align: right"]0.68[/TD]
[TD][/TD]
[TD][/TD]
[TD="align: right"]0.05[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD="colspan: 2"]M967114 I1290[/TD]
[TD][/TD]
[TD]M737081[/TD]
[TD][/TD]
[TD][/TD]
[TD]M325047[/TD]
[TD="colspan: 2"]KO1, I-L68[/TD]
[TD="colspan: 2"] Model Yamnaya[/TD]
[TD][/TD]
[TD]M828815[/TD]
[TD]Rise552[/TD]
[/TR]
[TR]
[TD]Iranian Neolithic[/TD]
[TD]10 kya[/TD]
[TD][/TD]
[TD="colspan: 3"]Kvalinsk HG (I0122+I0124+I0433)[/TD]
[TD]Hungarian, Tiszaszőlős-Domaháza[/TD]
[TD]7.7 kya[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD]Ulan iV, Yamnaya[/TD]
[TD]4.5 kya[/TD]
[/TR]
[TR]
[TD]Run time[/TD]
[TD="align: right"]7.91[/TD]
[TD][/TD]
[TD]Run time[/TD]
[TD="align: right"]8.15[/TD]
[TD][/TD]
[TD]Run time[/TD]
[TD="align: right"]9.43[/TD]
[TD][/TD]
[TD] Run time[/TD]
[TD][/TD]
[TD][/TD]
[TD]Run time[/TD]
[TD="align: right"]9.08[/TD]
[/TR]
[TR]
[TD]S-Indian[/TD]
[TD="align: right"]6.13[/TD]
[TD][/TD]
[TD]S-Indian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]S-Indian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] S-Indian[/TD]
[TD] 1.66[/TD]
[TD][/TD]
[TD]S-Indian[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]Baloch[/TD]
[TD="align: right"]62.71[/TD]
[TD][/TD]
[TD]Baloch[/TD]
[TD="align: right"]20.2[/TD]
[TD][/TD]
[TD]Baloch[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] Baloch[/TD]
[TD] 30.67[/TD]
[TD][/TD]
[TD]Baloch[/TD]
[TD="align: right"]33.24[/TD]
[/TR]
[TR]
[TD]Caucasian[/TD]
[TD="align: right"]24.97[/TD]
[TD][/TD]
[TD]Caucasian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Caucasian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] Caucasian[/TD]
[TD] 6.74[/TD]
[TD][/TD]
[TD]Caucasian[/TD]
[TD="align: right"]6.58[/TD]
[/TR]
[TR]
[TD]NE-Euro[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]NE-Euro[/TD]
[TD="align: right"]71.08[/TD]
[TD][/TD]
[TD]NE-Euro[/TD]
[TD="align: right"]80.37[/TD]
[TD][/TD]
[TD] NE-Euro[/TD]
[TD] 52.35[/TD]
[TD][/TD]
[TD]NE-Euro[/TD]
[TD="align: right"]56.02[/TD]
[/TR]
[TR]
[TD]SE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]SE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]SE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] SE-Asian[/TD]
[TD] -[/TD]
[TD][/TD]
[TD]SE-Asian[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]Siberian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Siberian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Siberian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] Siberian[/TD]
[TD] -[/TD]
[TD][/TD]
[TD]Siberian[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]NE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]NE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]NE-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] NE-Asian[/TD]
[TD] -[/TD]
[TD][/TD]
[TD]NE-Asian[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]Papuan[/TD]
[TD="align: right"]0.35[/TD]
[TD][/TD]
[TD]Papuan[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Papuan[/TD]
[TD="align: right"]0.53[/TD]
[TD][/TD]
[TD] Papuan[/TD]
[TD] 0.12[/TD]
[TD][/TD]
[TD]Papuan[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]American[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]American[/TD]
[TD="align: right"]6.89[/TD]
[TD][/TD]
[TD]American[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] American[/TD]
[TD] 4.69[/TD]
[TD][/TD]
[TD]American[/TD]
[TD="align: right"]2.46[/TD]
[/TR]
[TR]
[TD]Beringian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Beringian[/TD]
[TD="align: right"]1.7[/TD]
[TD][/TD]
[TD]Beringian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] Beringian[/TD]
[TD] 1.16[/TD]
[TD][/TD]
[TD]Beringian[/TD]
[TD="align: right"]0.75[/TD]
[/TR]
[TR]
[TD]Mediterranean[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Mediterranean[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Mediterranean[/TD]
[TD="align: right"]18.59[/TD]
[TD][/TD]
[TD] Mediterranean[/TD]
[TD] 0.93[/TD]
[TD][/TD]
[TD]Mediterranean[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]SW-Asian[/TD]
[TD="align: right"]3.88[/TD]
[TD][/TD]
[TD]SW-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]SW-Asian[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] SW-Asian[/TD]
[TD] 1.05[/TD]
[TD][/TD]
[TD]SW-Asian[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]San[/TD]
[TD="align: right"]0.18[/TD]
[TD][/TD]
[TD]San[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]San[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] San[/TD]
[TD] 0.05[/TD]
[TD][/TD]
[TD]San[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]E-African[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]E-African[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]E-African[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] E-African[/TD]
[TD] -[/TD]
[TD][/TD]
[TD]E-African[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]Pygmy[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Pygmy[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD]Pygmy[/TD]
[TD="align: right"]0[/TD]
[TD][/TD]
[TD] Pygmy[/TD]
[TD] -[/TD]
[TD][/TD]
[TD]Pygmy[/TD]
[TD="align: right"]0[/TD]
[/TR]
[TR]
[TD]W-African[/TD]
[TD="align: right"]1.78[/TD]
[TD][/TD]
[TD]W-African[/TD]
[TD="align: right"]0.13[/TD]
[TD][/TD]
[TD]W-African[/TD]
[TD="align: right"]0.5[/TD]
[TD][/TD]
[TD] W-African[/TD]
[TD] 0.59[/TD]
[TD][/TD]
[TD]W-African[/TD]
[TD="align: right"]0.95[/TD]
[/TR]
[/TABLE]
About farming in Yamnaya. I would say that Iranian variety of crops were not suitable for large scale agriculture, and better results of food production was made by herding. It is a different story with modern Ukraine where modern variety of crops grow much better.