Teff won't do as well in poorly drained soil. I've grown Teff in a couple different soil types from loamy to fairly heavy clay and had good results, thou it has been on hills. So the water can move down in the soil profile.  Neighbor grew teff this year his first time,, and put it on flat clay type soils which don't drain nearly as well. His teff does definitely look "different', has a real yellow tint to it, and doesn't look nearly like Teff I've seen. His crop was barely 2 feet high with more yellowish leaves, nothing like the 3-  3 1/2 foot high stuff I grow with extremely slender stalks and "whispy" looking heads.

Teff is actually a grain. Started eon's ago in Ethopia region was raised like wheat so yes there's a "head" on Teff thou it's so fine and wispy it's hard to see the individual seed kernals. There is something like 1.6 Million seeds per pound. Teff is actually sold in health food stores as "teff pearls".   

Teff doesn't compete well with other plants, if it's "shaded" under the canapy of other plants it's not going to do well at all. I'm not so sure about what your "seed guy" knows about Teff,,,  I've grown teff to "head stage" cut and got grow back. The "down side" for me is I am right on the very edge of what is a good climate for growning Teff.  I can get one good cutting but the weather "kraps out" on me before the Teff can snap back.  Teff behaves like Sudan Grass,  just sits there till the temp gets to 80-90 degrees.  100 degrees and moisture,, and you can literally watch the Teff grow.  Those temps "here" thou only last thru August. 

Planting Teff is only possible once ALL danger of freezing has passed. 31 degrees and Teff WILL die. On the plus side of that fall direct seeding into Teff is very easy as the teff WILL die.  My "seed guy" found out I knew more than he thought,, he was trying to convince me I needed to shoot the teff with Glyophospate in conjunction with seeding a field down to timothy. Told him "skip the Glyo the teff is already dying I checked this morning, it was 31 degrees last night". He didn't believe me,, drove out and looked for himself.  :-))

Teff is EXTREMELY shallow rooted,,  1/2 inch is the maximum for roots. Just turning the tractor 3 pt sprayer around in the field will tear out the roots. I usually turn and use the tractor brakes to avoid the front tires "skidding",,,  leaves funky little dead 'arcs" when the front tires skid..

LOL... by now growing teff is sounding kinda difficult ??  Well compared to stuffing corn seed in the ground spraying a few times and going fishing,, it's a little more tuffer...

Plus side of Teff.  It has 2x the protein of timothy, and is a complex carbohydrate so it has a very "sweet" taste to  horses, indeed all animals. And Teff is Highly digestible and paleteable.  I have several buyers with "horses with medical issues" and Teff is the only thing their animals do well on and doesn't create medical problems.

 Now I grow some fairly Good timothy (goes export) and had buyers buy timothy and teff at the same time. They were "leary" and wasn't sure their animals would eat the teff.  They returned to buy more teff. They had been switching their animals feed,,,  Timothy,, then tried the Teff,, switched back to timothy,,,,,  Animals refused to eat the timothy. People actually tried to get their animals to switch back for a couple days leaving only timothy for the animals to eat. Animals still refused the timothy.

I sell Teff for $200+ / per ton and don't for a second feel like I should be backing up to buyers when accepting the money.  Teff takes more careful irrigation than Timothy, I always spray soon as possible after that 1st irrigation, cutting Teff is also a "careful" affair,, can't just "skid" the hay equipment around,, and as stated earlier,, I blow out the windrows within 24 hours of cutting with a swather.. then rake rows back together right in front of the baler.  

For me,, "here" yes Teff takes more management skills.  Can't just throw out the seeds and go fishing..  I planted mid June, irrigated once, sprayed field, irrigated 2 more times, and would be cutting the Teff right now, Except the dang weather decided it should rain on Labor Day weekend....  (miserable rain,, love rain it just never rains when it should)

 I suppose some will be grousing,, too much work,, no money in it,, but,, considering all my inputs,, Teff returns more per acre than wheat does.

Don't hesitate if you've got more questions.