A warning to all - play nicely. Thank you.

@L4L

Better to cool it. The rudeness is certainly unwelcome. 
Let’s play nicely shall we? 

1.f4 g5

I admit to having played this in online blitz. I don’t believe it, but one can find that Birds players can sometimes be one trick ponies (see also: BDG). As such, often not familiar with the various g4 gambits versus the Dutch, and suitable responses.

Is it ‘sound’? No. Is it advisable v titled players OTB? No. Does it work v 2300-2400 players in lichess at 2-1 and 3-2? Yes, but that’s practical stuff, not theoretical judgment.   

I agree that 4.d4 is better for white, but I am having trouble with A3. 1.f4 e5 2.fxe5 Nc6 3.Nf3 g5 4.d4 g4 5.Ng5 d5 6.Nc3 Be7 7.e4 (these last two plies are my best guess) dxe4 8.Ngxe4 Qxd4 and here you have 9.Bb5+ (?). Stockfish gives  simply 9.Qxd4 Nxd4 10.Bd3.
Edited:

After 1.f4 e5 2.fxe5 Nc6 3. Nf3 g5 the computers at this stage are in agreement that 4.d4 is best. After 4….g4  5.Ng5  we have the stem position.   There are three variations to consider, A1. 5…Bh6, A2. 5…d6  and A3. 5…d4.
Variation A1.
The analysis of this variation by GM Kosten (Chess publishing.com) or Lakdawala can be very substantially improved. In fact White has a near winning advantage in this line. 
After  6. Ne4 Bxc1 7. Qxc1 Nxd4  8.Nbc3 Qh4+ Now  instead of 9.Nf2 as played in T.Bohn- K.Ziimmermann, Germany 2003, and endorsed by Lakdawala, White plays 9.g3 Qh5  and now 10.Qf4!  gives White a  winning advantage.(Stockfish depth 41 + 2.59).

In Kaissiber 36# (2010) after 1.f4 e5 2.fxe5 d6 3.exd6 Bxd6 4.Nf3 Nf6 5.Nc3, Hergert considered 5...0-0 insufficient in view of 6.e3 Ng4 ("?" Hergert) 7.Ne4 Nxh2 8.Nxh2 etc.

Here is a new idea: (5...0-0 6.e3 Ng4 7.Ne4) 7...Nc6! 8.Nxd6 Qxd6! and Black has space, development and open lines. White has a pawn and a cramped position. 

Sample lines:

9.Be2 Bf5! 10.0-0 Nb4 with attack

9.c3 Bf5! 10.d4 Rfe8 with attack

9.Bc4 Bf5! 10.d3 (10.0-0 Be4 =+) Rae8 equal/unclear

9.Bd3!? is Stockfish's favourite evaluating it to a very slight 
White edge, but who would want to play that, locking in ones owns pawns? 

My feeling is that Black has good compensation, but I may have overlooked something - or am I just over estimating Blacks pluses?

By the way, does anyone know about any examples of 5.Nc3 in practical play since 2010? In my database it seems that 5.d4 remains the most popular choise (and Black fares well with around fifty percent). 5.e3 has been seen twice (2-0). 

Stefan Buecker wrote:Thanks. On 4...g5 Volker Hergert had considered both 5.g3 and 5.c3, because the former is the main line in most sources, and because he believed that 5.c3! +/- was strong and close to a refutation of 4...g5. The intended contrast (between the main line 5.g3, which doesn't give White much, on one side and the unusual 5.c3! +/- on the other side) was later spoiled by me, when I found a hidden rescue for Black at a moment when much of the layout work was already done. It seemed too late to include 5.d4. We still believe that Kaissiber #36 shows convincingly that Black has an easier life after 4...Nf6, in comparison to 4...g5, when both 5.g3 and 5.c3 appear to involve more risks for Black. But the difference between g5 and Nf6 is not as big as it had seemed at first. The "!" after 4...Nf6 on page 30 can be seen as just a relic of the earlier version. 

In 1989-91 I had invested much time on studying the Bird and From's Gambit, for a work on 1.f4 which never reached the shops. But I had always preferred 5.g3 or 5.c3!? over 5.d4, because the latter seemed a bit boring. Obviously I was mistaken, the new ideas later developed for White make it difficult for Black to equalize. 



STEFAN, I didn't know you had analyzed 5.c3!?, but I noticed this move from a GM who lives in my city. I don't know Hergert's point of view in Kaissiber 36 either, but look following games which I consider very interesting:

A) 5..Nc6 6.Qa4 ( 6.d4!? g4 7.Ng5 ) 6..g4 7.Nd4 ( 7.Qe4+ Qe7 8.Qxe7+ Ngxe7= ) 7..Qh4+ 8.g3! Bxg3+ 9.Kd1 Bd7 10.Bg2 Bf2?! ( 10..Nd8! see analysis in C ) 11.Ncx6 bxc6? ( 11..Ne7 12.Qf4! ) 12.Bxc6 ( 12.Rf1! and 13.Qf4 ) 12..Rd8 was Meca (2065)-Melchor (2085), Barcelona, 2001 and now I think 13.Bxd7+ Rxd7 14.Qf4! is adventagous for White ( anyway first player won in 29 moves with 13.d3 etc. )

B) 5..g4 6.Qa4+ ( unique ) 6..Nc6 ( 6..Bd7 then perhaps 7.Qe4+ Ne7 8.Ne5 ) 7.Nd4 Bd7 ( 7..Qh4+ trasposes to A ; 7..Nge7 8.Nb5 += ) 8.Nb5 ( maybe better 8.Nxc6 Bxc6 13.Qxg4 ) 8..Nf6 ( 8..Be5!? and if 9.d4 a6! ) 9.Nxd6+ cxd6 10.d3 with initiative ( menacing Bg5 or Na3 and Nc4/Nb5 ) as Guerrero (2212)-Oms (2104), Barcelona, 2002 ( finally, 1-0 )

C) 5..g4 6.Qa4+ Nc6 7.Nd4 ( trasposing to A ) 7..Qh4+ 8.g3! Bxg3+ 9.Kd1 Bd7 10.Bg2 Nge7?! ( 10..Nd8! as pointed by R.Guerrero is the correct move. There are dotzens of lines, I'll give some as example: 11.Nb5 - 11.Qc2 Bd6 - 11..Be5!? - 11..Bd6 13.Nxd6 cxd6 14.Qd4 with initiative; 11..Bxb5 12.Qxb5+ c6 13.Qc5 - 12.d4 c6 - or 12..a6 13.N1a3 and 12..Qf2 - 13.dxe5 cxb5 in all them very complicated game ) 11.hxg3 Qxg3 12.Bxc6 bxc6 (!?) 13.Qa5!? trying to avoid ..h5. Finally game ended draw after hard battle, but logically White should to have win, Melchor (2040)-Guerrero (2212), Barcelona, 2002

Anyboby knows more games of theoretical intherest ?

A recent game with the From  and definitely one to play through from Black's point of view!
What about Black's 6...Be7!?


Regards,
Nik