Q:

Was the gene seed harvested during the Horus Heresy?

I'm reading a number of Warhammer 40k universe stories and I've noticed something odd which I'm hoping someone can explain.
In Helsreach a big deal is made of harvesting the Gene Seed of the fallen Astartes. My understanding is that these are the gene organs which if salvaged can then be implanted into future recruits.
However I've now read five of the Horus Heresy novels and as far as I can remember it's not been mentioned once. Did the Astartes Legions need to harvest the gene seed of their fallen brothers during the Horus Heresy or did it become more important later?

A:

It became much more important post-Heresy
TLDR: It's mainly because Chaos Space Marines need to steal Loyalist Marine gene-seed to produce new Marines, their gene-seed is too corrupted to produce new Marines the old fashioned way. Loyalist Marines really don't like it when that happens.

If we consider how a gene-seed is created: a gene-seed is implanted in a candidate and assuming the candidate survives long enough, a second gene-seed begins to grow and is then harvested and implanted in another candidate and so on and so on...
Pre-Heresy
There were 18 legions, before setting out on the Great Crusade each legion had many hundreds of thousands of battle brothers, each producing additional gene-seeds.
Additionally, each legions Primarch was alive and an entire legion could be created as long as he was alive.
Post-Heresy
Once the loyalist Primarchs were dead (missing) and the founding legions were divided into much much smaller chapters, individual gene-seeds became very important. Whereas a legion would have an almost inexhaustible supply, a chapter would not. It's at this point that genetic banks were setup on Terra to produce gene-seed for each new Chapter and each Chapter became very protective over their gene-seed, guarding them ferociously and recovering them from dead Marines at all costs.
Chaos Space Marines cannot produce gene-seed in the same fashion as Loyalist Marines. Their gene-seed becomes corrupted over time due to exposure to Chaos. Their only option is to raid Loyalist Chapter monasteries or scavenge from the dead, thereby increasing even further the value of existing gene-seed.
Also, out-of-universe
Some authors probably just like talking about gene-seeds more than others. I guess logically in any post-Heresy battle, especially vs Chaos, Space Marines should always be striving to retrieve their fallen brothers gene-seed. But I'm sure there's many 40k novels that rarely mention them, if at all.

A:

It was done Pre-Heresy and during it
@Daft 's answer is true, however, I would also like to add in that there was still the harvesting of gene-seed by the Space Marine Legions.
Here is an example from "Horus Rising" by Dan Abnett, during the pacification of the planet Sixty-Three Nineteen by the Luna Wolves (emphasis mine).

Marjex, one of the Legion’s apothecaries, was bending over him. He glanced up as Loken approached, and shook his head.
‘Who is it?’ Loken asked.
‘Tibor, of Second Squad,’ Marjex replied. Loken frowned as he saw the devastating head wound that had stopped Tibor.
‘The Emperor knows his name,’ Loken said.
Marjex nodded, and reached into his narthecium to get the reductor tool. He was about to remove Tibor’s precious gene-seed, so that it might be returned to the Legion banks.

Another in "The Flight of the Eisenstein" by James Swallow (emphasis mine).

 Out there in the shoal of destruction, Stormbirds on funerary details scoured the engagement area for Astartes who had been blown into the dark during boarding operations. Those found would be interred as heroes, once the progenoid glands in their corpses had been harvested. The precious flesh-matter from the dead would serve the Legion in their stead, passing on to strengthen new initiates when the next round of recruitment began.

In "Fallen Angels" gene-seed is also harvested specifically for the survival of the later legion (emphasis mine).

 A disturbingly long line of dead battle brothers were stretched out along the roadway to their left; the bodies were being tended to by one of the ground force’s two Apothecaries,  collecting the gene-seed for the future of the Legion.

Why was it still done Pre-Heresy?
It is also important to note how new gene-seed is actually made. Progenoid glands are one of the gene-seed implanted within an astartes which takes tissue from the astartes himself to duplicate one of the organs. This takes a few years to create a fully mature gene-seed. So the harvesting of gene-seed from fallen brothers has always been important for a legion/chapter because it's the way to reproduce new gene-seed. So even when there was plenty of resources for the legion's to produce new gene-seed it was still precious enough to harvest it.

