Seems there's scientific evidence that supports the semen increasing effect of some types of maca.
From wiki:
"A recent review states "Randomized clinical trials have shown that maca has favorable effects on energy and mood, may decrease anxiety and improve sexual desire. Maca has also been shown to improve sperm production, sperm motility, and semen volume.""
Review from some research organization:
"Black maca has better effects on sperm production than yellow maca which has only moderate effects. Red maca, however, has no effect on sperm production. However, red maca has been shown to reduce prostate size in rats in which prostate hyperplasia had been induced with testosterone enanthate; yellow maca has shown moderate effects here, whereas black maca has not shown any effects."
http://content.karger.com/ProdukteDB/produkte.asp?doi=10.1159/000264618 -ah weird it only let me see this once without an account.
One study from Pubmed:
"RESULTS: Treatment with Maca resulted in increased seminal volume, sperm count per ejaculum, motile sperm count, and sperm motility. Serum hormone levels were not modified with Maca treatment. Increase of sperm count was not related to dose of Maca.
CONCLUSION: Maca improved sperm production and sperm motility by mechanisms not related to LH, FSH, PRL, T and E2."
http://www.ncbi.nlm.nih.gov/pubmed/11753476