  The scale of alpha-element yields is difficult to predict from theory because
of uncertainties in massive star evolution, supernova physics, and black hole
formation, and it is difficult to constrain empirically because the impact of
higher yields can be compensated by greater metal loss in galactic winds. We
use a recent measurement of the mean iron yield of core collapse supernovae
(CCSN) by Rodriguez et al. (RMN23), $\bar{y}_{\rm Fe}^{\rm cc} =0.058 \pm 0.007
M_\odot$, to infer the scale of alpha-element yields by assuming that the
plateau of [alpha/Fe] abundance ratios observed in low metallicity stars
represents the yield ratio of CCSN. For a Kroupa IMF and a plateau at
[alpha/Fe]=0.45, we find that the population-averaged yields of O and Mg per
unit mass of star formation are about equal to the mass fractions of these
elements in the sun. The inferred O and Fe yields agree with predictions of the
Sukhbold et al. (2016) CCSN models assuming their Z9.6+N20 neutrino-driven
engine, a scenario in which many progenitors with $M<40M_\odot$ implode to
black holes rather than exploding. The yields are lower than assumed in some
models of galactic chemical evolution (GCE) and the galaxy mass-metallicity
relation, reducing the level of outflows needed to match observed abundances.
For straightforward assumptions, we find that one-zone GCE models with
mass-loading factor $\eta\approx 0.6$ evolve to solar metallicity at late
times. By requiring that models reach [alpha/Fe]=0 at late times, and assuming
a mean Fe yield of $0.7M_\odot$ per Type Ia supernova, we infer a Hubble-time
integrated SNIa rate of $1.1\times 10^{-3} M_\odot^{-1}$, compatible with
estimates from supernova surveys. The RMN23 measurement provides one of the few
empirical anchors for the absolute scale of nucleosynthetic yields, with
wide-ranging implications for stellar and galactic astrophysics.
