“Afrikaners is plesierig, dit can julle glo (Afrikaners are fun, that you can believe),” runs the chorus of the rustiest chestnut in Afrikaans folk music. It isn’t heard, much less proven, in “The Harvesters,” South African writer-director Etienne Kallos’ muscular, mood-rich debut feature. Unusual within the annals of its national cinema for its searching examination of the country’s once-dominant, now-dwindling white Afrikaner population, this sternly moving, vividly shot rural drama draws quasi-Biblical resonance from its tale of teenage foster brothers locked in a familial and cultural power struggle on a remote farmstead. That a low-key queer undercurrent courses through the conflict somewhat broadens the festival and distribution prospects of the film, the fine social divisions of which will nonetheless be unfamiliar to many outside viewers; in a Cannes edition heavy on auspicious debuts, this is among the most excitingly complete.

It says much about the out-of-time nature of life in the Bible belt of South Africa’s central Free State province — a flat expanse of scrubby yellow plains and maize fields, a million miles and yet not a million miles from the American Midwest — that it’s initially hard to tell what period Kallos’ film is even set in. Only once a cellphone appears in this rigorously visualized world of dateless khakiwear and ascetic midcentury furnishings can we tell for sure that we’re not still in the apartheid era, when white Afrikaans families like the one portrayed here were most prioritized and protected by ruling politicians. Now, nearly a quarter-century into the country’s rocky democracy, they seem quaint relics of a dark past, endangered by the very insularity of their community and, more directly, a drastic uptick in farm murders across the country. If not necessarily as racially driven as the infamous “kill the boer, kill the farmer” campaign of decades past, it’s a clear and rising threat, awareness of which young matriarch Marie (a superb Juliana Venter) drums into her young children with startling bluntness.

“There are so few of us left,” she explains morosely to her eldest child Janno (Brent Vermeulen), a quiet, dutiful 15-year-old boy whose extreme gentleness of nature is an unspoken but palpable concern to his parents — in a society where conservative models of masculinity still reign supreme. “Make his blood strong, make his seed strong,” she repeats in the near-incantatory prayer that opens the film, putting more faith in the Lord to keep Afrikaner blood running than in the boy himself. Janno, meanwhile, daren’t admit to anyone (perhaps not even himself, just yet) that his nascent sexual interests don’t extend to spreading his seed: In his tidy, sombre bedroom, he fantasizes about more-than-friends contact with a strapping rugby teammate.

It’s a secret that subtly becomes a bruise-tender point of ambiguous negotiation when Janno’s devoutly Christian parents welcome an underprivileged new boy into their family. Pieter (Alex Van Dyk) is around Janno’s age, though an adult lifetime’s worth of suffering flashes in his thin, flinty gaze: A sickly street orphan with a precocious history of crime and drug addiction, he’s unwillingly sent from a halfway house to Janno’s family farm, in the hope that some country air and straight-and-narrow Afrikaner discipline will right his path. While Janno’s opposite in many respects, Pieter is swift to identify the one thing that binds them: Neither boy will ever fully belong in this corn-fed, church-ruled, heritage-obsessed world of alpha males and nuclear families.

That’s not a cue for bonding, however, but for hostile, self-protecting competition, complicated by prejudices that may be eye-opening to audiences unversed in differences of culture and class within South Africa’s minority white population. Kallos’ literate, sharply calibrated script — partially inspired by the Free State-oriented work of the late, celebrated “African Gothic” playwright Reza de Wet — is brilliantly attentive to lexical contrast and code-switching, though it’s often in loaded silence that the characters reveal themselves most acutely. As a portrait of Afrikaans masculinity at a painful contemporary crossroads, “The Harvesters” would fill a strong double bill with Oliver Hermanus’ searing 2011 film “Beauty,” which likewise debuted in Un Certain Regard at Cannes.

Unerringly precise casting, meanwhile, ensure that the film’s milieu is as richly inhabited as it is written. Vermeulen and Van Dyk, both extraordinary novice finds, articulate the boys’ individual and combined insecurities with restrained but bone-deep feeling, their characters almost merging in volatile ways as they begin to truly see each other. As for the Free State, an unromantically robust landscape that has thus far received more notable literary than cinematic evocation, it becomes a breathing, burning reflection of the boys’ own frustrations through Michal Englert’s brilliant, weather-soaked widescreen lensing and Barri Parvess’ impeccably observed production design. Together with Kallos, they conjure a grey-and-grass palette, spiked with oxide yellow light, for this fractious, earthy parable: The increasingly abandoned world of “The Harvesters” could seemingly ossify or bloom at any moment, taking the lonely, slowly awoken Janno with it.