FROM THE FARM REPORT: GETTING THE PROTEIN – ENERGY BALANCE RIGHT FOR GROWING HEIFERS

Once dairy calves are weaned, how we balance metabolizable protein (MP) and metabolizable energy (ME) in the diet becomes a key factor in shaping the kind of growth they achieve. It’s not just about how fast they grow —it’s about what kind of tissue they’re putting on. A heifer that grows too quickly on a low-protein, high-energy diet can end up storing excess fat in her mammary gland instead of developing the milk-producing tissue she’ll need later in life.

Research from Albino et al., (2015) showed that Holstein heifers gaining around 2.2 lb per day (1 kg/d) needed diets supplying more than 43 g of MP per Mcal of ME to keep mammary development on track. Below that threshold, fat started replacing valuable parenchymal tissue. The latest (NASEM, 2021) guidelines line up with those findings, suggesting that a 440-lb heifer (200 kg) should receive roughly 46 g of MP/Mcal, dropping to about 39 g/Mcal by 880 lb (400 kg). However, studies by Albino et al. (2015, 2017) indicate that once daily gains reach 1 kg, that lower value isn’t enough — holding the diet above 43 g/Mcal is a safer bet.

When the MP:ME ratio slips too low, heifers may keep gaining weight, but not the “right kind” of tissue. Weller et al. (2016) found that high-energy diets boosted hormones like IGF-1, which support growth, but also increased fat synthesis in the udder when protein wasn’t keeping pace. Earlier work by Silva et al.,(2002) made the same point: it’s not how big they get, but how much body fat they carry that affects future milk production.

And the long-term stakes are real. Too much fat in the mammary gland during the “allometric” growth phase — from roughly 3 to 10 months of age — has been consistently linked with lower milk yield in the first lactation (Capuco et al., 1995; Sejrsen and Purup, 1997). On the other hand, heifers fed to maintain lean growth and adequate protein supply reach breeding size sooner and calve earlier — without hurting future milk output.

So how can nutritionists raise the MP:ME ratio without overloading the diet with crude protein? One effective tool is rumen-undegradable protein (RUP). Feeds like heat-treated soybean meal, blood meal, or fish meal let more protein escape rumen breakdown and reach the small intestine, boosting MP supply efficiently. In a study by Silva et al. (2018), heifers fed diets where about 50% of their CP came from RUP gained faster and converted feed more efficiently, with no negative effects on mammary development.

The takeaway: keep an eye on the MP:ME ratio, not just total gain. Aiming for at least 43 g of MP per Mcal of ME supports lean tissue growth, sets the mammary gland up for success, and helps future milk yield. Using more RUP in postweaned heifer diets can achieve that balance while keeping crude protein — and nitrogen losses — under control. It’s a small adjustment with big payoffs for both herd performance and sustainability.

— Marcos Marcondes

mmarcondes@whminer.com