By the time you are reading this, most spring-calving herds will have finished the main mating period. The bulls might still be out, but many of next season’s pregnancies are already there, quietly developing in early summer. Over the next few weeks, pasture quality will start to drop, and intakes will reduce. Hot, humid conditions will push cows to seek shade and, in many parts of New Zealand, facial eczema (FE) risk will start to build as we head towards drier summer weather. All of this affects the liver, hormones, and energy supply that the new embryo depends on for development. So cows may look fine, while the true damage shows up months later as more late calvers and higher empty rates. Early pregnancy losses easy to miss Research shows that many pregnancy losses in pasture-grazed herds occur in the first month after insemination, often in the first week. This is long before you would expect to see a return to oestrus. Any sudden heat stress in the first week of pregnancy can impact the conception rate. As the cow has to deal with toxins and a shortage of minerals, the body quietly shifts reproduction down the priority list. As a result, many embryos are lost before they can implant. Heat, behaviour and minerals Heat stress is a combination of two factors: heat and humidity. Heat stress can become an issue with temperatures as low as 20°C and humidity levels as low as 60%. In November, North Island temperatures can get over 25°C, and relative humidity can quickly build to over 80%. Heat changes how cows behave and eat. Once heat stress takes hold, farmers see their cows drinking more, along with a drop in milk. As temperatures creep into the low 20s, especially when humidity is high, cows spend less time grazing and more time around water, and standing and panting. Intakes drop, and rumen function comes under stress, so cows become fussier about when and what they eat. From a mineral point of view, that matters because any effective supplement programme relies on cows eating the ration put in front of them. Simple steps in hot weather At a basic level, make sure cows have easy access to plenty of clean water. Time milkings so they can feed mainly during the cooler parts of the day, and, where possible, provide adequate shade on hot days. Support this with sodium (salt) to help metabolic function and water intake, and with magnesium and suitable rumen buffers to support rumen function, ensuring feed and mineral intakes stay as steady as possible. Mycotoxins make heat stress harder Feed quality is essential at this time. If feed contains mycotoxins, heat stress becomes a bigger problem. Many mycotoxins cause blood vessels to constrict, which reduces the animal’s ability to deal with environmental heat. One of the worst in this respect is the common ryegrass ergot toxin, which is present in many summer pastures. Several toxin binders, as well as seaweed extract products, can be added to feed to help reduce the impact of these toxins FE: a liver problem first Spring is also when the foundations for FE mitigation are laid. Even if you are not seeing affected cows, FE is first and foremost a liver disease. Cows graze spores of the fungus Puesdopithomyces toxicarius from dead litter at the base of the pasture, and these spores release the toxin sporidesmin, which damages the liver and bile ducts. Why visible FE signs appear later The familiar sunburned skin is a consequence of the gradual buildup of liver damage, not the starting point. If liver function and capacity are supported early in the season, the liver will have better capacity to repair damage. On the other hand, if the diet is poor and mineral levels are already low, the liver has very little capacity to repair damage from summer toxins and FE. That damage can then escalate quickly. Planning the next six weeks Treat December and January as the time to protect the liver before spore counts spike. Stay updated with local monitoring, talk with your vet, advisor or consultant about when to start zinc, and choose a delivery method that fits how you feed cows through summer. Minerals that support liver function and fertility To recover from summer toxins, the liver requires adequate levels of zinc, iodine, and selenium for good glutathione production. Strong liver function also supports reproduction, as it drives the delivery of energy and protein to the cow. Providing a good supply of copper, zinc, selenium, iodine, and manganese supports the liver, which in turn supports milk production, reproduction, and overall cow health. Feeding cows to hold pregnancies through summer Summer changes the quality of pasture. Dry matter increases, protein and energy levels are lower, and at the same time, there is often a temptation to reduce the level and quality of supplementary feeds. Instead, aim to keep adequate daily intakes that match your soil, pasture and animal requirements, rather than simply matching the reduced level of production. This approach will keep production up for longer and give cows the nutritional support they need to maintain pregnancies. The work you put into shade and water, mineral delivery, FE management and trace element balance through January and February protects the pregnancies you already have and helps more of those cows return to the shed next spring. Chris Balemi is managing director of Agvance Nutrition #CHRIS_BALEMI