Highland cattle will, in fact, survive on virtually any feed or pasture condition available; however, mere survival will not produce good growth rates, and does not imply that the animals will thrive in all conditions.
All cattle have certain basic nutritional requirements for each stage of growth, and if these requirements are not supplied, that animal will suffer in one way or another. The cow that is deprived of adequate levels of protein and carbohydrates may not produce enough milk for her calf; that same calf may fail to grow and develop until it is weaned and placed on better feed. The chronically underfed animal will be small for its age, poorly developed with under size, weak horns, and may very well not breed or rebreed.
Occasionally, there will be times when the cow literally starves her calf to death because of poor milk production. Other times, some cows produce so much milk that their calves may be the biggest in the herd, but the cows appear to be near starvation themselves. These cows need more feed, but sometimes it is not possible to supply them with enough in a practical manner. If the condition of the cow is extremely poor, the calf may need to be weaned early. Creep feeding the calf will certainly put more weight on the calf, but it does little for the cow. Calves may replace some of their grazing requirements with grain supplements, but they will nurse their dam just as heavily with or without creep feed.
The quality of the feed is extremely important. High quality grass hay can have more nutrients in it than second cutting alfalfa that has been rained on two times. Corn stover is excellent filler, and fine for the mature brood cows not producing milk, but it will not provide adequate nutrition for cows due to calve, lactating cows or growing young stock. It is possible to starve cattle to death even with food right in front of them.
Moldy feed can also have detrimental effects. Economically, there may be times when some moldy feed has to be fed, but this is never without risk. Toxins in moldy feed can cause any number of problems, including abortion, fetal deformity, liver disease, intestinal upsets, laminitis and death.
There is a wealth of information on cattle nutrition available through the university extension services. Feed dealers also will often assist in formulating balanced rations for livestock. Take advantage of every resource available, and don't assume that Highland cattle can take care of themselves.