Nutrition is the most important limiting factor of beef herd reproductive performance. An understanding of the principles underlying the nutritional management of breeding females is necessary, including a working knowledge of the different energy measuring systems commonly used and their applications for different classes of animals, activities, and feedstuffs. (See Nutrition: Beef Cattle.) Increasing stocking rate tends to cause increased gain per unit land area but can result in decreased gain per animal. The key is to have these two factors in balance so that pasture land is optimally grazed and gain per animal is adequate. Overgrazed pasture is detrimental to the environment and can severely reduce gain per animal.
Nutritional requirements vary throughout the year. The most critical periods for reproduction are immediately precalving, when fetal growth is maximal along with lactation preparation, and early postcalving, when maximal lactation is combined with the need for rebreeding. (Also see Management of Reproduction: Cattle.)
Environmental conditions can strongly influence the nutritional requirements and intake of cattle. For example, cold weather increases energy needs, whereas hot or inclement weather can reduce foraging opportunity. The quality and quantity of range forage varies greatly throughout the year and between years, influenced by moisture, soil fertility, plant species, and grazing pressure. Seasonal changes in the nutrient density of rangeland forages are mainly associated with the degree of plant maturity. In general, the greatest nutritional value of plants develops before maturity. Good nutritional management involves matching, as far as possible, the nutrient requirements of cows and nutrient density of the pasture by careful consideration of factors such as the types of animals involved, stocking rates, plant species available, season of grazing, fertilization, and grazing methods used.
Accurate and timely determination of the nutritional status of grazing animals represents a challenge for beef producers, because many variables can influence a cow’s response to a given level of nutrition. The use of a BCS is an effective indirect method to determine nutritional status in breeding females. The BCS represents a subjective assessment of body fat (or energy reserves) that is strongly related to female reproductive performance. BCS, and changes in BCS, appear to be more reliable indicators of nutritional status than is body weight or changes in body weight, which can vary with gut fill and pregnancy status. In addition, BCS can often be assessed more conveniently than body weight can be measured. BCS is both repeatable and accurate in experienced hands. It is best done through visual appraisal, reinforced by palpation of body regions most likely to demonstrate fat deposits. Group observations of BCS made from a distance when animals are in the pasture or paddock are less accurate than those made when animals are nearby in the pen or chute.
BCS varies throughout the year and should be monitored regularly. In the 1–9 BCS system widely used in North America, the reference standard for beef females is a BCS of 5, which represents an average, moderately fleshed cow that is neither fat nor thin. However, the BCS for optimal female efficiency varies with breed and operation and may be higher or lower. In general, cows should calve when they are between condition scores 5–6 (heifers 6–7) and then regain the weight lost at calving and gain slightly until breeding. It generally takes ~2 mo to gain 1 score (75–100 lb [34.1–45.5 kg]) for nonlactating cows under pasture conditions. Care should be taken not to rely on averages, because these can mask variations that might adversely affect herd fertility.
Whereas BCS at calving has been long proved to impact reproductive success, recent studies have demonstrated that an increasing BCS from after calving to breeding is just as important or can be more important than precalving BCS in terms of herd fertility. The concept of increasing BCS just before breeding is especially true of yearling heifers. These heifers are often developed on more of a feedlot type ration during the winter and then turned out to pasture at breeding time. This causes a decrease in available energy, which can significantly decrease first-service conception rate. Solutions to this are to feed a lower energy ration during the development stage and to add additional energy to the diet after pasture turn-out.
The BCS of females at calving can provide much information about their rebreeding prospects. However, assessment of BCS at this time provides a relatively short length of time in which to meet targets if cows are too thin. Assessment of BCS in females at breeding should provide the most accurate prediction of herd fertility, because it is done just before the predictive event. The disadvantage is that there is no opportunity to correct significant shortfalls in time to affect the current breeding season. Assessment of BCS at the time of pregnancy checking has the advantage of not requiring a separate animal handling. It also allows considerable time to remedy obvious deficiencies before calving and subsequent rebreeding. The disadvantage is that, although it can provide clues to explain current pregnancy patterns, it is again too late to remedy them. The ideal time to assign BCS to cows is ~2–3 mo precalving. This gives ample time to move cows to an optimal precalving BCS, because BCS at calving is highly correlated to herd fertility. It is best if someone other than the owner (eg, veterinarian, extension specialist, etc) does the BCS evaluation of the herd, because the owner sees the cows every day and is less likely to see changes.