Nutrition in Cattle Reproduction
Nutrition is one of the most important management factors in reaching calf crop goals and in attaining a short calving season every year in beef breeding herds. The limiting nutrient related to reproduction in beef and dairy cattle is usually energy; although dairy cattle are usually fed rations that supply adequate energy during lactation, genetic drivers for milk production inevitably lead to a period of some negative energy balance postpartum. The level of energy and body condition before calving primarily influences when a beef cow returns to estrus, whereas level of energy after calving primarily influences subsequent conception. Feed requirements vary during the reproductive cycle (see Nutritional Requirements of Beef Cattle).
There are four periods of beef cow nutrient requirements, and generally three for dairy cows. Period 1 is the interval from calving to breeding; it is ~70–90 days and is the period of greatest nutritional demand. The dairy cow is at maximal milk production and recovering from the stress of parturition. During this period, she is expected to be ready to breed.
Period 2 is the interval from rebreeding to weaning the beef calf; it is ~120–150 days in beef cows. Periods 2 and 3 overlap in dairy cows and are not as easily separated as in beef cattle. The beef cow should gain weight while still milking. Although some dairy cows maintain body weight, many high producers continue to lose weight during this period.
Period 3 is from weaning to 50 days before calving; it lasts ~100 days and is the period of least nutritional demand. The beef cow has only to maintain her condition and continue fetal development. The dairy cow should be managed to gain or lose body weight during the last months of lactation to achieve target body condition ready to enter the stable dry period.
Period 4 is a critical stage and is the 50 days before calving; it is during this time that 75% of fetal growth occurs. Cows are usually not lactating during this "dry period." Cow condition at calving is critical to rebreeding; the onset of estrus after calving is delayed in cows that lose weight or are thin and not gaining during late pregnancy.
Dairy cows (see Nutritional Requirements of Dairy Cattle) are usually fed for optimal milk production throughout their 305-day lactation. It is assumed they will lose weight during heavy lactation (the early months) and regain the loss during the remainder of lactation. Dairy cows should not be overfed during the dry period because of a genetic predisposition to sacrifice body condition to maximize milk production through insulin resistance. This leads to the increased probability of metabolic diseases, eg, type II ketosis and fatty liver disease (see Ketosis in Cattle and see Fatty Liver Disease of Cattle), during early lactation as insulin resistance leads to excessive fat mobilization from adipose tissue storage and overwhelms the capacity of lipoprotein transport mechanisms in the bovine liver to transport and metabolize lipid. In addition, dairy cows should be fed to minimize the incidence of calving-related disorders (eg, dystocia, hypocalcemia, and retained fetal membranes), including control of dietary cation-anion balance (DCAB), which have a negative effect on fertility and health postpartum.
The amount of cow feed required per pound or kg of calf weaned is fairly constant, although larger cows require more feed for maintenance than smaller cows. Cows that give more milk require more feed, generally with a higher level of protein. Increased milk is produced at the expense of reproduction when feed is not adequate to meet all needs.
The protein requirement of young growing stock and heavy-milking cows is often a limiting factor, while mature dry cows are often overfed protein. Heifers must be fed adequately from weaning to breeding if they are to calve at 2 yr of age; this target is critical for herd economics, because before this point the absence of beef calf or milk for sale represents significant investment and risk.
To provide the essential nutrient requirements during various stages of the reproductive cycle, major forages and homegrown cereals should be analyzed to monitor nutrient content and actual value. Variation in amounts of trace minerals is common between and within different geographic areas. Globally, different systems are used to determine energy levels of the ration, such as the metabolizable protein (MP) or "Feed into Milk" (FiM) models or the Total Digestible Nutrient System and the California Net Energy System. All are commonly used, and application should be tailored to fit the individual operation.
Even within nutritional need categories, cattle benefit from feeding and handling in subgroups: lightweight heifers at weaning need to gain more than heavier heifers to reach puberty by breeding season; first-calf heifers require special attention from both an energy and competition standpoint if they are expected to breed and conceive at the proper time. These heifers are still growing, as well as lactating, and they may not have the rumen capacity to meet postcalving energy needs on forage alone. Monitoring of growth rates is important to achieve successful rearing targets. Supplemental feeding of both high-energy and high-protein feeds to first-calf heifers may be required for optimal reproductive potential. Calves from first-calf beef heifers may be weaned 30–40 days earlier than calves from cows in the main herd to allow the heifer more time to grow and recover from demands associated with lactation.
Thin, old, and small cows may not compete favorably with heavier cows within the same herd and often benefit from being fed as a separate subgroup.
Lactating dairy cattle are usually fed according to milk production. They may be fed concentrate on an individual basis or divided into groups according to milk production and fed an appropriate total mixed ration.