Nutrition is one of the most important management factors in reaching calf crop goals and in attaining a short calving season every year in breeding herds of beef cattle. The limiting nutrient related to reproduction in beef and dairy cattle is usually energy. Although dairy cattle typically are fed rations that supply adequate energy density during lactation, genetic drivers for milk production inevitably lead to a period of some negative energy balance postpartum, and dry-matter intake may be critically limiting during the transition period (historically described as 2 weeks before and 2 weeks after calving/freshening).
Energy status and body condition before calving affect mainly the timing of when a beef cow returns to estrus. Subsequent conception relies on many factors, including prior energy status, as well as current nutrition, infectious disease, and semen quality. Feed requirements vary during the reproductive cycle.
There are four periods of beef cow nutrient requirements, and generally three such periods for dairy cows:
Period 1 is the interval from calving to breeding; lasting ~70–90 days, it 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. Beef cows should gain weight while still milking. Although some dairy cows maintain body weight, many high milk producers continue to lose weight during this period.
Period 3 is the interval from weaning to 50 days before calving; lasting ~100 days, it is the period of least nutritional demand. Beef cows need only maintain their condition and continue fetal development. Dairy cows should be managed to gain or lose body weight during the final months of lactation to achieve a target body condition that makes them ready to enter the stable dry period.
Period 4 is a critical stage consisting of the 50 days before calving; 75% of fetal growth occurs during this time. 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 weight during late pregnancy.
Dairy cows are usually fed to attain optimal milk production throughout their typical lactation period of 305 days. It is generally assumed that they will lose weight during peak 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, primarily through insulin resistance. This tendency increases the probability of metabolic diseases—eg, type II ketosis and fatty liver disease—during early lactation because 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 to avoid negative effects on fertility and health postpartum.
Relative to the size of the calf at her side, the amount of cow feed required prior to weaning is fairly constant. However, larger cows require more feed for maintenance than do smaller cows, and in general, cows that give more milk require more feed with a higher concentration 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 calves and heavy-milking cows can be a limiting factor for maintenance of cow condition prior to weaning. Mature, dry cows, on the other hand, are often overfed protein. The environmental impact of feeding excess crude protein in both dairy and beef cattle rations is prompting a review of optimal inclusion amounts because ammonia and nitrous oxide emissions are associated with global warming potential. Interest in lower crude-protein rations with targeted amino acid profiles is growing, in order to optimize sustainable production.
Heifers must be fed adequately from weaning to breeding if they are to calve at 2 years old. This target is critical for herd economics because before this point, the absence of beef calf or milk for sale represents a sizable investment and risk without a return. This target is also critical in managing the sustainable production of beef or milk because environmental impact is usually measured per unit of production. Poor reproductive performance in either beef or dairy production inevitably is associated with a greater number of unproductive heifer replacements, which negatively affects the environment without the benefit of food production.
To provide the essential nutrient requirements during various stages of the reproductive cycle, the nutrient content of major forages and homegrown cereals should be analyzed and confirmed. Variation in amounts of trace minerals is common between and within different geographic areas. Different systems are used globally to determine the energy content of feeds, including the metabolizable protein and Feed into Milk models, as well as the total digestible nutrient system and the California net energy system. All are commonly used; the choice should be tailored to fit the individual operation.
Even within nutritional-need categories, cattle benefit from being fed and handled in subgroups. For example, lightweight heifers at weaning need to gain more than heavier heifers to reach puberty by breeding season. First-calf heifers require special attention, focusing on potential energy losses due to social competition, 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. A third subgroup might be thin, old, and small cows, who may not compete favorably with heavier cows within the same herd; they often benefit from being fed separately.
Monitoring of growth rates is important to achieve successful rearing targets. Supplemental feeding of both high-energy and balanced protein feeds to first-calf heifers may be required for optimal reproductive potential, with careful consideration of rumen health regarding acidosis and dry-matter intake. 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; this earlier weaning may also help this group to calve before the older cows do.
Lactating dairy cattle are usually fed according to milk production. They may be fed concentrate on an individual basis, or they may be divided into groups according to milk production and fed an appropriate total mixed ration. Single ration systems are also used for the entire milking herd. Once established, disruption of social groupings should be avoided, as it will negatively impact reproductive success (such as estrus expression). However, inappropriately high-energy total mixed rations in late lactation may lead to fat cows, dystocia, and fatty liver syndrome, and regrouping to better tailor the rations may be necessary in such cases.
