Reproductive loss in high-producing dairy cattle: where will it end?

Forage quality can be assessed in terms of the animal performance that is elicited when the forage is offered to the animal. Animal performance is the product of supply, nutrient and energy concentration, intake, digestibility, and metabolism. Typically, forage quality is measured assuming that the forage is freely available and that deficient nutrients other than protein and energy are provided in a supplement. Of the remaining factors affecting forage quality, intake is the most important one affecting animal performance. Reid (1961) listed the amount of forage eaten as the first factor limiting the usefulness of forage-testing schemes. His list also included the effect of concentrates on the amount of forage eaten, degree of selective feeding, and effect of physical form as important information not provided by the forage testing systems used at that time. Unfortunately, intake, substitution rates, selection, and physical form still are not a part of routine forage evaluation systems in the 1990's. Animal performance depends on the intake of digestible and metabolizable nutrients. Of the variation in digestible dry matter (DDM) or digestible energy (DE) intake among animals and feeds, 60 to 90% is related to differences in intake, whereas only 10 to 40% is related to differences in digestibility (Crampton et al., 1960; Reid, 1961). Intake generally accounts for twice as much variability in DDM intake as does digestibility (Milford, 1960; Ingalls et al., 1965). Milford and Minson (1966) observed that DDM intake is more closely correlated with dry matter intake (DMI) than any other feed or animal characteristic. Similarly, high correlations would occur between DMI and metabolizable (ME) or net energy (NE) intake, and between DMI and overall animal performance when forages are fed. The primary importance of intake in assessing forage quality does not mean that digestibility or metabolizability of nutrients are invariant or unimportant. Metabolizability can vary depending on the amounts and ratios of absorbed nutrients, the individual and interacting biochemical pathways of nutrient conversion and the type of animal production (MacRae et al., 1985; Blaxter, 1989). However, differences in the efficiency of converting DE to ME are small compared to differences in intake and digestibility, and the metabolic conversion of DE to ME or NE is typically considered to be relatively constant within type of production, such as maintenance, growth, pregnancy, or lactation (Moe and Tyrrell, 1976; NRC, 1989). Digestibility of forages is more variable than metabolizability, and feces typically are the greatest loss of ingested nutrients and energy. Extensive research has been devoted to measuring digestibility and relating it to feed characteristics because digestibility can be accurately measured with relative ease compared to DMI. Although intake is more important than digestibility in assessing forage quality, progress in understanding the basic factors that affect intake has been hampered by our inability to measure it accurately and to separate the influences of animal and diet on intake. Thus, we routinely use availability information (DE, ME, andNE)

Regulation of Forage Intake

The effect of embryonic death rates in cattle on the efficacy of estrus synchronization programs

Voluntary dry matter intake (VDMI) of forages by ruminants may be limited by distention resulting from restricted flow of digesta through the gastrointestinal tract. An animal's capacity for fill depends on the weight and volume of digesta that causes distention and the flow rate of digesta from the organ in which distention occurs. The reticulorumen is generally regarded as the site in the gastrointestinal tract for which distention limits VDMI with high-fill diets, although evidence suggests that distention of the abomasum may also limit VDMI. Linear decreases in VDMI have been noted with increasing amounts of inert fill inserted into the reticulorumen, but results have not been consistent across several experiments. Reduction in VDMI depends on the extent to which intake is limited by fill before insertion of inert fill; hence animals with high energy requirements consuming relatively low-energy, high-fill diets are affected to the greatest extent. Because NDF generally ferments and passes from the reticulorumen more slowly than other dietary constituents, it has a greater filling effect over time than non-fibrous feed components and has been found to be the best single chemical predictor of VDMI. However, many other factors affect fill, including particle size, chewing frequency and effectiveness, particle fragility, indigestible NDF fraction, rate of fermentation of the potentially digestible NDF, and characteristics of reticular contractions. These factors are only partially accounted for in models that have been developed to predict VDMI. Increased accuracy of prediction of VDMI is expected as models continue to evolve.

Physical constraints on voluntary intake of forages by ruminants.

Influence of supplemental fats on reproductive tissues and performance of lactating cows.

Management of replacement beef heifers should focus on factors that enhance physiological processes that promote puberty. Age at puberty is important as a production trait when heifers are bred to calve as 2-yr-olds and in systems that impose restricted breeding periods. Calving by 24 mo of age is necessary to obtain maximum lifetime productivity. Because the reproductive system is the last major organ system to mature, factors that influence puberty are critical. The influence of environment on the sequence of events leading to puberty in the heifer is dictated largely by the nutritional status of the animal and related effects on growth rate and development. Management strategies have been designed to ensure that heifers reach a prebreeding target weight that supports optimum reproductive performance, and consequences of inadequate or excessive development have been evaluated. Those strategies are based on evidence linking postweaning nutritional development with key reproductive events that include age at puberty and first breeding, conception, pregnancy loss, incidence and severity of dystocia, and postpartum interval to estrus. Management alternatives that ultimately affect lifetime productivity and reproductive performance of heifers begin at birth and include decisions that involve growth-promoting implants, creep-feeding, breed type and(or) species, birth date and weaning weight, social interaction, sire selection, and exogenous hormonal treatments to synchronize or induce estrus. Basic and applied future research efforts should converge to match in a realistic manner the production potential of the animal with available resources. Strategies that incorporate consideration of nutrition, genetics, and emerging management techniques will need to be tested to enable producers to make decisions that result in profit. This review evaluates the current status of knowledge relating to management of the replacement beef heifer and serves to stimulate research needed to enhance management techniques to ensure puberty at an optimal age.

https://www.vetfood.theclinics.com/article/S0749-0720(13)00066-2/pdf

Management considerations in heifer development and puberty.

Evaluating recipient and embryo factors that affect pregnancy rates of embryo transfer in beef cattle.

Twenty-eight Hereford x Angus cows (4 yr of age) were used to determine the effects of pre- and postpartum dietary energy on performance and reproductive function in suckled beef cows. The experiment was designed as a 2 x 2 factorial with cows receiving either 70 (L) or 150% (H) of NRC recommended level of dietary energy before and(or) after parturition, resulting in four treatment combinations (L-L, L-H, H-L, H-H). Prepartum diets were fed for approximately 110 d, and postpartum diets were fed until either 10 d after the second postpartum ovulation or 150 d postpartum for those cows that failed to ovulate. Cows receiving low compared with high levels of energy before calving lost more (P less than .01) weight, body condition, subcutaneous fat, and longissimus muscle area before parturition and had calves with lighter (P less than .01) birth weights. Cows receiving low compared with high levels of energy postpartum lost more (P less than .01) weight, body condition, and longissimus muscle area after parturition. Low levels of energy before and after parturition decreased (P less than .01) milk production and calf weight at 70 d of age. Rates of cervical and uterine involution were unaffected by dietary energy treatments. Cows receiving high levels of energy prepartum had increased (P less than .01) mean concentrations and pulse frequency of LH in serum after parturition, and cows receiving high levels of energy postpartum had increased (P less than .05) pulse frequency of LH. Low levels of energy postpartum decreased (P less than .01) appearance rate of small (5.0 to 7.9 mm) and large (greater than or equal to 10 mm) follicles, and low levels of energy prepartum decreased (P less than .02) appearance rate of large follicles based on transrectal ultrasonography. Cows receiving high levels of energy prepartum had shorter (P less than .02) intervals from parturition to ovulation, and a higher (P less than .01) percentage of the cows that received high levels of energy postpartum ovulated by 150 d postpartum. In summary, prepartum level of dietary energy influenced birth weight and weight gain of calves, milk production, concentrations and pulse frequency of LH in serum, appearance rate of large follicles, and the interval to first ovulation. Postpartum level of dietary energy influenced milk production, weight gain of calves, pulse frequency of LH, appearance rate of small and large follicles, and the percentage of cows that ovulated after parturition.

Influence of dietary energy on follicular development, serum gonadotropins, and first postpartum ovulation in suckled beef cows.

Expanded use of artificial insemination in the beef cattle industry depends on successful application of treatments designed to synchronize estrus. Regulation of estrous cycles is associated with control of the corpus luteum (CL), whose life span and secretory activity are subject to trophic and lytic mechanisms. The advantages of melengestrol acetate (MGA) in estrous synchronization incorporate ease of administration, lower cost relative to other estrous synchronization products, and potential for use to induce estrus in prepubertal heifers. Treatments first designed to synchronize estrous cycles of normally cycling heifers by feeding MGA were imposed daily for 14 to 18 d at levels of .5 to 1 mg. The minimal daily effective dose required to inhibit ovulation was .42 mg. Longer feeding periods of MGA were associated with low fertility at the first synchronized estrus, but at the second estrus, conception was normal. Low fertility at the synchronized estrus resulted in development of alternative treatment practices, which combined feeding of MGA with injections or implants of estradiol-17 beta, estradiol cypionate, luteinizing hormone, human chorionic gonadotropin, pregnant mare serum gonadotropin, or oxytocin. Estrus was synchronized after MGA and estradiol-17 beta or estradiol cypionate treatments, but fertility was low. Short-term feeding of MGA (5 to 7 d) combined with prostaglandin F2 alpha or its analogs (PGF) on the last day of MGA reduced fertility at the synchronized estrus. The reduced conception at first service occurred in animals that began treatment after d 12 of the estrous cycle. However, feeding MGA for 14 d and then injecting PGF 17 d later avoided problems with reduced conception. Fertility of animals after this treatment was similar to that of contemporaries synchronized with Syncro-Mate-B. However, the length of the treatment period creates a need for increased management and may extend management beyond practical limits. Further research is warranted to address problems associated with reduced fertility after short-term treatment with MGA.

Control of the Bovine Estrous Cycle with Melengestrol Acetate (MGA): A Review

Twelve multiparous Simmental cows (584 kg) were used to determine the influence of calcium soaps of fatty acids (CSFA) incorporated in a range supplement on postpartum reproductive characteristics. Cows were assigned randomly to receive a control [C; containing grain sorghum (GS) and soybean meal (SBM)] or CSFA-based (containing Megalac [a source of CSFA], GS, and SBM) supplement. Supplements plus prairie hay were individually fed. Diets were isonitrogenous and met the NEm requirement for heavy-milking beef cows in early lactation. Supplement feeding and daily blood collection began at parturition. Calves were removed permanently from cows at 25 +/- 2 d postpartum. Duration of first postpartum estrous cycles was determined by both visual observations and changes in concentrations of progesterone in serum. Concentrations of LH in serum (15-min intervals for 6 h) were determined 12 h before and 48 and 96 h after calf removal. Concentrations of progesterone and estradiol-17 beta in serum were determined daily. Cows receiving CSFA had higher (P = .06) mean concentrations of LH than those receiving C (1.47 vs 1.12 +/- .13 ng/ml). Concentrations of estradiol-17 beta were lower (P less than .02) and serum progesterone were higher (P less than .02) between d 6 and 8 of the induced cycle in CSFA-fed cows. Plasma cholesterol was greater (P less than .01) in cows fed CSFA although plasma triglyceride concentrations were similar between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Larry R. Corah

Papers

Management considerations in heifer development and puberty.

Evaluating recipient and embryo factors that affect pregnancy rates of embryo transfer in beef cattle.

Influence of dietary energy on follicular development, serum gonadotropins, and first postpartum ovulation in suckled beef cows.

Control of the Bovine Estrous Cycle with Melengestrol Acetate (MGA): A Review

Effects of calcium soaps of fatty acids on postpartum reproductive function in beef cows.