Postpartum infertility is caused by four factors: general infertility, lack of uterine involution, short estrous cycles and anestrus. The general infertility component is common to any estrous cycle and reduces potential fertility by 20 to 30%. Incomplete uterine involution prevents fertilization during the first 20 d after calving but is not related to anestrus. Short estrous cycles prevent fertility during the first 40 d after calving by causing the cow to return to estrus before pregnancy recognition occurs. Anestrus is the major component of postpartum infertility and is affected by several minor factors: season, breed, parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy as well as two major factors: suckling and nutrition. These major factors have direct effects on anestrus but also interact with one or more other factors to control postpartum anestrus. Physiological mechanisms associated with anestrus involve blockage of the GnRH "pulse generator" in the hypothalamus, but other pathways also must be involved because bypassing the pulse generator is not an effective treatment for all cows. The primary cause of anestrus probably is different for different stages of anestrus. The mediating mechanisms for anestrus are not involved with prolactin, oxytocin, the adrenal or direct neural input from the mammary gland but are at least partially involved with blood glucose and the endogenous opioid peptide system. Management options to decrease the impact of anestrus and infertility include: 1) restrict breeding season to less than or equal to 45 d; 2) manage nutrition so body condition score is 5 to 7 before calving; 3) minimize effects of dystocia and stimulate estrous activity with a sterile bull and estrous synchronization; and 4) judicious use of complete, partial or short-term weaning.

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.

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The objectives of this study were to estimate heritability for scrotal circumference (SC) and semen traits and their genetic correlations (rg) with birth weight (BRW). Semen traits were recorded for Line 1 Hereford bulls (n = 841), born in 1963 or from 1967 to 2000, that were selected for use at Fort Keogh (Miles City, MT) or for sale. Semen was collected by electroejaculation when bulls were a mean age of 446 d. Phenotypes were BRW, SC, ejaculate volume, subjective scores for ejaculate color, swirl, sperm concentration and motility, and percentages of sperm classified as normal and live or having abnormal heads, abnormal midpieces, proximal cytoplasmic droplets (primary abnormalities), bent tails, coiled tails, or distal cytoplasmic droplets (secondary abnormalities). Percentages of primary and secondary also were calculated. Data were analyzed using multiple-trait derivative-free REML. Models included fixed effects for contemporary group, age of dam, age of bull, inbreeding of the bull and his dam, and random animal and residual effects. Random maternal and permanent maternal environmental effects were also included in the model for BRW. Estimates of heritability for BRW, SC, semen color, volume, concentration, swirl, motility, and percentages of normal, live, abnormal heads, abnormal midpieces, proximal cytoplasmic droplets, bent tails, coiled tails, distal cytoplasmic droplets, and primary and secondary abnormalities were 0.34, 0.57, 0.15, 0.09, 0.16, 0.21, 0.22, 0.35, 0.22, 0.00 0.16, 0.37, 0.00 0.34 0.00, 0.30, and 0.33, respectively. Estimates of rg for SC with color, volume, concentration, swirl, motility, and percentages of live, normal, and primary and secondary abnormalities were 0.73, 0.20, 0.77, 0.40, 0.34, 0.63, 0.33, -0.36, and -0.45, respectively. Estimates of rg for BRW with SC, color, volume, concentration, swirl, motility, and percentages live, normal, and primary and secondary abnormalities were 0.28, 0.60, 0.08, 0.58, 0.44, 0.21, 0.34, 0.20, -0.02, and -0.16, respectively. If selection pressure was applied to increase SC, all of the phenotypes evaluated would be expected to improve. Predicted correlated responses in semen characteristics per genetic SD of selection applied to SC were 0.87 genetic SD or less. If selection pressure was applied to reduce BRW, the correlated responses would generally be smaller but antagonistic to improving all of the phenotypes evaluated. Predicted correlated responses in SC and semen characteristics per genetic SD of selection applied to BRW were less than 0.35 genetic SD.

Two experiments were conducted in consecutive years to determine the effects of prepartum nutrient level and postpartum ruminally undegraded protein intake on nutrient status, milk production, subsequent calf production, and reproductive performance of 126 crossbred, primiparous beef heifers. Prepartum treatments were low nutrient intake (LN) (approximately 2.5 kg of TDN, .5 kg of CP animal-1.d-1 and maintenance nutrient intake (MN) (5 kg of TDN, 1 kg of CP animal-1.d-1), which were fed for 75 d before parturition. Two postpartum protein supplements were formulated to provide 250 g/d of ruminally degradable protein (RD) and one to supply ruminally undegraded protein (UD) at 250 g/d of additional UD CP compared to the RD supplement. Cholesterol was lower (P less than .01) in heifers given UD than in heifers given RD. Blood urea nitrogen was higher (P less than .01) for UD-fed heifers than for RD-fed heifers and was higher in LN heifers (P less than .06) than in MN heifers. Milk production did not differ (P greater than .11) as a result of LN, MN, UD, or RD. Postpartum cow weight gain was greatest (P less than .01) for UD and LN heifers. The percentage of heifers bred during the first estrous cycle of the breeding season was greater (P less than .02) for UD than for RD. Overall, prepartum nutrition did not interact with postpartum protein supplement, nor did it have any effect on postpartum interval, whereas UD increased cow weight gain postpartum and reduced postpartum interval.

Multiparous beef cows (n = 7) were used to evaluate peripartum changes and interactions among body temperature (BT) and circulating progesterone (P4), estradiol-17beta (E2), triiodothyronine (T3), cortisol, thyroxine (T4), and 13,14-dihydro-15-keto-prostaglandin F2alpha (PGFM) concentrations. Electronic temperature monitors were placed under the obliquus abdominis internus muscle of the left flank, and BT was measured using radiotelemetry every 3 min for 10-s periods from 144 h before to 24 h after calving. Environmental temperatures (ET) were recorded hourly. Body and environmental temperatures were averaged, separately, within 8-h periods. Blood samples were collected every 8 h, and hormone concentrations were measured. Time of day affected BT (P < .01), at 0300 cows had the lowest BT, at 1900 the highest, and at 1100 values were intermediate. Body temperature remained relatively constant (P > .10) from 144 to 56 h before calving and from 8 to 24 h after calving but decreased (P < .01) from 48 to 8 h before calving. Precalving BT was affected (P < .01) by ET, but hour-before-calving (time) had the greatest effect on BT during the 48 to 8 h immediately preceding parturition (b' = .41, P < .01) and was independent of ET effects. Before the BT decrease, cows gestating heifers had lower (P < .01) BT than cows gestating bulls. Plasma E2, PGFM, T3, and T4 concentrations before the precalving decrease in body temperature were greater (P < .03) in cows gestating bull rather than heifer calves. Approximately 30% of the variation (R2) during the temperature decrease was explained by plasma hormone concentrations; PGFM (b' = -.30, P < .05) and T3 (b' = -.22, P < .10) had the most significant effects. In conclusion, BT of the cow before the precalving decrease was affected by ET and sex of calf. However, the prepartum BT decrease was independent of these variables, and seemed partially endocrine-induced.

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