Considerations in Multiple Trait Evaluation1

SYMPOSIUM: MULTIPLE T R A I T E V A L U A T I O N Considerations in Multiple Trait Evaluation 1 R O B E R T W. B L A K E Department of Animal Science Texas A&M University College Station 77843 ABSTRACT

of this paper are to summarize briefly traits of potential importance and to focus on alternative measures and research needs to help elucidate genetic pathways of dairy cattle performance. Hopefully, the "grist" of future multiple trait models will be information reflecting more closely the biology of the cow, than has yet been possible.

Traits of dairy cattle known to contribute to genetic merit are discussed briefly. Focus is on alternative measures and research to reflect more closely genetic pathways of performance because knowledge is expanding about cellular, organ, and animal function. Improved measurement of physiological traits of economic characters portends increased genetic control of animal productivity by considering more alternative traits. Research should clarify multiple trait selection programs to maximize profit. The null hypothesis to test is that selection for milk is optimal index selection for milk income and costs of production.

G O A L S OF S E L E C T I O N

INTRODUCTION

My task is to assess characteristics of dairy cattle that merit consideration either directly (e.g., trait in a selection index because of a substantial product of economic weight by heritability; ah ~) or indirectly in programs of genetic improvement (e.g., correlated character of uncertain modus operandi). Improved profit potential is by optimal selection of multiple traits, some cryptically manifested, which requires accurate estimates of parameters describing interrelated biological systems of the dairy cow. Knowledge is expanding about biological mechanisms of cellular, organ, and animal function; and this portends improved genetic control of animal productivity. Many (33, 65, 68, 73, 83, 84, 113, 117, 119) have defined breeding objectives and traits contributing to genetic merit. Objectives

Received October 26, 1983. 1Technical Article No. 18862 of the Texas Agricultural Experiment Station, College Station 77843. Project 2491, a contributing project to Southern Regional Project $49, Genetic Methods of Improving Dairy Cattle for the South. 1984 J Dairy Sci 67:1554--1566

Most can agree to a universal selection goal of maximum profit by producing economic foods from our livestock. However, attainment of a breeding goal of maximum profit is precluded by inadequate knowledge about its constituent traits and their proper weights. Young (119) emphasized objective measurement of correlated responses to selection for milk that contribute also to profit, and attention was focused on inputs to milk production, especially labor for individual care of cows. However, comprehensive profit functions are difficult to construct because accurate data on inputs are scarce, expensive to obtain, and may not be applicable to all herds; also, opinions vary regarding economic values for some traits receiving substantial selection pressure (e.g., type score). To improve estimates of profit potential of dairy cows, Young (121) suggested 1) that herds enrolled in Dairy Herd Improvement (DHI) programs could report number of days milk was discarded because of mastitis treatment, 2) that those herds report all inseminations to estimate accurately services per conception for cows and bulls, and 3) that selected cooperator herds maintain extensive records of inputs influencing profit potential of cows. Discarded milk and veterinary and labor costs of cow care were identified as fundamental economic measures of cost of production (121). Legates (61) discussed profitability functions for dairy cattle and warned that measures to model a specified management system may be weighted less optimally than by selection index

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SYMPOSIUM: MULTIPLE TRAIT EVALUATION methods. This is wise counsel, because profit is the sum of prices'or costs times elements of the underlying production functions, and the selection index (multiple trait) approach provides opportunity to model the respective production functions accompanied by their proper economic weights. Smith (109) discussed potential important cumulative effects of several traits, each with small individual influence by their ah 2, on efficiency of the selection index and the greater impact of relative economic weights if traits are correlated (genetically, phenotypically) unfavorably rather than favorably. Smith (109) noted the insensitivity of selection index efficiency to small changes of economic weights such as those reflecting market adjustments and refinements of management. A vision for multiple trait methodology might be inclusion of genetic and phenotypic covariances of intermediate biochemical pathways comprising a particular trait with a clear economic value only for the trait itself. Such additional information may help improve accuracy of predicting correlated responses. T R A I T S TO P R E D I C T P R O F I T

Numerous traits have been utilized to obtain a breeding objective of profit, and McDaniel (68) discussed basic steps to choose traits to maximize profit. Andrus and McGilliard (4) predicted profit per year of herd life from lifetime records of 111 Holstein cows. By order of importance, actual milk per year of herd life (predominant trait), cases of mastiffs per year, percent milk fat, and live freshenings per year were associated with 66% of the variance of actual profit. Reduced accuracy (R 2 = 50%) of predicting profit by substituting mature equivalent milk for actual yield underscored the need to measure real inputs and outputs (4). As an alternative to single trait selection on milk yield of first lactation, Cassell and McDaniel (25) indicated, "Some information relative to later lactation performance appears critical to genetic progress for lifetime profit in the dairy cow" (p. 8). The study of Bakker et al. (5) indicated that stayability and yields of milk and fat of first lactation, to represent lifetime performance, might be useful in sire evaluations for profitability, although it remains to be seen just how stayability improves prediction of lifetime profit when its genetic variance is nil.

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Because of few data to estimate covariances among many traits related to profitability, Pearson (83) suggested a practical breeding objective composed of traits each accounting for I>10% of the variance of profit. Miller and Pearson (82) presented an excellent discussion of comprehensive breeding goals associated with profit per unit time. Traits suggested for a profit index were yields of milk, fat, and protein, somatic cell (SC) count as a proxy for discarded milk (as in 121), body weight, calving interval, age at first calving, conformation, and calving difficulty (84). Freeman (33) cautioned against selecting for too many traits because of unknown relative economic values, and he indicated that traits other than milk income possibly warranting sire selection were dystocia, stayability, and conformation. Balaine et al. (6) submitted profit per day of herd life as the preferred expression for net economic merit of dairy cattle because of large correlations with income - expense (r = .98) and total profit (income minus expense, r = .76 to .87). Correlations of profit per day with constituent traits were .44 to .48 for yields of milk, fat, and protein; .27 for cost of feed energy; - . 2 1 for mastiffs treatments; .24 for live calf weight; and <.2 for salvage value, growth, inseminations, and herd life. Repeatability of profit per day of herd life (t~.25) was less than for milk yield (7). Net income per day was greater for daughters of sires chosen only for milk versus those selected by a combination of udder index, percent daughters culled in first lactation, and minimum restrictions on Predicted Difference (PD) milk and repeatability (85). First lactation milk yield, percent milk fat, days in lactation, number of inseminations, and age at calving were used by Lin and Allaire (63) to construct a selection index to maximize genetic gains in profit to 72 mo of age. Heritability of the selection index was .59 + .09. Although selection for profit to 41 mo of age (h 2 = .46 + .09) was 7% more efficient than the selection index, a profit function was always more efficient (2 to 14%) than milk yield of first lactation. Genetic gain in profit to 72 mo was achieved either by reducing age at first calving (h 2 = .18) by 1 mo or by increasing milk in first lactation by 500 kg (63), in general agreement with Hansen et al. (44). As more is learned about profitability and its production constituents, multiple trait evaluaJournal of Dairy Science Vol. 67, No. 7, 1984

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tions may need to account for intermediate optima to achieve maximum gains of profit (73). Lin and Allaire (63) indicated that " . . . commercial dairymen need to be concerned more with maximizing the profitability of their cattle than maximizing yield regardless of economic consequences" (p. 1970). Toward this objective, Allaire (2) presented a mate selection plan to maximize net genetic merit of offspring by combining optimally parental genotypes when traits selected are related nonlinearly with the merit index. Miller and Pearson (73) pointed out that it would be surprising if genetic gain failed to exhibit a diminishing rate of return. In other words, alternative plans which produce genetic gains near the maximum possible are likely to cost more than alternatives which produce gains at a lower rate of annual improvement. (p. 288). •

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This potential conflict between genetic gain (production) and economic maxima was demonstrated by Wilcox et al. (118), who evaluated Holstein artificial insemination (AI) sires under alternative selection goals and herd management by linearly combining net present value transmitting abilities for milk income and type weighted by relative expected net incomes from each trait• Optimal sire selections that maximized return on investment in semen were seldom those with premier PD for milk income and type (i.e., Total Performance Index) due to prohibitive semen cost (118). The input cost of alternative genetic improvement programs needs more attention to assess net economic benefits. Vinson (113) cited extensive literature in a discussion of secondary traits (defined as contributors either indirectly to differences among cows for yields of milk, milk components, and meat, or directly to cost of production). Among traits of reproductive performance for males and females, dystocia, mastiffs, milking ease, conformation, and longevity, it was considered (113) that 1) selection for fertility should await improved understanding of mechanisms controlling it (e.g., genotypeenvironment interactions affecting calving interval, differences between heifers and cows), 2) dystocia should be evaluated by first and later parity traits and by consideration of direct and maternal effects, 3) " . . . breeding strategies cannot be developed without a clear underJournal of Dairy Science Vol. 67, No. 7, 1984

standing of the biological variation among cows in SC numbers in the m a m m a r y gland" (p. 376), 4) economic values for ease of milking that are correlated positively with milk yield (15) and for conformation are small, and 5) indices of profitability should be developed in lieu of longevity (stayability) as a selection criterion. Labor Inputs. Although the need is great, few studies have monitored labor inputs to produce milk and profit. The report by Hansen et al. (45) of Minnesota's project to evaluate heritable costs of cow care indicated that more total labor and expense accompanied greater expected net incomes, excluding differences in discarded milk, from cows selected for milk compared to controls. In separate studies by Shanks et al. (106, 107), more net returns, in spite of increased health costs, from highest yielding cows further strengthen recommendations of intensive selection for milk to increase net profitability of dairy cattle. Recall that profits are differences between functions describing total revenues and costs. If economies of size of the dairy cow arise principally from increased milk yield (e.g., increased apparent feed efficiency implies no increase of feed per unit of milk), then greater costs of labor and health should be expected to accompany greater yields of milk and profit. Variations of inputs to milk harvest have been examined (15, 16, 18), and results agreed with health care cost of dairy production. Highest yielding cows required additional labor and machine inputs, but differences of economic and real time among cows were dominated by increased milk (17, 18). Attention to udder traits, especially udder support, may help restrict increasing milking costs from selection for milk (16). Beef. Beef is an important by-product of dairy cattle. However, Cartwright (24) did not consider it prudent to transfer selection intensity from milk to beef traits. For example, selection to improve dressing percent probably would reduce capacity of the gastrointestinal tract; and this may conflict with increased milk yield. Research Needs. Future dairy cattle breeding programs are expected to include more alternative traits than are selected currently (35, 65, 117), especially those governing costs of production. More accurate estimates of economic values, genetic correlations among

SYMPOSIUM: MULTIPLE TRAIT EVALUATION traits, and heritabilities and repeatabilities of less defined traits such as disease resistance (especially mastitis), reproductive traits of males and females, and linearized type were identified as important topics for research (117). Information describing the genetic covariance structure among inputs to dairy production will be useful in simulation studies to evaluate profits from alternative genetic improvement programs that may interact with herd management (e.g., combinations of A] and natural service to restrict economic risks from reproductive performance). For example, Allaire (3) did not find an interaction between cow replacement rate and genetic trend in milk up to 1% of the mean per year. Gahne (35) pointed out the potential benefit from genetic studies of endocrine characters if we learn how to account for rates of secretion and clearance of circulating hormones and their variations in time. At an earlier symposium Legates (60) exhorted "If researchers are to find really new facts and forge ahead, they must be more specific and innovative in attempts to measure the desired traits" (p. 852). SELECTION FOR M I L K Direct Response

Positive genetic trends from selection experiments have shown animal breeding theory to be a reliable technique to improve production, and this is in spite of our substantial ignorance about the biological systems. See (27, 48, 70) for evidence of selection responses in milk solids utilizing genetic methods of the last 20 yr. Metand et al. (70) referenced several studies of the direct response to selection for milk. Powell et al. (96) documented a positive genetic trend for milk of <1% of mean yield per year in commercial dairy herds and indicated relatively greater economic importance of genetic gains compared to other husbandry techniques to improve milk yield. Adjustments for days open were compared to mature equivalent (ME) milk records as an alternative to evaluate transmitting abilities of sires (112), and advantage was little for removal of the environmental effect of days open. Feed Efficiency

Results from recent studies of feed efficiency may be most important to illustrate a phys-

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iological crossroad among biological systems of the cow: how milk yield increases by selection. Custodio et al. (28) found no evidence of pleiotropy between utilization of the immediate diet of protein, starch, and fiber and actual or transmitting ability for fat corrected milk (FCM) yield of cows with genetic abilities reflecting the Holstein breed (X = 229 kg, SD = 192 kg USDA Cow Index milk). Because high yields were associated with tissue catabolism and because gross energy efficiency increased with actual and additive genetic ability for milk, selection for milk was inferred to be dependent upon the tissue balance system. Research suggestions in (14) were to elucidate potential genetic aspects of endocrine pathways and mechanisms comprising the system of feed input - tissue balance - milk yield. Sejrsen and Neimann-Sorensen (104) suggested selection for appetite to restrict negative energy balance, but this is infeasible because of the prohibitive cost of obtaining feed intake information. Also, increased appetite as measured by dry matter intake in first half of lactation may not be the primary genetic pathway for increased milk (8, 14, 28). Bauman and Elliot (8) indicated that changes of body composition in ruminants have not been studied, but decreases of visceral fat and adipocyte size parallel estimates of negative energy balance. They (8) stated, " . . . it is clear that the animal's capacity for sustained copious milk production and for extensive mobilization of body fat reserves go hand-in-hand" (p. 443) and concluded that synthesis and mobilization of Iipids are apparently reciprocal processes. Endocrine secretions linking milk yield, tissue balance system, and ovarian function were identified in (8). Low serum progesterone during lactogenesis apparently stimulates mammary epithelial growth as measured by increased insulin receptors independently of cell number and may cause the animal to catabolize adipose tissue (decrease insulin receptors) in support of accelerating milk secretion. Other hormones that may promote homeorhetic ["orchestrated changes for the priorities of a physiological state" (p. 440), (8)] support of milk production via tissue catabolism are prolactin and growth hormone (8, 39, 86, 87, 115). Custodio et al. (28) suggested that "gene~ c parameters will be important for monitori g the rate and identifying the limits of catabc c Journal of Dairy Science Vol. 67, No. 7, 1984

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subsidy of milk yield in cattle selected intensively for milk" (pp. 1945-1946). Possible quantity and rate dimensions for accessing tissue reserves to support high milk yields were proposed in (14). The hypothesis of genetic gain in milk by tissue catabolism from growth hormone could be examined by exogenous treatment of cows from control lines of selection experiments. Control cows of inferior genotype for milk would receive growth hormone throughout lactation to simulate peak yield and lactation performance of the selected line. If results support growth hormone as a principal gene product from selection for milk, additivity could be tested by close response studies. Onset of lactation is dependent on rapid flux of calcium to the mammary gland, especially for exocytosis from the Golgi apparatus of cytoplasm in casein micelles (8, 78). Analyses of mammary fluids from nonlactating cows at Texas A&M showed normal concentrations of milk solids, dissociated casein micelles, and no detectable calcium. It is intriguing that, in addition to secretion, calcium is also linked to 1) enzyme regulation of intracellular processes such as energy metabolism via specific cellular proteins (e.g., calmodulin; 101, 110), 2) potential influence of growth hormone on intestinal absorption (36), 3) steroidogenesis (75, 101) and luteolysis (101), and 4) potential expression (by other processes resident in cytoplasm) of maternal cytoplasmic inheritance of milk yield and days open (9). Although knowledge is meager about biological systems of the cow, recent advances of biomedical research make it clear that, as Miller and Pearson (73) reminded, whether by design or default, any selection is index selection. Milk Composition

Basic economic evaluations of milk components are in (13, 49). Although direct selection would be effective to increase protein content of milk, Mbah and Hargrove (64) showed that incentives to emphasize milk protein are proportional to its price. Everett et al. (30) provided guidelines to new assessments of milk components to increased genetic trend for profit. The major limitation of measuring and selecting for yield of a new component is cost of analysis if genetic parameters are similar to those of other composition traits of animals. Journal of Dairy Science Vol. 67, No. 7, 1984

Concentration of casein proteins determines the yield of manufactured dairy products from milk. Variation of casein relative to total protein (casein number) 1) is economically valuable (19), 2) is repeatable with an additive genetic basis (19, 46, 47) but apparently with small heritability (h 2 = .08 + .03; 47), and 3) may be useful also to monitor udder health because secretion (exocytosis) of casein is impaired by infection (72). An inexpensive measure of casein content of milk could have two-fold value: to assess the value of milk for manufacture (79) and to evaluate cows for mastiffs resistance in a manner more clearly interpretable than by SC count (71, 72). DISEASE RESISTANCE AND HEALTH

Young (120) suggested screening young bulls for AI sampling programs by independent culling level based on mastiffs and health histories of their dams to augment selection against disease in addition to that already applied indirectly via selection for milk. In agreement with (45, 106, 107), Miller et al. (74) found no difference in the incidence and cost of mastitis or SC count between two groups of Holsteins differing by 685 kg milk in first lactation. Miller (71, 72) discussed the uncertainty regarding biological interpretation of SC counts, especially the potential tactical error of selection to minimize SC because highest counts accompany clinical infections. He (72) envisioned a "battery of automated tests" to detect mastiffs: concentrations of lactose and some enzymes, and SC count. Given the progress in genetic evaluation of profit discussed earlier and heritability estimates ~.10 for mastitis scores and SC count (37, 71, 72, 108), casein number might be a useful measure of mastiffs resistance to include in a selection index if percent casein or serum protein could be analyzed inexpensively. PHYSIOLOGICAL CHARACTERS

Quantitative genetic theory presumes genie control of biochemical processes (e.g., enzyme activity, intermediate protein products) causing phenotypic expression. That traits can be predicted accurately across generations shows that the general mode of gene action is understood partially. However, specific pathway contributions of the many genes aggregated

SYMPOSIUM: MULTIPLE TRAIT EVALUATION into a trait generally are unknown or understood poorly. Therefore, one might summarize, to the credit of animal breeders, that realized genetic responses are by usefully accurate inference through a "black box" of biochemical pathways. Land (58) described animal performance in a trait as a function of the fluxes or rates of biochemical activity of its pathways. Because of reciprocal cancelling changes among individual fluxes comprising the system, implying large negative genetic covariances, he pointed out the trivial consequences of selecting to increase the enzyme activity of a single biochemical process. Instead, the study of. physiological characters [i.e., intermediate set of rate constants for fluxes comprising economically important traits such as lactation and reproduction] was suggested to accelerate genetic change "through a greater understanding of variation and covariation" (p. 210) (58). Similarly, Lerner (62) suggested a "cybernetic model" (p. 65) of buffered feedbacks favoring "balanced phenotypes", where " . . . those in which extreme deviations for one trait are compensated for by some form of deviation in another" (p. 13). He (62) distinguished " . . . between additively genetic control of the traits under selection and the nonadditive basis for fitness" and emphasized that " . . . a correlated response . . . [of reduced] . . . fitness is an obligatory consequence of selection for extreme morphological phenotypes" (p. 94). Gorski (39) emphasized study of circulating hormones and their interactions with tissues; the concentration of receptor sites was submitted as a possible reason for genetic differences in milk yield of dairy cows. Thyroxine degradation in bulls was associated with FCM yields of their daughters (54), but single plasma samples to measure thyroxine, triiodothyronine, and insulin were useless to predict daughter milk yield (82). Frequent measures of the serum residual from hormone secretion and binding to tissue receptors are necessary to monitor true endocrine feedback patterns (98). Much research has been devoted to the reproductive complex (34) because of its primordial importance to productivity and genetic advance. Fifteen years ago, Meadows (69) emphasized the need to decrease calf losses and to improve fertility. Since then, genetic and environmental aspects of dystocia have been studied widely (22, 88, 89, 90, 91, 92, 93, 95),

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and methods of genetic evaluation of dairy sires have been developed (34, 95) and implemented to restrict calf losses, especially from primiparous dams. Other recent research has been devoted to reproductive performance of lactating cows. The following discussion focuses on reproductive performance of males and females, endocrine feedback mechanisms, and associations between reproductive performance and milk yield. Relationship Between Male and Female Fertility

Because heterosis is common for traits of reproduction, Land (57) suggested development and maintenance of distinct strains of male and female lines of general livestock to meet demands across time in various production environments of the world. For dairy cattle it would be helpful to improve or to restrict decay of female reproductive performance by selecting for corresponding characters in males. Although heritabilities of testicular size and consistency of Holstein bulls were substantial (h 2 = .3 to .6) and related to semen fertility, little is known about the relationship between fertility traits in males and females (26). Studies with sheep indicate a link between physiological characters of male and female reproductive performance. Land (56) proposed testis growth as a measure of circulating gonadotropins which may relate also to the onset of ovarian activity; Land and Carr (59) observed that three breeds of sheep ranked the same for concentration of luteinizing hormone (LH) accompanying testicular hypertrophy from hemicastration as for ovulation rate. In this study and those of Ricordeau et al. (99) and Hanrahan et al. (41), sampling was probably too infrequent to characterize accurately the intrinsic pulsatile pattern of release of LH (98). Carr and Land (23) submitted the possibility of using the amplitude and pattern of pulsatile release of LH in conjunction with testis growth to select for improved reproductive performance. Islam et al. (51) estimated genetic correlations of .25 to .50 between testis weight and ovulation rate in mice but without change in litter size. Bindon and Turner (12) discussed the possibility that LH differences in young female lambs may aid selection for ovulation rate if gonadal development is subject to early stimulation by LH. Journal of Dairy Science Vol. 67, No. 7, 1984

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Walkley and Smith (114) examined index selection utilizing testis size, ovulation rate, and gonadotropin to improve litter size. They (114) suggested that size of testis may be useful to improve size of litter but cautioned the unreliable prediction of correlated responses. Sustarsic and Wolfe (111) studied the production and release of LH in adult male mice from a 4 x 4 diallel experiment. Parallel to the discussion by Land ( 5 8 ) o f reciprocal changes in flux among segments of a biological process (e.g., "down-regulation"), negative heterosis was observed in two of six crosses for serum and pituitary LH and in five of six crosses for induced release of LH (111). However, most genetic variation was additive and represented 37, 29, and 8% of total variance for pituitary LH, induced LH release, and serum LH. Because of its intrinsic pulsatile release, serum LH (single sample) was influenced more by environmental factors than other LH characters and was the only one unaffected by nonadditive genetic variation. If gonadotrophic hormones of males and females are parallel (i.e., development and performance of gonad tissue) and have a genetic basis, then research may be warranted to evaluate the relationship of testis traits and gonadotropin of AI sires with the reproductive performance of their daughters. Research and cooperator herds may be used complementally to study postpartum interval to initial ovarian activity and function by milk progesterone, days open, and service periods of daughters of AI sires and relationships with testis and LH measurements. The possibility is open for AI organizations to sponsor collection of this kind of information as part of young sire programs and to examine the relationship with sire proofs for conception rare (67). The high cost (ca. $3/sample) of radioimmunoassay for progesterone precludes field use, although if dairies were close to a laboratory and demand were high as in Great Britain, the cost could be similar to that of somatic cell analysis. Endocrine Feedback Mechanisms

Rahe et al. (98) found distinctive pulsatile patterns of serum LH concentration throughout the estrous cycle of the dairy heifer. It appears necessary and important to sample blood on a frequent basis (~<15 rain) to establish clearly Journal of Dairy Science Vol. 67, No. 7, 1984

patterns of repeated pulses of LH throughout the day. Therefore, it is unlikely that patterns of LH were characterized accurately in the studies cited previously by single, daily, hourly, 30-min, or 20-rain sampling intervals. Further, a feedback effect of ovarian steroids of estrogen and progesterone on the pattern of plasma LH concentration was suggested because pulsatile patterns had the same mean LH concentrations throughout the estrous cycle (98). Procknor et al. (97) also found a pulsatile pattern for progesterone that temporally and uniformly lagged the pulsatile pattern of plasma LH. Fitz (31) studied the number of LH receptors and concentration of adenyl cyclase [enzyme activation of cyclic adenosine monophosphate production, which is directly related to steroidogenesis] in corpora lutea in various stages of development and regression. Adenyl cyclase concentrations increased with number of receptors during growth of corpus luteum (eL) tissue. Serum progesterone decreased with regression of the eL. However, the number of LH receptors on the CL decreased following initiation of CL regression; and adenyl cyclase decreased synchronously with regression of the CL (31). This suggests that a decreasing number of LH receptors is an unlikely mechanism for CL regression. However, possible involvement of adenyl cyclase in CL regression is suggested by the parallel relationship between them. Results like these suggest the critical need for research to unravel mechanisms and potential genie components of the reproduction complex. Freeman (34) identified improved understanding of the biology of reproduction as a prerequisite for its selection. Taken collectively, reproductive function is associated with feedback mechanisms involving patterns of hormonal secretion and enzyme mediation. The report of Bell et al. (9) that maternal cytoplasmic effects influenced days open further supports this supposition. Enhancement of genetic improvements of performance potentials impinges on improved understanding of these physiological mechanisms. Reproductive Performance and Milk Yield

Reproductive performance and its relationship with milk yield have received much recent study in the US (10, 11, 42, 43, 44, 55, 105) and Europe (52, 53, 66, 94). Common r e -

SYMPOSIUM: MULTIPLE TRAIT EVALUATION productive traits in recent research are days open, conception rate, and service period. Heritability of days open was estimated near zero, and, in contrast to conclusions in (112), days open was considered an environmental effect needing adjustment, especially in selecting sires of the next generation of bulls (103). Philipsson (94) pointed out that, in spite of low heritability ratios, additive genetic variation for reproductive function is substantial and requires attention along with selection for milk to avoid long-term losses of fertility. Laben et al. (55) observed longer intervals to first insemination and conception for high compared to low yielding cows, but performance varied, especially among herds where some high yielding herds also had short calving intervals. Results in (100) indicated no difference in reproductive performance between daughters of sires selected intensely for milk and those selected less intensely. Genetic correlations between milk yield and reproductive performance were antagonistic in recent US studies (11, 43, 105). Contrary to Philipsson's (94) conclusion of substantial genetic variation of reproductive traits, Hansen et al. (42) found it was nearly nil and concluded (44) that the antagonism may be inconsequential. Fonseca et al. (32) and Gonzalez et al. (38) do not support antagonism between milk yield and interval to initial ovarian activity (ovulation) or length of the first postpartum estrous cycles of Holstein or Jersey. Frequencies of estrus at initial ovarian activity were <50% in (32, 38). Therefore, the pleiotropic mechanism between milk yield and reproductive performance may not be mediated by ovarian activity, but it may result from subliminal or absent estrous behavior. A simple requirement for a behavioral mechanism linking high milk yield and reduced reproductive performance as measured by increased service period (11)is an unequal probability of estrus [related to energy balance (milk yield)] among ovulations in the early postpartum period. Gross energy efficiency and tissue balance in (28) and days to first ovulation and calculated daily energy balance (13 Holsteins; 21) were correlated negatively. As discussed, several endocrine and enzyme changes may be involved in homeorhetic support of high milk yield in early lactation. Hansen et al. (44) found and Freeman (34) also discussed the apparent interaction of

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genotype by environment (signs of genetic correlations differed) of fertility of heifers and cows with milk yield of first lactation. If the physiological environment for the reproduction genotype is unfavorable during lactation but favorable in heifers, then it may be argued that changes of energy balance and endocrine patterns accompanying lactation may constitute a gradient of physiological environments restricting reproductive performance of cows. Similarly, a negative relationship between postnatal environment and reproductive performance in litter bearing species has been suggested (76, 77). Hansen et al. (44) offered the possibility of highest heifer fertility among daughters of highest PD sires because of large body size or early maturity, and this could be associated endocrinologically with secretion of growth hormone. A total merit index of FCM yield, heifer service period, and constant days open was examined (44), but the corresponding economic value was required to be large. Holmann et al. (50) estimated differences in average income over feed cost for a single calving interval for Holsteins. Average cost per day open was nil for calving intervals extended from 12 to 13 or 15 mo with income-maximizing feeding throughout lactation and 65 days dry. In summary, genetic mediation of physiological characters comprising the response pathways of economically important traits has been identified. Because many traits compete for the same resource base of nutrients and endocrine and enzyme regulators, it is not surprising that unfavorable genetic covariances could arise between them. Genetic manipulation of productive responses (direct, correlated, heterotic) will be enhanced when genic pathways are understood and information is available to model connected variance-covariance structures representing biological systems of the cow. Research efforts should clarify whether selection for milk is near optimal index selection for milk income and costs of production (e.g., reproduction, mastiffs, herd life). LONGEVITY AND CONFORMATION

Other things equal, the longer the payback period on an investment, the greater the profit; and when applied to the dairy cow, payback period may be an adequate measure of stayability. Longevity or stayability represents a Journal of Dairy Science Vol. 67, No. 7, 1984

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desirable quality of cows to sustain profitable production that 'when accumulated across cow lifetimes and discounted results in greatest net returns (net present values) for those surviving longest. Burnside and Wilton (20) pointed out the desirablity, in spite of generation intervals extended by selecting for longevity, of sires with "high production proofs and relatively low percentages of daughters culled for low production, udder problems, and reproduction in early life" (p. 845). Several nonyield traits have been considered associated with longevity: traits thought to impart resistance to disease, injury, or physiological imbalance (80); the "workability" traits of milking speed, disposition, udder and feet and leg problems (1); and type score (29, 81). Agyemang et al. (1) found little justification for selection for workability traits based on small sire components of variance [<2% of total for all traits except slow milking speed (2.6%)] and heritabilities < 10%. De Lorenzo et al. (29) found PD for type score unrelated to stayability, but type was helpful for predicting stayability for highest yielding Holstein cows, which may have resulted from specific culling criteria. "Sire differences in the individual type traits are not related to other than random stayability differences" (p. 1284) (29). Norman et al. (81) evaluated contributions of type traits to predict profit per day of productive life of Jersey cows. Yields of milk and milk fat in first lactation were 1.9, 2.7, and 4.1 times as important as final score in predicting total days in milk, lifetime gross earnings, and profit per day of productive life. Type traits that contributed to the prediction of profit in Jerseys were final score, suspensory ligament, and mammary system (81). Improved prediction of profit by type traits probably will come from physical measurements associated with cost of cow care, such as measurement of heritable hoof characteristics (40). A major research need, to deliver to dairy producers clear selection recommendations to maximize net profit, is to evaluate type economically (116). White and Vinson (116) recommended relative emphasis on milk and type of 3:1 or 6:1 for registered breeders and 9:1 or 20:1 for most commercial breeders to optimize genetic gains in these traits. Ruff et al. (102) studied factors influencing sale price of registered Holstein cattle and found the standard Journal of Dairy Science Vol. 67, No. 7, 1984

partial regression coefficient for milk yield 5 to 7 times larger than for final score of cows when prices were predicted by a model containing genetically independent effects of cow and service sire. Wilcox et al. (118) submitted a method to maximize net profit from sire selections for specified weightings (indices) of milk income and type score under alternative situations of reproductive management. SUMMARY

The purpose of genetic improvement of livestock is to meet societal goals and to maximize profits. Indices to maximize profit such as profit per day of herd life to 41 or 72 mo of age are comprised principally by milk yield, mastitis losses, milk composition, days in milk, and number of lactations initiated by birth of a live calf. Data from DHI programs that could contribute to profitability estimates of cows are days of discarded milk, days of mastitis treatment, and a complete history of inseminations. A major limitation to construction of useful profitability indices is accurate estimation of economic weights for the constituent traits (e.g., type) and of genetic parameters influencing profits (e.g., labor, health, cow care). The assessment of mastitis susceptibility may become more accurate with development of automated, inexpensive tests for lactose, enzymes, and casein from monthly DHI milk samples. Endocrine measurements (e.g., progesterone, prolactin) complementing insemination and estrous information also could be derived from DHI milk samples. Biomedical advances will provide measurements forming new avenues to control direct and correlated responses to selection. Current understanding of physiological-endocrinological mechanisms that compete for limited resources is the basis for expecting a myriad of genetically antagonistic relationships among traits. Gonadotropin patterns (e.g., LH) of mammals appear heritable, and a potential parallel relationship may exist between sexes. Feedbacks among hormone systems that manifest pulsatile patterns (e.g., LH, progesterone) and apparent relationships with gonad function (e.g., testis growth, ovulation rate) indicate research needs to evaluate genetic pathways controlling reproductive performance. Relationships between sire (e.g., testis growth,

SYMPOSIUM: MULTIPLE TRAIT EVALUATION g o n a d o t r o p i n p a t t e r n , c o n c e p t i o n rate) and d a u g h t e r (e.g., intervals and p a t t e r n s o f ovarian activity by milk p r o g e s t e r o n e , days o p e n , service p e r i o d , c o n c e p t i o n rate) r e p r o d u c t i v e f u n c t i o n deserve study. ACKNOWLEDGMENT

The h e l p f u l discussion and criticisms o f E. J. Pollak a n d R e x M c M a h o n are a c k n o w l e d g e d gratefully.

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