Milk and fat production in dairy cattle influenced by advanced ...

Proc. Nati. Acad. Sci. USA Vol. 86, pp. 993-996, February 1989 Genetics

Milk and fat production in dairy cattle influenced by advanced subclinical bovine leukemia virus infection (age/seropositive/seronegative/prsstent lymphocytosis/breeding values)

MING-CHE Wu, ROGER D. SHANKS, AND HARRIS A. LEWIN* Department of Animal Sciences, University of Illinois, 126 Animal Sciences Laboratory, 1207 West Gregory Drive, Urbana, IL 61801

Communicated by Charles R. Henderson, November 7, 1988 (received for review February 1, 1988)

ABSTRACT Genetic potentials (pedigree-estimated breeding value) for milk and for fat were compared in cows grouped according to subclinical stage of bovine leukemia virus infection. Genetic potential for milk production was significantly greater in seropositive cows with persistent lymphocytosis (662 ± 72 kg) and in seropositive hematologically normal cows (554 ± 34 kg) than in seronegative herdmates (418 ± 53 kg). When 305-day twice-daily-milking mature equivalent milk production records for the current lactation were adjusted for genetic potential, bovine leukemia virus-infected cows that were hematologically normal had significantly greater milk production than did seronegative herdmates, suggesting that early bovine leukemia virus infection was positively associated with milk yield. Genetic potential for fat production was significantly greater for cows with persistent lymphocytosis (21 ± 2 kg) than for other seropositive (16 ± 1 kg) and seronegative herdmates (13 ± 2 kg); however, 305-day twice-daily-milking mature equivalent fat production for the current lactation was not significantly different between the groups. Thus, cows with persistent lymphocytosis did not produce fat according to their genetic potential. As an apparent consequence of tendencies for greater milk yield and less fat production, milk fat percentage was significantly reduced in cows with persistent lymphocytosis (3.33 ± 0.09%) and other seropositive cows (3.48 ± 0.05%) relative to seronegative herdmates (3.67 ± 0.07%). These results suggest a need to reevaluate the economic impact of bovine leukemia virus infection on the dairy industry.

Bovine leukemia virus (BLV) is a type C retrovirus that causes a chronic B-cell proliferative disease in cattle (1) and is considered an important model for human T-cell leukemia virus type I infection because of many shared molecular and

biological features (2, 3). The essential difference between these two oncogenic viruses is that the primary target cell of BLV infection is the B lymphocyte (4). The pathogenesis of BLV infection proceeds in a stepwise fashion. Cattle infected with BLV will produce antibodies to the major BLV envelope glycoprotein (BLV-gpSl) (5), usually followed by the appearance ofantibodies to the viral core protein, p24 (6). Approximately 30% of BLV-infected animals develop a persistent lymphocytosis (PL) (7), which is characterized by a dramatic increase in the percentage and absolute number of B cells in peripheral blood (8, 9). About one-third of the B cells in cattle with PL have BLV provirus (10). The PL stage of BLV infection identifies animals that are at increased risk for developing lymphosarcoma (7), the terminal stage of the disease, and recent studies have shown that susceptibility to B-cell lymphocytosis is under the control of the bovine major histocompatibility complex (11, 12).

The economic impact of BLV infection in the United States has been estimated at 44 million dollars per year, primarily because of slaughterhouse condemnation of lymphosarcomatous carcasses (13). The effects of subclinical BLV infection on milk production and reproductive performance have been investigated but remain largely unknown (14, 15). An important question that has not been addressed is whether differences in genetic potential influence susceptibility to BLV infection and disease progression. Furthermore, previous estimates of the effect of BLV infection on milk production have not considered the effect of advanced subclinical stages of BLV infection, such as PL, on production traits. The objectives ofthis study were to determine the association between breeding values and serological status for BLV infection and progression to PL and to evaluate the effect of BLV infection on milk production, with corrections for genetic differences between animals if warranted. In the herd we studied, cows with high genetic potential for milk production and for fat production were more likely to be seropositive to BLV-gp51 than were herdmates with low genetic potential for these production parameters. Moreover, BLV-infected cows with PL did not reach their genetic potential for fat production and had a significantly lower average milk fat percentage than that of their seronegative herdmates.

MATERIALS AND METHODS Experimental Animals. Data were collected on 219 Holstein-Friesian dairy cows, sired by 50 bulls, that ranged in age from 2 to 10 years. Cows were maintained at research facilities operated by the Department of Animal Sciences, University of Illinois. Cows were bled for detection of serum antibodies to BLV-gpSl and for leukocyte differentials within a 1-month period for 2 consecutive years. Further details on animals used in this study have been described (12, 16). Detection of BLV Infection. Serum samples were tested in duplicate for precipitating antibodies to commercially prepared BLV glycoprotein(s) (Pitman-Moore, Washington Crossing, NJ) by using the standard agar gel immunodiffusion (AGID) test (5). This antigen preparation contains BLV-gp51 and a small amount of the Mr 24,000 viral core protein. The AGID test is the most widely used method for detection of BLV infection (17), although more sensitive assays have been reported (18). False negatives in recently exposed animals are the main source of incorrect classification (18). In the present study, a cow was considered seronegative to BLV-gpSl only if AGID results were negative in consecutive years. Hematology. Leukocyte counts per microliter of whole blood were determined on an automated cell counter Abbreviations: AGID, agar gel immunodiffusion; BLV, bovine leukemia virus; PEBV, pedigree-estimated breeding value; PL,

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persistent lymphocytosis; 305-2X-ME, 305-day twice-daily-milking mature equivalent. *To whom reprint requests should be addressed. 993

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Proc. Natl. Acad. Sci. USA 86 (1989)

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(Coulter). Coverslip blood films were stained with a WrightGiemsa-based stain (American Scientific Products, Ft. Lauderdale, FL), and 200 cells were counted under oil immersion at x 1000 magnification. Hematological status was classified as normal or PL according to the European Community's Leukosis Key (9). Statistical Analysis. Milk, fat, and calving information was obtained from monthly Dairy Herd Improvement Association summaries. Pedigree estimates of breeding values (PEBV) for milk and fat production, the 305-day twicedaily-milking mature equivalents (305-2X-ME) for milk and fat production, the percentage of fat in milk, and the calving interval were matched with hematological data for each cow. The PEBV, based entirely on information of relatives, is an estimate of genetic merit. The PEBV was calculated as the sum of the estimated predicted difference of the sire and the estimated average transmitting ability of the dam and was expressed as the expected deviation (in kilograms) from herdmates. Breeding values, milk production data, and calving interval were analyzed independently by one-way analysis of variance (19) with the knowledge that confidence limits would be approximate, since variances were unequal because amounts of production information on relatives were unequal and individuals were interrelated. The model was: yij

=

g

+

BLVi

+

egg

where Yy was the PEBV for milk, PEBV for fat, 305-2X-ME for milk, 305-2X-ME for fat, fat percentage, or calving interval; ,. represented the general mean; BLVi was the ith group of BLV infection status; and eij was the residual effect. Cow age was also fit in additional models to better interpret the partial confounding between age and BLV infection. The final model for evaluation of the effect of BLV on performance (adjusted for genetic differences) was:

Yyk

=

A

+

BLVi

+

TERMj

+

f3PEBVyk

+

eik,

where Yuk, tt, and BLVi were as defined in the previous model, TERMj reflected whether the lactation record ended with subsequent calving or removal of the animal from the herd; P3 was the regression of yijk on PEBV milk for 3052X-ME milk and PEBV fat for 305-2X-ME fat; and ek was the residual effect. All two-way interactions were tested and were not significant and, thus, not included in the final model.

Each analysis

was squares means were

performed independently, and least compared.

RESULTS Pedigree Estimate of Breeding Value for Milk and Fat in Seropositive and Seronegative Cows. For all cows tested over the 2-year period, 74% (163 of 219) were seropositive to BLV-gp5l, and 18% (30 of 163) ofthese infected cows had PL (Table 1). As expected, prevalence of seropositivity to BLV-gp5l and PL increased with age (1, 16). Genetic potentials for milk and fat production (PEBVs), as estimated from production records of relatives, were compared in cows grouped according to subclinical stage of BLV infection. The mean PEBV for milk production was significantly greater for both groups of BLV-infected cows when compared with seronegative cows (Table 1). The mean PEBV for fat production was significantly greater for cows with PL than for cows without PL. For cows ranked and grouped independently by the PEBV for milk and PEBV for fat, there were trends toward increased frequency of PL within the seropositive cows from low to high PEBV groups (Table 2). When only serological status was considered (either seropositive or seronegative), the presence of antibodies to BLV-gpSl was positively associated with the high-PEBV groups compared with low-PEBV groups for milk production (X2 = 5.12, P < 0.05; relative risk = 2.4) and for fat production (X2 = 3.69, P < 0.06; relative risk = 2.1). Further analysis of the relationship between BLV infection status'and breeding values indicated that animal age at the time of sampling was also a major component of the variation in breeding values (data not shown). PEBVs for milk and fat production increased with age, except in the oldest age group (>5 years). Partitioning by age class and analyzing differences in PEBVs for milk between BLV infection categories resulted in two interesting comparisons. For 2- to 3-yr-olds, seropositive hematologically normal cows (n = 43) had an average PEBV for milk production that was 144 kg greater than the corresponding value of seronegative herdmates (n = 35, P < 0.10). A statistical comparison with the PL group could not be made because there was only one cow with PL in this age group. For the 3- to 4-yr age group, cows with PL (n 11) had an average PEBV for milk production that was 246 kg greater than the average PEBV of seropositive hematologically normal (n = 45) and seronegative (n = 9) =

Table 1. Genetic potential and production traits for the current lactation in cows classified according to BLV-gp5l serology and hematological status

Trait

Seronegative; hematologically normal 56 418 ± 53* 13 ± 2*

Seropositive Hematologically PL normal 30 133 662 ± 72t 554 ± 34t 21 ± 2t 16 ± 1*

No. of cows PEBV for milk, kg PEBV for fat, kg 305-2X-ME, kg 7609 ± 279t 7612 ± 144t 6945 ± 203* Milk production 248 ± 10* 262 ± 5* 255 ± 7* Fat production 3.33 ± O.09t 3.48 + 0.OSt 3.67 ± 0.07* Fat, % Adjusted 305-2X-ME, kg 7498 ± 270*t 7614 ± 139t 7081 ± 198* Milk production§ 243 ± 9* 262 ± 5* 258 ± 7* Fat productions 3.32 ± 0.09t 3.48 ± 0.05t 3.68 ± 0.07* Adjusted fatI % 418 ± 16* 416 ± 7* 410 ± 12* Calving interval, days 4.8 ± 0.2t 3.9 ± O.lt 3.2 ± 0.2* Age, yr < values are All 0.05). (P different significantly are with symbols row different *ttValues in the same expressed as least-squares means ± SEM. §Adjusted by the PEBV for milk. lAdjusted by the PEBV for fat.

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Table 2. Association between genetic potential for milk and fat production and seroreactivity to BLV-gp5l and PL

Seronegative; hematologically Group* PEBV for milk production High Medium Low PEBV for fat production

PEBVt

normalt

(979 ± 20) (530 ± 20) (92 ± 20)

13 (18%) 18 (25%) 25 (34%)

Seropositive Hematologically PL§ normaFt 46 (63%) 46 (63%) 41 (56%)

14 (23%) 9 (16%) 7 (15%)

14 (23%) 46 (63%) 13 (18%) (30 ± 1) High 10 (14%) 43 (59%) 20 (27%) (16 ± 1) Medium 6 (12%) 44 (601%) 23 (32%) (2 ± 1) Low *Cows were stratified into three equal-sized groups of 73 after ranking by PEBV. The group names "high," "medium," and "low" are thus relative to the genetic profile of this herd. tLeast-squares means ± SEM. SNumbers in the column are actual numbers of individuals in PEBV groups. Numbers in parentheses in the column are the percentages of cows in high, medium, or low PEBV groups. §Numbers in the column are actual numbers of individuals in PEBV groups. Numbers in parentheses in the column are the percentages of seropositive cows with PL in each of the PEBV groups.

(P < 0.06). For older age groups (4-5 and >5 yr), seropositive cows always had a numerically greater average PEBV for milk than seronegative herdmates had, but standard errors were large because of small class sizes, and differences were not significant. Consistent trends between age groups were also found for the PEBV for fat production. In 3- to 4-yr-old cows, the average PEBV. for fat was 8.6 kg greater for cows with PL than for seropositive hematologically normal and seronegative herdmates (P < 0.05). In older age groups, seropositive cows always had greater average PEBVs for fat than seronegative herdmates had, but sample sizes were too small to detect a significant difference. Thus, trends in PEBVs within an age group were consistent with results pooled across all age groups (Table 1). These results show that cows with superior genetic potential for milk and fat yields were more susceptible to subclinical progression of BLV infection. Effect of BLV Infection on Milk and Fat Production. For the current lactation, 305-2X-ME milk production was significantly greater in both groups of BLV-infected cows when compared with seronegative herdmates (Table 1). These results were generally consistent with what was predicted by the PEBVs. The 305-2X-ME milk production for cows with PL was nearly identical to that for other seropositive herdmates, although a tendency towards greater milk production was expected based on PEBVs. The 305-2X-ME fat production for both groups of BLV-infected cows was not different from that of their seronegative herdmates. The lack of a significant difference in 305-2X-ME fat production for BLVinfected cows was counter to what was expected based on PEBVs. These results show that cows with PL did not produce fat according to their genetic potential. The combination of trends for greater milk yield and less fat for both groups of BLV-infected cows contributed to significantly lower milk fat percentages for BLV-infected cows relative to seronegative herdmates (reductions of 0.19% and 0.34%, respectively, for seropositive hematologically normal and PL cows

cows).

Adjustment of Lactation Records for Genetic Potential. When lactation records were adjusted for PEBVs for milk, BLV-infected hematologically normal cows had significantly greater 305-2X-ME milk production for the current lactation than did seronegative herdmates (Table 1). There was also a trend toward increased milk production in cows with PL relative to seronegative herdmates (P < 0.12). These results suggest a positive effect of early BLV infection on milk yield. After adjustment for PEBVs for fat production, cows with PL tended to produce less fat on the average compared with their

seronegative herdmates. However, fat percentage was significantly lower for both groups of BLV-infected cows when compared with seronegative herdmates. These results suggest a significant change in milk composition as an effect of subclinical progression of BLV infection. Effect of BLV Infection on Calving Interval. There was no effect ofBLV infection on calving interval. Hence, the effects of BLV infection on 305-2X-ME milk and fat production were not confounded by pregnancy status.

DISCUSSION Genetic potentials for milk and fat production (PEBVs) were compared in cows grouped according.to subclinical stage of BLV infection. The AGI1D test for antibodies to BLV-gpS1 was used to measure exposure to BLV, and lymphocyte counts provided a means of identifying cows with advanced subclinical BLV infection (7). Statistical analyses demonstrated that cows with superior genetic merit were more likely to be seropositive (Table 2) and that BLV-infected cows were, on the average, genetically superior to their herdmates (Table 1). One interpretation of these results is that the physiological stress demanded by high genetic potential for milk production (20) is an important factor in determining susceptibility to BLV infection and progression to PL. Studies of human patients infected with human immunodeficiency virus strongly suggest that genetic factors (21) and physiological stress (22) influence the onset of clinical disease. The effects of stress on the immune system are known to be generally immunosuppressive (23), and susceptibility to PL probably has an immunological basis. Recent studies documenting an important role for the bovine major histocompatibility complex in seroconversion and susceptibility to PL support this hypothesis (11, 12). Hence, the BLV model might yield important information for our understanding of the role of physiological and genetic factors in the pathogenesis of retrovirus infections in outbred species such as man. Whether the low frequency of seropositivity in cows with low genetic potential is indicative of true genetic resistance to BLV infection or merely delayed seroconversion because of lower physiological stress requires a prospective study of seroconversion in cows with low genetic potential. The most dramatic effect of BLV infection on production parameters was in cows with PL. Although the PEBV for fat production was highest for cows with PL, 305-2X-ME fat production was lowest. These results show that cows with PL did not attain their genetic potential for fat production in their

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most recent lactation. A decline in mammary fat synthesis might be a result of altered energy balance from increased demands for more energy to maintain the lymphocytotic state. Milk fat percentage, expected to be constant across all groups, was significantly reduced in both groups of BLVinfected cows as compared with seronegative herdmates. In cows with PL, reduced milk fat percentage was clearly a consequence of a tendency toward more milk and lower fat production. In seropositive cows that were hematologically normal, decreased fat percentage seemed to be more a function of increased fluid milk than an absolute decline in fat synthesis because PEBV-adjusted milk production was significantly increased while PEBV-adjusted fat production was approximately equal, as compared with values for seronegative herdmates. Consistent with previous studies (14, 15), reproductive performance, as assessed by calving interval, was found not to be affected by BLV infection. However, these earlier studies did not detect significant effects of BLV infection on milk production traits. Importantly, cows with PL were not separated from other seropositive cows in previous studies. Hence, cows classified as "seropositive" must have included all exposed animals, from those with very recent infections to those with PL and tumors. Furthermore, these investigators determined the effects of BLV infection on milk production traits by adjusting milk production records to "3.5% fatcorrected milk;" thus, fat production and fat percentage were not directly analyzed. In the present study, seropositive cows were subgrouped into those with PL and those that were hematologically normal, and differences in fat production and fat percentage in these groups were examined. These essential differences in experimental design and data analysis might explain differences in results and conclusions drawn from our study and previous endeavors. If confirmed in other herds, our findings would have two important practical consequences. (i) BLV might cause greater economic liability to dairy producers than previously estimated (13) because of the high prevalence of BLV infection in U.S. dairy herds (24) and the decreased percentage of milk fat in cows with PL. The higher prevalence of BLV infection among genetically superior animals suggests that a greater economic impact may be felt in elite herds. Furthermore, production in herds with older animals would be most affected by BLV, because PL develops primarily in cows 3 years of age and above (7). (ii) Sire evaluations in herds that have a high prevalence of BLV infection and PL would be less accurate because BLV infection introduces a significant amount of environmental variation in milk production traits, and not all cows within a contemporary group are BLV-infected. Heritability estimates for milk production parameters could also be affected by prevalence of BLV infection. Study of additional herds is clearly warranted, and a reevaluation of the impact of BLV infection on the dairy industry may be necessary. We thank Dr. J. Weller for helpful suggestions on the data analysis and J. Beever, G. McCoy, T. Nolan, and J. Stewart for technical

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