The ability of mammary tissue from prepubertal bulls and heifers of beef and dairy breeds to respond to hormonal stimuli through synthesis and secretion of milk proteins was studied. Experimental animals were six to eight month old Angus and Holstein cattle. All subjects were injected with estradiol and progesterone for seven days and slaughtered on day 15. Mammary tissue was explanted and cultured for 96 h in basal medium (B) which contained hormones necessary for maintenance, or stimulatory medium (P), further supplemented with prolactin. Selected cultures were incubated for 24 h in B or P medium containing 3H-amino acids. Concentrations of non-labeled alpha-lactalbumin (Alac), 3H-Alac, and 3H-total protein (TP) were determined in media and in explant homogenates. Among cultures of bull mammary tissue, Angus explants secreted greater overall quantities of 3 H-TP and 3H-Alac than Holstein explants (p<.05). Secretion of Alac was also greater in Angus cultures at two of eight treatment periods (p<.01). Concentrations of all three protein fractions were likewise enhanced in homogenates of Angus explants for at least three of four treatment periods (p≤.05). Presence of prolactin in medium stimulated secretion of Alac (p<.005), and accumulation of all three fractions in explants (p<.10). Holstein heifer explants secreted more Alac at three of eight treatment periods than Angus explants (p<.0005). Overall secretion of ³H-TP and ³H-Alac also was elevated in Holstein over Angus females (p<.10), as were concentrations of all three fractions in homogenates (p≤.01). Presence of prolactin had no direct effect on any protein parameters in female tissue. I conclude that mammary tissue of immature bulls and heifers can be hormonally induced to express it's genetic merit for milk production (based on breed differences), through synthesis and secretion of milk proteins. Prolactin stimulated protein production in bulls but not in heifers. These findings indicate that similar methods of stimulating mammary tissue to produce milk proteins may be adaptable for commercial evaluation of genetic potential for milk production, especially in young bulls. / M.S.

Forty-two high producing Holstein cows were paired by body condition and mature equivalent milk production and fed either a high or low energy complete ration ad libitum during the last 16-20 weeks of lactation. Cows fed the high energy ration ate more feed, produced more milk and gained more body condition than cows fed the low energy ration. Cows were fed to maintain condition during the dry period. During weeks 3-15 of the next lactation, half the cows in each condition group (fat or thin) were fed a mixed ration with soybean meal (SBM) as the major protein source. The remaining cows were fed a ration with dried brewers grains (DBG) as the main protein source. / Master of Science

Reproductive data from 30 research herds were on 31,132 breeding periods of 11,347 dairy cows. Cows were sired by 1,101 sires and had 66,184 services to 1,320 service sires. Several measures of reproductive pe.rformance were calculated. These included conception rate, number of services, service period length, days open, age at first breeding, calving interval, days between services, and return to estrus lag. First, second and third service period were each analyzed separately, while fourth and later service periods were pooled. Heritability was estimated using the sire component of variance and the estimate of the total variance derived from MIVQUEO and maximum likelihood analyses. The data set was restricted to daughters of sires used in multiple herds. Heritability estimates were less than .07 for all traits in the heifer service period except age at first breeding (.2 by maximum likelihood and .13 by MIVQUEO). Similarly, with the exception of conception rate, none of the measures of reproduction had heritabilities greater than .05 for all three remaining service period groups. Conception rate measured as a trait of the male (service sire) ranged from .08 to .135 for second and third service periods. Conception rate as female trait (sire) had heritabilities ranging from .09 to .249 for second and third service periods. Low heritability estimates obtained in this and other studies suggest that large progeny or service sire groups will be necessary to identify the small genetic differences between bulls. Many genetic and phenotypic correlations were forced positive due to a part-whole relationship or due to the fact they were simply different bounds for the same measure. A few correlations were in the range from .50 to .90, but many were not significantly different from zero due to large approximate standard errors. Repeatabilities based upon pairwise comparisons were in the range from 0 to .13. Repeatabilities for the reproductive performance of virgin heifers with first parity ranged from .01 to .06 and were generally smaller than later parities. Repeatabilities based upon repeated measures on the same cow ranged from 0 to .12. Predicted Differences for female (sire) and male (service sire) reproduction were calculated by Best Linear Unbiased Prediction. This analysis included 207 bulls which were in the data both as sire and service sire. Correlations between proofs for male and female reproduction ranged from -.13 to .13. These results suggest limited genetic relationships between male and female fertility. / Ph. D.