Later Stages Of Lactation Research Articles

Effects of Controlled Compensatory Growth on Mammary Gland Development and Lactation in Rats

The objective of this study was to examine the effect of compensatory growth nutritional regimen on mammary gland growth and lactation. One hundred twenty-two Sprague Dawley female rats (35 days of age) were randomly assigned to either a control or a stair-step compensatory nutrition (SSCN) feeding regimen or an alternating 2-2-3-3-week schedule beginning with 40% energy restriction for 2 weeks followed by re-alimentation (control diet) for 2 weeks. Pup weight gain and milk yield were improved 8% and 8 to 15%, respectively, by the SSCN regimen. The gene expression of β-casein was 2.3-fold greater in the SSCN group than in the control group during early lactation, but they were greater at all stages of the second lactation. The gene expression of insulin-like growth factor-1 was 40% lower in the SSCN group than in the control group during early lactation of the second lactation, but during late lactation it was 80% greater than in the control group. The concentration of serum corticosterone tended to be higher in the SSCN group during the late stage of the first lactation. These results suggest that the stair-step compensatory nutrition regimen improves lactation performance and persistency by modulation of cell differentiation and apoptotic cell death.

Milk ejection in dairy cows at different degrees of udder filling.

Occurrence of milk ejection and course of milk removal were investigated in 18 dairy cows at milking intervals of 4, 8 and 12 h in early, mid or late lactation. Milk ejection occurred fastest in early lactation at a milking interval of 12 h and was delayed at short milking intervals and in late lactation. Storage capacity of the udder was estimated and the actual milk yields of experimental milkings were calculated as a percentage of storage capacity, i.e. degree of udder filling. It was shown that the occurrence of milk ejection after the start of teat stimulation is a function of udder filling. The relationship between the degree of udder filling and the delay from the start of milking until commencement of milk ejection followed a linear regression curve. Changes in occurrence and course of milk ejection have to be considered in practical milking, mainly in late stages of lactation and after short milking intervals. In automatic milking systems where variable and sometimes extremely short milking intervals occur, the duration of pre-milking udder preparation should be adapted to the expected milk yield at each individual milking procedure.

Computing Mating Bull Fertility from DHI Nonreturn Data

Animal model methodology was used to compute yearly measures of relative fertility of Holstein AI mating bulls based upon 70-d nonreturn of first breedings as reported to U.S. DHIA from 1988 through 1997. Estimated Relative Conception Rates (ERCR) were computed for bulls with a minimum of 50 first breedings in a single year using variance ratios 45.5 for mating bull, 45.5 for animal genetic effects, and 31 for permanent environment. The model assumed repeatability across lactations of 0.05 and included fixed effects of herd-year-month bred and classes of parity, early lactation energy-corrected milk and days open when bred. Estimates of fertility were greater for breedings to cows that were young, had low early lactation production, and were in late stages of lactation. ERCR were expressed as difference in nonreturn from the average AI mating bull of herdmates. Values ranged from –18 to +13. For ERCR computed from a minimum of 1000 breedings, 90% were within four units of zero. Early ERCR computed from a few breedings in a single year were tested for ability to predict later ERCR computed from a minimum of 1000 different breedings. Early ERCR computed from 300 or more matings accurately predicted later independent ERCR. For yearly estimates each based upon a minimum of 1000 breedings, 8% changed more than three units, and 4% declined more than three units. Correlations between ERCR and predicted transmitting abilities protein and type production index were significant but accounted for little variance. Correlations between ERCR and other traits were not significant.

Alternative housing for sows and litters: 2. Effects of a communal piglet area on pre- and post-weaning behaviour and performance

We conducted an experiment to determine how allowing piglets access to a communal area affects pre-weaning and post-weaning behaviour and performance. Litters were housed in pens with sows in crates until piglets reached 11 days of age. For 10 experimental cohorts, barriers between the pens of three litters were removed allowing piglets to mingle freely in a central area. For the 10 control cohorts (also three litters each), litters were not allowed to mix before weaning. There was little aggression when experimental litters were allowed to mix, and piglets played and rested in mixed-litter groups. Time spent in the common area increased from approximately 20% just after mixing (12 days old) to approximately 45% at day 26 (week 4; P<0.001). Experimental piglets were nursed less often than controls, especially during the later stages of lactation ( P<0.01). However, there was no difference in the amount of creep feed consumed or rate of weight gain before weaning. After weaning, experimental piglets ate more solid food than controls (approximately 70 g/day more during the second week after weaning, P<0.05), and showed a non-significant tendency to gain more weight (approximately 50 g/day). As a second comparison, piglets were either kept in single litter groups after weaning, or mixed with other litters from the same cohort. When control litters were mixed after weaning, piglets spent almost 60% more time in aggressive behaviour than when litter-mates were kept together, but no such difference occurred for the experimental litters (interaction P<0.05). Thus, the experimental housing treatment allows for mixing at an early age (much as occurs in nature) with little apparent conflict, provides piglets with a socially enriched pre-weaning environment, and helps avoid fighting between mixed piglets at weaning. The system also slightly lowers demands on the sow during lactation (in terms of a reduced nursing frequency) and slightly increases intake of solid food by piglets after weaning.

Expression of growth hormone receptor 1A messenger ribonucleic acid in liver of dairy cows during lactation and after administration of recombinant bovine somatotropin.

The mRNA for growth hormone receptor is transcribed from at least three different promoters in cattle. The first promoter (P1) is liver-specific and transcribes growth hormone receptor mRNA containing exon 1A (growth hormone receptor 1A). The second and third promoters (P2 and P3) are active in a variety of tissues and transcribe growth hormone receptor mRNA containing exon 1B and 1C. The objective was to characterize P1 activity by measuring the amount of growth hormone receptor 1A mRNA in liver of dairy cows at different stages of lactation as well as after administration of recombinant bovine somatotropin (rbST). In study 1, liver RNA was isolated from Holstein cows during the dry period (nonlactating, n = 6) and during early (n = 6), mid (n = 6), and late (n = 11) stages of lactation. Six of the late-lactation cows received injections of rbST (25 mg/d) for 7 d prior to collection of liver tissue. In study 2, lactating Holstein cows received either no infusion (control, n = 10) or continuous infusion of rbST (29 mg/d, n = 10) for 63 d. The amount of growth hormone receptor 1A mRNA was decreased in early- and mid-lactation cows compared with late-lactation cows or nonlactating cows (study 1). Administration of rbST increased growth hormone receptor 1A mRNA (studies 1 and 2). The total amount of growth hormone receptor transcribed from alternative promoters (growth hormone receptor P2 and P3) remained unchanged during different stages of lactation or in response to rbST. We conclude that changes in liver growth hormone receptor mRNA in lactating dairy cattle primarily depend on growth hormone receptor P1 activity.

Lactation-dependent down regulation of leptin production in mouse mammary gland

Lactation-dependent regulation of leptin expression in mouse mammary gland and parametrial adipose tissue was estimated by RT-PCR analysis for virgin, pregnant, lactating and post-lactating mice, and the serum and milk leptin levels of these mice were also determined by ELISA. Leptin gene expression in mammary gland as well as in adipose tissue was obviously detected before pregnancy, markedly decreased to 30–50% after parturition and kept at the low level during lactation period, and restored to the original level after weaning. The leptin concentration of milk collected just before weaning was about two-fold higher than that of the milk collected at mid-lactating stages. The serum leptin levels of the mid- and late-lactating mice were not significantly higher than those of non-pregnant mice. These results suggested that the lactation-induced down regulation of leptin was associated with autocrine/paracrine action of leptin in mammary and adipose tissues, and that the milk leptin, especially at the latter stages of lactation, was not only ascribed to diffusive transport from maternal blood stream, but also regional production and secretion by mammary epithelial cells. This possible production of leptin by mammary epithelial cells was further supported by the fact that leptin was expressed by cultured cells of mammary epithelial cell line, COMMA-1D, in a manner negatively dependent on the lactogenic hormones.

Two-generation reproductive toxicity study of plant stanol esters in rats.

Plant stanol esters are intended for use as an ingredient in food to reduce the absorption of cholesterol from the gastrointestinal tract. Consumption of plant stanol esters has a demonstrated diet-derived public health benefit, as shown by numerous clinical studies. Plant stanol esters are ring-saturated analogs of common dietary sterols that are transesterified with fatty acids from vegetable oils such as canola oil. The reproductive and developmental toxicity of plant stanol esters was investigated in male and female Wistar rats during F0 and F1 generations using dietary concentrations of 1.75, 4.38, and 8.76% stanol esters (equivalent to 1, 2.5, and 5% total stanols). No adverse treatment-related effects were noted on reproductive performance of male or female rats in any dose group. Increased food consumption was observed in high-dose F0 generation males throughout the entire premating period and in F1 males at specific time periods during the premating period. This increase in food consumption was also observed in F0 generation females (mid- and high-dose groups) and F1 generation females (low-, mid-, and high-dose groups) at specific time periods throughout the 10-week premating period. At different intervals throughout the gestation and lactation periods, increased food consumption was observed in F0 generation females of the mid- and high-dose groups, while increased food consumption was noted in F1 generation females of the mid- and high-dose groups during gestation, but not during lactation. Such increases in food consumption are expected as a result of the animals' attempt to compensate for the reduced caloric value of the test diet compared to controls. No adverse developmental effects were noted in F1 or F2 pups of the low- and mid-dose groups based on evaluation of the following parameters: litter size, pup mortality, pups weights, and sex ratio. However, a treatment-related effect on body weight and body weight change was observed in both F1 and F2 male and female pups of the high-dose group, particularly during the latter stages of lactation (postnatal days 14 and 21) in F1 pups, and during the majority of the lactation period (postnatal days 4-21). Lower body weight in the high-dose pups is attributed to a reduction in the caloric value of the test diet compared to control. The pups, unlike adult animals, are particularly sensitive to reductions in caloric value of feed since they are in a rapid growth phase of their development. It is likely that they could not increase their food consumption enough to adequately meet their caloric and nutritional needs. In conclusion, dietary concentrations of up to 4.38% plant stanol esters (equivalent to 2.5% total stanols in the diet) are not associated with adverse effects on reproduction, pup mortality, pup body weight, or pup body weight change.

Effect of pregnancy and lactation on lipoprotein and cholesterol metabolism in the rat

Origins of hyperlipidemia and cholestasis that occur during pregnancy were investigated by examining expression of key elements related to plasma and hepatic cholesterol metabolism during pregnancy, lactation, and post-lactation in the rat model. Among major findings were: during pregnancy, the activities of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase, acyl coenzyme A:cholesterol acyltransferase, acyl coenzyme A:diacylglycerol acyltransferase, cholesterol 7α-hydroxylase, cholesterol ester hydrolases, low density lipoprotein receptors, LRP, and mdr2 were significantly lower or similar to non-pregnant controls while SR-B1 was elevated. Once lactation began, reductase, cholesterol acyltransferase, 7α-hydroxylase activities, low density lipoprotein receptors, and mdr2 increased while SR-B1 decreased. In later stages of lactation most hepatic elements returned to near control levels. Plasma cholesterol levels were higher than control at birth and during lactation with increase in LDL-size particles. By 24 h post-lactation, plasma triglycerides were 3.7-fold higher while cholesterol remained unchanged. Very large lipoproteins were present while LDL-size particles were now absent. Hepatic cholesterol acyltransferase had decreased to 27“% of control while diacylglycerol acyltransferase increased 3-fold and low density lipoprotein receptors doubled. Most elements were normalized 3 weeks after weaning except for LRP and low density lipoprotein receptors which were elevated. These studies provide an integrated picture of expression of key elements of hepatic and plasma cholesterol metabolism during pregnancy and lactation and advance understanding of hyperlipidemia and cholestasis during these states. —Smith, J. L., S. R. Lear, T. M. Forte, W. Ko, M. Massimi, and S. K. Erickson. Effect of pregnancy and lactation on lipoprotein and cholesterol metabolism in the rat. J. Lipid Res. 1998. 39: 2237–2249.

Comparative organophosphate-induced effects observed in adult and neonatal sprague–dawley rats during the conduct of multigeneration toxicity studies

Five organophosphates: tribufos, oxydemeton-methyl, fenamiphos, coumaphos, and trichlorfon were evaluated for their potential to produce reproductive and neonatal toxicity following continuous dietary exposure during multigenerational reproduction toxicity studies in the Sprague–Dawley rat. Dietary concentrations were selected to demonstrate parental effects in the high dose and provide for a no-adverse effect level at the low dose. There were no clinical signs observed in the adults or neonates during either generation. Significant effects on body weight and food consumption, when observed, were typically observed only with the highest dietary concentration and were greater in the second generation. Reproductive effects, including decreased fertility and mating indices, were only observed with test compounds and at dietary concentrations demonstrating effects on body weight and/or food consumption. Similarly, pup body weight was also affected by those test compounds that produced significant maternal effects during lactation. Significant inhibition of parental cholinesterase activities (plasma, erythrocyte, and brain) was similarly observed in both generations with all test compounds, with at least the highest concentrations. In general, females demonstrated greater enzyme inhibition than the males. For example, mean PChe inhibition considering both generations and all test compounds was 74% for the females, whereas inhibition was 51% in the males. Effects on cholinesterase activities in the neonates (Lactation Day 4) were, for most test compounds, below 10% at the highest dietary concentration. However, by Lactation Day 21, inhibition of enzyme activity (considering all test compounds at the highest concentration and all enzymes) was approximately 30%. The increase in inhibition is attributed to the consumption of the treated feed during the latter stages of lactation. Considering the relative maternal (termination) and neonatal (Lactation Day 4) cholinesterase effects at the highest dietary concentration, it was observed that the effects in the neonate were, for all organophosphates tested, significantly less than those observed in the dam.

Expression of the genes encoding CCAAT-enhancer binding protein isoforms in the mouse mammary gland during lactation and involution.

Transcription factors belonging to the CCAAT-enhancer binding protein (C/EBP) family have been implicated in the activation of gene expression in the mammary gland during lactation. We have therefore investigated the detailed expression profile of the C/EBP family during lactation and involution of the mouse mammary gland. The expression of C/EBPbeta and C/EBPdelta mRNA was low during lactation, increased dramatically at the beginning of involution and remained constant thereafter. In contrast, C/EBPalpha mRNA expression was relatively high during the early stages of lactation, declined to low levels during the late stages of lactation and at the start of involution, and increased again during involution. Electrophoretic mobility-shift assays showed a close correlation between the expression of the C/EBP genes and the functional C/EBP DNA-binding activity and, additionally, demonstrated the participation of heterodimers, formed from among the three proteins, in DNA-protein interactions. The DNA-binding activity of the activator protein 1 (AP1) family of transcription factors was also induced during involution. These results therefore point to potentially important regulatory roles for both the C/EBP and the AP1 family during lactation and involution of the mammary gland.

Somatic Cell Score as Predictor of Daily Milk Yield in Holsteins

The purpose of our Study was to determine if variation in milk yield at later Stages of lactation can be explained by expressions of early lactation somatic cell score (SCS) and if the prediction of future yield within lactation can be improved by including SCS among the predictors. Three data sets (n &gt;600.000 each) were: milk yield with sample days near 20, 50 and 140. Stepwise regression was used requiring F statistic (P &lt; .01) for any SCS variable to stay in the model. Separate analyses were run for 12 combinations of four seasons and first three parities for each data set. Selection of SCS variables was not consistent across seasons or parities. Coefficients of determination (R2) ranged from 54 to 74% with higher values for higher days in milk (DIM) and earlier lactations. The inclusion of SCS expressions in the prediction equations improved R2 by &lt; 1% . SCS was associated with milk yield on sample day, but the association was not strong enough to improve the prediction of future yield when other expressions of milk yield were taken into account.

Physiologic-chemoattractant-induced migration of polymorphonuclear leukocytes in milk.

The somatic cell count (SCC; leukocytes and epithelial cells) in milk is used as an indicator of udder health status. A SCC above the regulatory standard is generally considered as an indication of mastitis. Therefore, milk with a SCC equal to or greater than the regulatory limit cannot be sold to the public because it is unsuitable for human consumption. This study was performed to determine whether SCC levels above the regulatory limit observed in goats during late lactation are a physiologic or a pathological response of the goat mammary gland. Differential counts of cells in nonmastitic goat milk samples during late lactation revealed that approximately 80% of the cells were polymorphonuclear leukocytes (PMNs). In addition, microchemotaxis assay results indicated that normal nonmastitic late-lactation-stage goat milk is significantly higher (P < 0.001) in PMN chemotactic activity than early-lactation-stage goat milk, with a mean chemotactic activity of 14.9 and 42.7/mg of protein for early and late lactation stages, respectively. Physicochemical analyses also suggest that the PMN infiltration observed in normal late-lactation-stage goat milk is due to a PMN chemotactic factor(s) that is different from the PMN chemotactic factor(s) present in mastitic milk. Interestingly, the PMN chemotactic factor in late-lactation-stage goat milk is highly acid resistant (pH 2), suggesting that the factor is able to survive the highly acidic gastric environment and may therefore be important in the augmentation of the immune systems of sucklings. These results indicate that the chemotactic factor(s) present in the milk of normal late-lactation-stage goats is nonpathological and may play a physiologic regulatory role in mammary gland involution. Hence, the regulatory standard for goat milk needs to be redefined in order to reflect this.

Thyroxine, Triiodothyronine and Reverse-Triiodothyronine Concentrations in Blood Plasma in Relation to Lactational Stage, Milk Yield, Energy and Dietary Protein Intake in Estonian Dairy Cows

Average levels of thyroxine (T4), triiodothyronine (T3) and reverse-triiodothyronine (rT3) in blood plasma of 159 Estonian Red and Estonian Black and White cows were 55.2 nmol/L, 1.78 nmol/L and 0.25 nmol/L respectively. Animals were grouped according to stage of lactation. The T4 level was significantly lower during the early stage of lactation (45.1 nmol/L), compared with later stages, but increased as the stage of lactation progressed (late stage of lactation - 56.7, dry cows 64.3 nmol/L). The T3 level was significantly higher at the late stage of lactation (1.93 nmol/L) compared with the early stage of lactation (1.71 nmol/L) and level in dry cows (1.71 nmol/L). rT3 showed a trend similar to that found for T4 (lowest plasma concentration in early lactation, 0.19; highest in the dry period, 0.33; late lactation, 0.24 nmol/L). Levels of all thyroid hormones were negatively related to the daily milk yield (T4-r = -.51, rT3-r = -.47, calculated thyroid index as rT3 x T3 x T4-r = -.52, for all p < .0001; T3-r = -.32, p < .01). Plasma thyroid hormone concentrations were affected by energy and dietary protein intake. Differences were found in thyroid hormone levels between the 2 breeds and between summer and winter holding periods for Estonian Red cows, which could all be explained by differences in the feeding level and daily milk yield. It is suggested that maintaining low levels of thyroid hormones in early lactation may be one of the dairy cow's mechanisms of reducing metabolic demand.

5. Differences in β-lactoglobulin concentrations between morning and afternoon milkings at early, mid and late stages of lactation

5. Differences in β-lactoglobulin concentrations between morning and afternoon milkings at early, mid and late stages of lactation

Changes of gangliosides and sialic acid in human milk during lactation

To elucidate the possible biological function of gangliosides in human milk, the content and composition of gangliosides in human milk during lactation were examined. Gangliosides were rich in human milk at the earlier stages of lactation. Themajor ganglioside was GD3. The content of GD3 decreased, while that of GM3 increased in the later stages of lactation. GDla, and poiysialogangliosides were rich in the milk at 6 days of postpartum. Levels of sialic acid and sialidase, a ganglioside-hydrolyzing enzyme, were also studied. The content of sialic acid and sialidase activity were decreased in the later stages of lactation. As the results, GD3, GDIa, polysialogangliosides, sialic acids (lipid-bound and protein-bound), and sialidase were rich in human colostrum and these components may play a role in immune system.

Mammary Growth and Regression -Regulation of Milk Synthesis-

The mammary gland is an unique organ present only in the mammal. The mammary gland, grown and differentiated during pregnancy, begins to synthesize milk following parturition. The lactating mother continues actively to supply milk to the young for a certain period, but milk synthesis terminates at the late stage of lactation. In various species, the changing profile of milk production is known as the lactation curve. The increase or decrease in milk production has merits for both the young and the mother. At the termination of milk synthesis, the mammary gland undergoes regression and most mammary epithelial cells disappear from the gland through the process of apoptosis. The mammary gland returns the state identical to that before pregnancy. It is well known that prolactin regulates mammary gland functions; growth, differentiation, milk synthesis and regression. The mammary gland modulates sensitivity to prolactin by changing the number of prolactin receptors of the mammary epithelial cell. As described here, milk production is regulated primarily by the level of the prolactin receptor gene expression. It is believed that as a result of the law of the survival of the fittest, the mammary gland acquired the ability of regulating its own functions by modulating the prolactin receptor gene expression. Milk production of dairy cattle has been improved during the last two centuries. By manipulating prolactin receptor gene expression, it will be possible to further improve milk production in dairy cattle.

Effects of abomasal infusions of sodium caseinate and of casein hydrolysates varying in the relative proportions of peptides and free amino acids on milk production in dairy cows.

Two experiments were conducted to test the hypothesis that responses of milk production to the abomasal infusion of casein or hydrolysates of casein can be influenced by the proportion of peptide-bound amino acids in the infusate. Eight lactating cows were used in two Latin square experiments with period lengths of 10 d. In Expt 1, the four cows were at a late stage of lactation and the four treatments were a basal diet of silage and a barley-soya supplement alone or with infusions into the abomasum of 180 g/d of sodium caseinate, an enzymic hydrolysate of casein or an acid hydrolysate of casein; all treatments supplied equivalent amounts of all the amino acids, this being achieved by addition of free amino acids as required. As infused, the three treatments contained 89 (caseinate), 40 (enzymic hydrolysate) and 15 (acid hydrolysate) % of their amino acids as peptides. The increase of milk production in response to infusion was small and there were no significant differences between the infusion treatments. In Expt 2, the cows received a basal diet of grass silage and a supplement containing feather meal as the main source of protein. Treatments were the basal diet alone and with three infusion treatments as in Expt 1, except that the amount infused was 230 g/d. The response to infusion was greater in this experiment, there were significant differences between infusions and the yield of milk protein with the infusion treatments was linearly related (P = 0.001) to the proportion of peptide-bound amino acids in the infusate. Moreover, there was a close inverse relation (P = 0.007) between the proportion of peptide-bound amino acids in the infusate and the concentration of total peptide-bound amino acids in blood plasma.

Effect of parity and milk production on somatic cell count, standard plate count and composition of goat milk

Fifteen milking does were randomly selected from the Alpine herd of the Langston University farm, with five each in Parities 1, 2, and 3 (P1, P2 and P3, respectively). Composite milk samples of equal volume of morning and evening milkings were taken once a month throughout the lactation period of 8 months. All samples were analyzed for somatic cell count (SCC), standard plate count (SPC), and chemical composition (percent fat, percent protein, percent lactose, and percent solids non-fat (SNF)). Milk production was recorded daily. All experimental does kidded in March and were dried off in October. No symptoms of mastitis were observed during this study. Overall mean SCC, SPC, percent fat, percent protein, percent lactose, percent SNF, and milk production were 1.2 × 10 6 cells ml −1, 9.5 × 10 3 colony forming units (cfu) per ml, 3.73%, 2.80%,4.42%, 7.85%, and 2.69 kg per doe per day, respectively. Parity of does did not affect the variables significantly except for milk production. Does of P1 produced less milk than those of P2 and P3 ( P < 0.05). Goat milk samples contained high SCC and exceeded the 1 × 10 6 ml −1 requirement of the Pasteurized Milk Ordinance (PMO) during most of the late lactation stages, although mastitic conditions were absent in any experimental goats during the lactation period. Histological and pathological tests on fresh tissues of udder halves revealed no changes in mammary gland change or other mastitic evidence. High SCC in goat milk appeared to be natural, particularly in the late stages of lactation. Goat milk showed low SPC, indicating high microbial quality. SCC had a positive relationship with SPC (0.44) and was negatively correlated with milk production (0.46) and fat content (0.41). As the lactation progressed, SCC and SPC increased ( P < 0.001) and milk production decreased ( P < 0.001). SPC was negatively correlated with percent fat, percent protein, percent SNF and milk production.

Effect of Disease on Length of Productive Life of French Holstein Dairy Cows Assessed by Survival Analysis

Data from a survey performed from 1986 to 1990 were analyzed to assess the effects of diseases on length of productive life of 3589 Holstein cows from 47 herds, using a proportional hazard model.The probability of a cow being culled, or hazard function, was supposed to be the product of an unspecified baseline hazard function and log-linear, time-dependent explanatory variables that possibly influence culling rate (Cox's regression). The effect of 16 health events was studied according to lactation number of occurrence. The model included adjustments for effects of herd-year-season (considered to be random), month of calving, stage of lactation, lactation number, reproductive performance, and milk production.The probability of a cow being culled increased in early and late stages of lactation in older cows, in low producing cows, and in cows with poor reproductive performance. Mastitis before the peak of lactation or during the dry period increased the rick (relative culling rate in first lactation, 1.3 and 4.0, respectively). Teat injuries and nontraumatic udder disorders had a large impact on longevity. Cows with late metritis or early abortion had poor survival. The decrease in median length of productive life could be over a standard lactation in particular cases.Expected survivor curves, computed after assumption of a priori values of covariates and their evolution over time, appear to be powerful tools for examining the effect of health disorders on length of productive life of cows.

Prediction of Cow Performance with a Connectionist Model

Since the main reason for disposal of dairy cows is low milk yield, implementation of an optimum selection program requires the prediction of cow performance with regard to production. The prediction of fat and protein content in milk are also rapidly becoming important factors for decisions related to breeding and herd policy. While, on average, traditional lactation models furnish good results, some improvement is possible when predicting the yield for an individual cow early in lactation. Artificial neural networks (ANNs), known to perform well in pattern recognition, may constitute an effective alternative to the traditional models. The objective of this research was to investigate how ANNs might be used to predict total milk, fat, and protein production for individual cows. Results indicated that ANNs generally performed at least as well overall as the model currently used by Canadian milk recording agencies, especially in the first third of lactation. This has important implications for early identification of superior animals. Predictions from both methods were relatively similar for the later stages of lactation. The addition of nontraditional data inputs such as average milk herd production and weight of cow improved the quality of prediction. Three different techniques were used to analyze the sensitivity of the ANN to different inputs, and their relative abilities are discussed. Results illustrate the potential effectiveness of ANNs in predicting milk yield and its composition and appear to justify further pursuit of this research.