Effect of heat stress during the dry period on mammary gland development

Heat stress during the dry period negatively affects hepatic metabolism and cellular immune function during the transition period, and milk production in the subsequent lactation. However, the cellular mechanisms involved in the depressed mammary gland function remain unknown. The objective of the present study was to determine the effect of heat stress during the dry period on various indices of mammary gland development of multiparous cows. Cows were dried off approximately 46 d before expected calving and randomly assigned to 2 treatments, heat stress (HT, n = 15) or cooling (CL, n = 14), based on mature equivalent milk production. Cows in the CL treatment were provided with sprinklers and fans that came on when ambient temperatures reached 21.1°C, whereas HT cows were housed in the same barn without fans and sprinklers. After parturition, all cows were housed in a freestall barn with cooling. Rectal temperatures were measured twice daily (0730 and 1430 h) and respiration rates recorded at 1500 h on a Monday-Wednesday-Friday schedule from dry off to calving. Milk yield and composition were recorded daily up to 280 d in milk. Daily dry matter intake was measured from dry off to 42 d relative to calving. Mammary biopsies were collected at dry off, −20, 2, and 20 d relative to calving from a subset of cows (HT, n = 7; CL, n = 7). Labeling with Ki67 antigen and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick-end labeling were used to evaluate mammary cell proliferation and apoptosis, respectively. The average temperature-humidity index during the dry period was 76.6 and not different between treatments. Heat-stressed cows had higher rectal temperatures in the morning (38.8 vs. 38.6°C) and afternoon (39.4 vs. 39.0°C), greater respiration rates (78.4 vs. 45.6 breath/min), and decreased dry matter intake (8.9 vs. 10.6 kg/d) when dry compared with CL cows. Relative to HT cows, CL cows had greater milk production (28.9 vs. 33.9 kg/d), lower milk protein concentration (3.01 vs. 2.87%), and tended to have lower somatic cell score (3.35 vs. 2.94) through 280 d in milk. Heat stress during the dry period decreased mammary cell proliferation rate (1.0 vs. 3.3%) at −20 d relative to calving compared with CL cows. Mammary cell apoptosis was not affected by prepartum heat stress. We conclude that heat stress during the dry period compromises mammary gland development before parturition, which decreases milk yield in the next lactation.     

Cell turnover and activity in mammary tissue during lactation and the dry period in dairy cows

Milk yield of the dairy cow follows a pattern termed the lactation curve. We have investigated the cellular background for this pattern. Seven mammary biopsies were obtained from each of 10 cows: at the end of lactation (d 347, equal to d 77 before next parturition); during the dry period at d 48 (4 d after dry off); 16 d before parturition; and during lactation at d 14, 42, 88, and 172. The fraction of proliferating (staining positive for Ki-67) alveolar cells was higher during the dry period (8.6%) than during lactation (0.5%). The fraction of apoptotic (staining positive by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) alveolar cells was higher immediately after dry off (0.37%) and in early lactation (0.76%) than during other periods (0.15%). The enzyme activities of fatty acid synthetase, acetyl CoA-carboxylase, and galactosyl transferase were approximately 12-, 11-, and 4-fold higher, respectively, during lactation than during the dry period. In conclusion, mammary cell proliferation is substantial in a period near parturition but otherwise low, and apoptosis is elevated at dry off and in early lactation. The increase in apoptosis in early lactation may be due to discarding nonfunctional or senescent cells or to removal of a surplus of newly synthesized cells. The activity of selected enzymes central for milk synthesis is probably not limiting for milk production.     

Inhibiting prolactin by cabergoline accelerates mammary gland remodeling during the early dry period in dairy cows

The inhibition of prolactin release using cabergoline, a dopamine agonist, is an effective strategy to accelerate the changes in mammary secretion composition after drying-off. The objective of this study was to determine how cabergoline may affect mammary tissue remodeling during early involution. Holstein dairy cows were treated with either a single i.m. administration of 5.6 mg of cabergoline (Velactis, Ceva Santé Animale, Libourne, France, n = 7) or placebo (n = 7) at the time of drying-off. Mammary biopsy samples were collected 1 wk before drying-off (d ?6), after 30 h of milk accumulation (d 1), and again 8 d following drying-off (d 8) to determine changes in gene expression, lactoferrin content, and cell turnover. Blood and mammary secretion samples were collected at d ?6 and again at d 1, 2, 3, 4, 8, and 14 following the abrupt cessation of lactation to evaluate indicators of blood-milk barrier integrity and other markers of mammary tissue remodeling. Cabergoline induced less SLC2A1, BAX, CAPN2, and IGFBP5 mRNA expression. In contrast, cabergoline did not modify changes in cell proliferation and apoptosis. Following the cessation of lactation, changes in mammary secretion composition (Na⁺ and K⁺) and blood lactose concentrations were indicative of a loss in the blood-milk barrier function in both treatment groups. Cabergoline treatment affected only Na⁺ and K⁺ concentrations at d 1, suggesting a moderate increase in tight junction permeability. The increase in the activity of MMP9 and in mammary epithelial cell concentration in mammary secretions was greater in cabergoline-treated cows than in control cows, suggesting more mammary tissue remodeling. The increase in lactoferrin immunostaining in the mammary tissue occurred earlier for cabergoline-treated cows than for control cows, and was essentially localized in the stroma. Changes in some key markers of mammary involution suggest that cabergoline accelerates mammary gland remodeling. Thus, a single injection of cabergoline after the last milking would facilitate drying-off by enhancing mammary gland involution.     

Elimination of selected mastitis pathogens during the dry period

We aimed to evaluate the elimination of 4 different mastitis pathogens, Streptococcus agalactiae, Mycoplasma bovis, Staphylococcus aureus, and Streptococcus uberis, from infected udder quarters during the dry period using quantitative PCR. The second purpose of this study was to evaluate the association between milk haptoglobin (Hp) concentration and the presence of udder pathogens (Strep. agalactiae, Staph. aureus, M. bovis, and Strep. uberis) in udder quarter milk samples before and after dry period. Aseptic udder quarter milk samples (n = 1,001) were collected from 133 dairy cows at dry off and at the first milking after calving from 1 large dairy herd. Bacterial DNA of Strep. agalactiae, Staph. aureus, Strep. uberis, and M. bovis in the udder quarter milk samples was identified with commercial quantitative PCR analysis Mastitis 4B (DNA Diagnostic A/S, Risskov, Denmark). Milk Hp concentration (mg/L) was measured from udder quarter milk samples. The elimination rates during the dry period for M. bovis, Staph. aureus, Strep. agalactiae, and Strep. uberis were 86.7, 93.6, 96.2, and 100.0%, respectively. The new IMI rate was 3.0% for M. bovis, 2.9% for Staph. aureus, 2.4% for Strep. agalactiae, and 3.1% for Strep. uberis. The milk Hp concentration was significantly higher in udder quarter milk samples with blood and in samples positive for Strep. agalactiae at dry off and for Staph. aureus postcalving. Elevated milk Hp concentration was not associated with the presence of M. bovis in the udder quarter milk samples. In conclusion, elimination of Staph. aureus, Strep. agalactiae, and Strep. uberis during the dry period was high; the elimination of M. bovis from infected udder quarters was lower, but probably spontaneous. Additionally, milk Hp concentration may be used as a marker for udder inflammation when combined with the bacteriological results at dry off and postpartum.     

Milk production of dairy cows treated with estrogen at the onset of a short dry period

The objective was to determine whether the use of estradiol-17beta (E2) at the initiation of short dry periods prevented an anticipated decline in milk production in the subsequent lactation. Lactating Holstein cows (n = 66) were dried at either 60 or 30 d before expected calving. Treatments in a 2 x 2 factorial arrangement included: D60 (n = 19, 60-d dry, no E2), D60 + E2 (n = 18, 60-d dry, E2), D30 (n = 15, 30-d dry, no E2), and D30 + E2 (n = 14, 30-d dry, E2). To accelerate mammary involution, estradiol-17beta (15 mg in 4 ml of ethanol) was injected subcutaneously daily for 4 d beginning 30 d before expected calving. Parturitions occurred between November 1995, and March 1996. Actual days dry for respective treatments were 57.3, 60.6, 33.9, and 33.8 +/- 1.7 d. Onset of parturition, calving difficulty, and cow health were not affected by E2. Actual 305-d milk yields for the lactation completed immediately before the experimental dry period were 10,318, 10,635, 10,127, and 10,447 +/- 334 kg, respectively; and were 9942, 9887, 9669, and 10,172 +/- 387 kg, respectively, for the lactation immediately following treatment. Respective pre- and posttreatment mature equivalent 305-d yields were 9574, 9861, 9812, and 9724 +/- 297 kg; 8987, 8843, 9126, and 9008 +/- 294 kg. Milk yields did not differ across treatments. Cows with a 34-d dry period were as productive as cows with a 59-d dry period. Estradiol-17beta had no effect, but perhaps should be evaluated with dry periods shorter than 34 d.     

Effect of dry period length on reproduction during the subsequent lactation

Days dry may influence reproductive measures such as days to first postpartum ovulation, days open, and pregnancy per artificial insemination (AI). Holstein cows (n = 781) from an approximately 3,000-cow commercial dairy operation were randomly assigned to 1 of 2 treatments with different targeted dry period (DP) lengths. Treatments were 1) a traditional DP of 55 d (T) or 2) a shortened DP of 34 d (S). All dry cows on T were fed a low-energy diet until 35 d before expected calving, and then at 34 d before expected calving, cows on T and S were fed a moderate energy diet until parturition. After parturition, all cows consumed the same diets that included a postcalving diet followed by a lactation diet. Actual days dry for each treatment were close to expected values, 34 and 56 d for S and T, respectively. Median days until first postpartum ovulation occurred sooner for S compared with T (35 vs. 43 d). The percentage of cows that were classified anovular by 70 d in milk (DIM) was more than 2-fold greater for cows on T than S (18 vs. 8%). Cows received AI after standing estrus starting at d 45, and the percentage of cows pregnant at 70 DIM tended to be greater for S than T; younger cows were similar (20.2 vs. 18.8%), but there was a difference between S and T in older cows (20.3 vs. 10.6%). Similarly, median days open tended to be fewer for cows on S than T. At 300 DIM, 85% of cows in both treatments were pregnant. Combining data from first and second service, pregnancies per AI were greater in older cows on S than T (32 vs. 24%). Thus, shortening the DP appeared to increase reproductive efficiency in older cows by shortening time to first ovulation, reducing numbers of anovular cows, and improving fertility. Future studies at more locations with varying reproductive management strategies are needed to confirm and provide the mechanistic basis for these results.     

Effects of photoperiod during the dry period on cellular immune function of dairy cows

Recent studies suggest that exposure of cattle to photoperiod can influence immune function. The objective of this study was to determine whether treatment of cows with short day photoperiod (SDPP; 8 h light: 16 h darkness) during the dry period alters immune function, relative to cows subjected to a long day photoperiod (LDPP; 16 h light: 8 h darkness). Multiparous Holstein cows (n = 39) were dried 62 d before calving and exposed to photoperiod treatment until parturition; thereafter, cows were exposed to natural photoperiod. General health was monitored weekly during the dry period and cellular immune function was examined monthly during the dry period and at calving. Concentrations of prolactin and cortisol were measured from 10 d before calving to 2 d after calving. The periparturient prolactin surge in plasma was greater in LDPP cows (54.6 ng/ mL) than SDPP (22.4 ng/mL). Relative to LDPP cows, neutrophil chemotaxis and lymphocyte proliferation were enhanced in SDPP cows during the dry period. Neutrophil chemotaxis averaged 142.5 and 178.8 cells/ well during the dry period for LDPP and SDPP, respectively. Lymphocyte proliferation during the dry period averaged 197.6 and 326.5% for LDPP and SDPP cows, respectively. Physiological characteristics of the cows were not affected by treatment during the dry period. However, differences between treatments were observed within 2 d of parturition. Potential implications of photoperiod management for cow health and well-being merit further investigation.     

Evaluation of antioxidant and proinflammatory gene expression in bovine mammary tissue during the periparturient period

The incidence and severity of mastitis can be high during the period of transition from pregnancy to lactation when dairy cattle are susceptible to oxidative stress. Oxidative stress may contribute to the pathogenesis of mastitis by modifying the expression of proinflammatory genes. The overall goal of this study was to determine the relationship between critical antioxidant defense mechanisms and proinflammatory markers in normal bovine mammary tissue during the periparturient period. Mammary tissue samples were obtained from 12 cows at 35, 20, and 7 d before expected calving and during early lactation (EL, 15 to 28 d in milk). Enzyme activities for cytosolic glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase were relatively low during the dry period, but increased during EL, whereas activity of thioredoxin reductase 1 did not change significantly as a function of time. In contrast, gene expression for these antioxidant selenoproteins and for heme oxygenase-1 gradually decreased as parturition approached and then increased during EL. The expression of intercellular vascular adhesion molecule-1 and vascular cell adhesion molecule-1 followed a similar trend where mRNA abundance gradually declined as parturition approached with a slight rebound in EL. Gene expression of the pro-oxidant, 15-lipoxygenase 1, which is known to increase during times of oxidative stress, also increased dramatically in mammary tissue from EL cows. Expression of the proinflammatory cytokines, IL-1β, IL-6, and IL-8 did not change significantly during the periparturient period. Strong positive correlations were found between several antioxidant enzymes (cytosolic glutathione peroxidase, thioredoxin reductase 1, and heme oxygenase-1) and vascular adhesion molecules (intercellular vascular adhesion molecule-1, vascular cell adhesion molecule-1) suggesting a protective response of these antioxidants to an enhanced proinflammatory state. Ability to control oxidative stress through manipulation of key antioxidant enzymes in the future may modify the proinflammatory state of periparturient cows and reduce incidence and severity of some diseases such as mastitis.     

Models for predicting dry matter intake of Holsteins during the prefresh transition period

The objectives of this study were to develop and validate a model for predicting dry matter intake (DMI) of Holsteins during the prefresh transition period. The original database (ODB) for model development was established by compiling parity, body condition score (BCS), and DMI data during the final 3 wk of gestation from 366 Holsteins fed 24 different diets that were used in eight experiments conducted at three universities. For model validation, a validation database (VDB) was established by compiling data from 333 prefresh transition Holsteins fed 25 different diets that were used in eight experiments conducted at five universities. Dry matter intake during the prefresh transition period was fitted to an exponential function: DMI(t) = a + pe(kt), where DMI(t) = DMI as a percentage of body weight (BW) at time t, a = asymptotic intercept at time--infinity, p = change in intake (kg) from the asymptotic intercept until parturition, k = rate constant influencing the shape of the curve, and t = day relative to parturition expressed as days pregnant--280. The model developed from the ODB predicted DMI of heifers in the VDB with satisfactory accuracy and precision. However, this was not true for cows, probably due to differences in BCS of cows and diets fed to cows from the two data sets. When a subset of cows was selected from each data set that had similar BCS (> 4.0) and were fed similar diets, accuracy and precision of the model predicting DMI was improved. Finally, both databases were combined to develop final models for predicting DMI of heifers and cows. Proposed models for predicting mean daily DMI of heifers and cows during the prefresh transition period were DMI(t) = 1.713-0.688e(0.344t) (R2 = 0.96) and DMI(t) = 1.979-0.756e(0.154t) (R2 = 0.97), respectively. Adjustment factors for animal and dietary factors were generated to demonstrate the plausibility of adaptive fitting of the prediction. The regression coefficients of prediction models (a, p, and k) were affected by BCS and dietary organic macronutrient concentrations.     

Effects of feeding intensity during the dry period. 2. Metabolic and hormonal responses

The metabolic response to different feeding levels during the dry period was studied in 24 multiparous dairy cows of the Swedish Red and White breed. The cows represented two lines, selected for high or low milk fat percentage, at the same amount of energy produced in milk. The cows were fed one of three different amounts of the same total mixed diet during the dry period, starting 8 wk prior to the expected parturition. The rations provided 71, 106, or 177 MJ metabolizable energy per day. After parturition the cows were offered another total mixed diet ad libitum for the first 12 wk of lactation. Glucose challenges were performed 3 wk prior to and 3 wk after parturition. Prepartum the glucose clearance rate was related to feeding level. Insulin response to the glucose challenge was reduced during the postpartum period, compared to the prepartum period. During about 6 wk prepartum, the insulin level in plasma was related to feeding level. At the sampling 3 wk prior to parturition the plasma level of leptin also was related to the feeding level. After parturition both leptin and insulin were reduced. In early lactation plasma leptin concentration was not related to adiposity as reflected by body condition scoring. It was suggested that lactation as such affected the leptin concentration in plasma.     

Overstocking dairy cows during the dry period affects dehydroepiandrosterone and cortisol secretion

Stressful situations trigger several changes such as the secretion of cortisol and dehydroepiandrosterone (DHEA) from the adrenal cortex, in response to ACTH. The aim of this study was to verify whether overstocking during the dry period (from 21 ± 3 d to the expected calving until calving) affects DHEA and cortisol secretion and behavior in Holstein Friesian cows. Twenty-eight cows were randomly divided into 2 groups (14 animals each), balanced for the number of lactations, body condition score, and expected date of calving. Cows in the far-off phase of the dry period (from 60 to 21 d before the expected calving date) were housed together in a bedded pack. Then, animals from 21 ± 3 d before the expected calving until calving were housed in pens with the same size but under different crowding conditions due to the introduction of heifers (interference animals) into the pen. The control condition (CTR) had 2 animals per pen with 12.0 m² each, whereas the overstocked condition (OS) had 3 interference animals in the same pen with 4.8 m² for each animal. On d ?30 ± 3, ?21 ± 3, ?15 ± 3, ?10 ± 3, and ?5 ± 3 before and 10, 20, and 30 after calving, blood samples were collected from each cow for the determination of plasma DHEA and cortisol concentrations by RIA. Rumination time (min/d), activity (steps/h), lying time (min/d), and lying bouts (bouts/d) were individually recorded daily. In both groups, DHEA increased before calving and the concentration declined rapidly after parturition. Overstocking significantly increased DHEA concentration compared with the CTR group at d ?10 (1.79 ± 0.09 vs. 1.24 ± 0.14 pmol/mL), whereas an increase of cortisol was observed at d ?15 (3.64 ± 0.52 vs. 1.64 ± 0.46 ng/mL). The OS group showed significantly higher activity (steps/h) compared with the CTR group. Daily lying bouts tended to be higher for the OS group compared with CTR group in the first week of treatment. The overall results of this study documented that overstocking during the dry period was associated with a short-term changes in DHEA and cortisol but these hormonal modifications did not influence cow behavior.     

Effect of dry period management on mammary gland function and its endocrine regulation in dairy cows

The objective of this study was to evaluate the effect of shortening the dry period on the mammary gland and the hormonal regulation of its functions. Holstein cows (n = 18) were assigned to a short dry period (SDP; 35 d; n = 9) or a conventional dry period (CDP; 65 d; n = 9). All cows were fed the same diets, with the exception that, during the dry period, the SDP cows received only the pre-calving diet for 35 d, whereas the CDP cows were fed a high-fiber diet from 65 to 28 d before calving and then received the same pre-calving diet as the SDP cows. Mammary gland functional capacity was evaluated at 70 days in milk, and mammary biopsies were taken in early and midlactation. Dry period length averaged 64.3 ± 1.1 and 31.9 ± 1.0 d for the CDP and SDP cows, respectively. The SDP cows had a lower milk yield and a lower energy-corrected milk yield compared with the CDP cows. The SDP cows also had a lower dry matter intake from wk 5 to 20 of lactation and tended to have lower plasma concentrations of β-hydroxybutyrate from wk 1 to 4. Prepartum serum progesterone and estradiol concentrations were unaffected by the dry period management. Serum growth hormone concentrations and milking-induced prolactin release were similar in both groups. However, during the period when the CDP cows were dry but the SDP cows were still being milked (wk −9 to −6), serum prolactin concentrations were higher in the SDP cows than in the CDP cows. The SDP cows had a lower milk BSA content than the CDP cows after the dry period and similar milk lactose concentrations, suggesting that their mammary tight junctions were closed following parturition and, therefore, that the later stage of their lactogenesis was not impaired by SDP management. In early and midlactation, mammary cell apoptosis and proliferation rates as well as mammary expression of genes involved in the function of this tissue were unaffected by the dry period management strategy. For cows in their second lactation, mammary gland functional capacity at 70 d in milk tended to be lower in the SDP cows. In conclusion, even though SDP management decreased milk production during the subsequent lactation, it did not affect mammary cell activity. Although direct evidence is still lacking, decreased mammary cell growth during the dry period is likely responsible for this negative effect. The higher prolactin concentrations in lactating cows during late gestation could be involved in this effect. More research is needed to test these hypotheses.     

Peripartal metabolism and production of holstein cows fed diets supplemented with fat during the dry period

Previous research from our laboratory demonstrated that cows fed supplemental fat throughout the dry period in an attempt to increase body condition score (BCS) had little hepatic lipid accumulation at d 1 postpartum compared with cows fed an isocaloric high-grain diet or a lower energy control diet. However, results were confounded by lower dry matter intake and loss of BCS by cows fed the fat-supplemented diet. Here, cows were fed a control diet (C) moderately high in nonfiber carbohydrates (NFC) or an isocaloric fat-supplemented, low NFC (F) diet to reassess the effects of supplemental fat throughout the dry period on peripartal lipid accumulation in liver. A more energy-dense, high-NFC diet supplemented with fat (CF) was also fed to test the efficacy of supplemental fat in a diet with similar carbohydrate composition but higher energy density. Intakes of dry matter and net energy for lactation were similar among treatments throughout the experiment, although diet x day interactions during the last 21 d before parturition indicated that cows fed CF decreased intakes more slowly. Cows gained similar amounts of BCS and body weight among diets prepartum, but cows fed C tended to lose more BCS and body weight around parturition. Milk production and milk components did not differ among treatments. Prepartum concentrations of glucose, insulin, total protein, nonesterified fatty acids, and mu-hydroxybutyrate in plasma were similar among treatments. Supplemental fat increased prepartum concentrations of urea and cholesterol in plasma. Postpartum concentrations of metabolites and insulin in plasma were similar among treatments. Concentrations of total lipid and triglyceride in liver increased at parturition, whereas hepatic glycogen concentration decreased, but concentrations were not different among treatments. Supplemental fat fed prepartum did not affect peripartal lipid accumulation in liver tissue and did not benefit postpartum milk production.     

The effect of a shortened dry period on intramammary infections during the subsequent lactation

Several recent studies have investigated the effect of shortened dry periods on milk production in the subsequent lactation. What is lacking from these studies is an understanding of the effect that a shortened dry period has on udder health. Four herds, 156 cows, were studied to determine if a shortened dry period (30 d) had a negative effect on mammary gland health during the subsequent lactation as opposed to cows assigned to a long, 45 or 60 d, dry period. Cows in 2 herds were assigned to either 30- or 60-d dry periods (group I), whereas cows in the other 2 herds were assigned to either 30- or 45-d dry periods (group II). Intramammary instillation of commercial preparations of cephapirin benzathine, 300 mg (dry cow formulation), was given to cows assigned a 45- or 60-d dry period length protocol, and 200 mg (lactating cow formulation) was administered to cows assigned a 30-d dry period. Differences in response variables to dry period length were compared within group. Cure rates for 60- vs. 30-d dry period cows were 72% (28/39) vs. 81% (30/37) and 74% (25/34) and 73% (27/37) for 45- vs. 30-d dry periods. Differences were not statistically significant for either comparison group. The majority of intramammary infections were caused by the minor pathogens, coagulase-negative staphylococci (n = 102) or Corynebacterium bovis (n = 11). Only 11 cows had intramammary infections by major pathogens. The herd average percentage of new intramammary infections ranged from 6 to 9% and did not differ among herds between treatment groups. Linear somatic cell counts were not significantly affected by dry period length during the first 6 to 7 mo of the subsequent lactation. Milk production did differ between groups. Mature equivalent milk production was greater in group I cows given a 60-d dry period (11,942 ± 2,059 kg) as opposed to those given a 30-d dry period (10,749 ± 2,321 kg). Cows given a 45-d dry period did not produce more milk than cows with a 30-d dry period in group II. Although shortening the dry period to 30 d did not have untoward effects on mammary gland health as measured by intramammary infections or milk somatic cell counts, production may be adversely affected when dry periods are shortened to 30 d.     

Continuous lactation in dairy cows: effect on milk production and mammary nutrient supply and extraction

Reports over the past decade have indicated that normal lactational performance can be achieved in genetically superior and high-producing dairy cows, even when the dry period between 2 lactations is omitted. The hypothesis tested in this experiment was that normal lactogenesis I and metabolic function may be achievable in continuously milked high-yielding dairy cows as a result of the genetic selection for lactation performance and hence longevity of mammary epithelial cells. The milk production and mammary nutrient uptake in response to omission of the dry period for cows with an expected peak milk yield higher than 45 kg/d were studied in 28 Holstein dairy cows managed without bovine somatotropin. Performance and metabolic parameters were followed in late gestation and in the following early lactation. Fourteen cows were milked continuously throughout late gestation, and another 14 dairy cows experienced a 7-wk prepartum dry period. Continuous milking during the prepartum period reduced milk production in the following early lactation period by >20%. The reduced milk production could not be readily ascribed to inefficiency of the mechanisms responsible for nutrient uptake by the lactating mammary epithelial cells, nor to systemic endocrine changes. This suggests that lowered mammary nutrient uptake must have been associated with reduced mammary blood flow, metabolic activity, or both, most likely as a result of disturbed lactogenesis I prepartum or lactogenesis II postpartum triggered by as yet unknown local mechanisms. Milk protein content was elevated by 0.4 percentage units in the continuously milked cows. The underlying reason is unknown, but given the current pricing system for milk, it deserves to be further investigated.     

Effect of heat stress during early,late, and entire dry period on dairy cattle

Cooling during the entire dry period abates the negative effects of heat stress postpartum, yet the temporal relationship of cooling (i.e., early or late dry period) to performance is unknown. We evaluated the effect of heat stress early, late, and for the entire dry period on subsequent performance. Cows were selected based on mature-equivalent milk yield and dried off 45 d before expected calving. Cows were blocked by parity, previous 305-d mature equivalent milk yield, and body weight (BW) and randomly assigned to cooling (shade, fans, and soakers; CL) or heat stress (shade; HT). Treatments included CL (n = 20) or HT (n = 18) during the entire dry period, HT during the first 3 wk dry and then CL until calving (HTCL, n = 21), or CL during the first 3 wk dry period and then HT until calving (CLHT, n = 19). Heat stress increased rectal temperature (RT; CL, 38.8; HT, 39.1 ± 0.04°C) and respiration rate (RR; CL, 52.9; HT, 70.5 ± 1.9 breaths/min) during the early dry period. In the late dry period, HT increased RT and RR relative to CL cows (RT = CL, 38.7; HT, 39.1; CLHT, 39.1; HTCL, 38.9 ± 0.05°C; RR = CL, 47; HT, 64; CLHT, 66; HTCL, 53 ± 2.1 breaths/min). During the early dry period, HT decreased dry matter intake (CL, 11.8; HT, 10.5 ± 0.35 kg/d) but dry matter intake did not differ among treatments during late dry period (HT, 10.7; HTCL, 11.1; CL, 11.2; CLHT, 10.1 ± 0.55 kg/d). Cows exposed to prepartum cooling during the entire dry period had increased dry matter intake compared with cows exposed to heat stress during the late dry period (CL vs. CLHT, 11.2 ± 0.55 and 10.1 ± 0.55 kg/d, respectively). Heat stress at any time reduced gestation length compared with cows under prepartum cooling during the entire dry period (CL, 277 vs. HT, 274; CLHT, 273; and HTCL, 274 ± 1.17 d). Dry period length decreased by approximately 4 d if cows were exposed to HT at any time. During the early dry period, HT decreased BW, whereas CL increased BW relative to that at dry-off (CL, 6.9; HT, −9.4 ± 3.7 kg). In the late dry period, we detected no differences in BW gain among treatments, but cows exposed to prepartum cooling for the entire dry period tended to have increased BW gain compared with HT and HTCL. Prepartum cooling during the early or late dry period alone partially rescued milk yield only in the first 3 wk of lactation (CL, 32.9; HT, 26.6; CLHT, 29.7; HTCL, 30.7 ± 1.37 kg/d). Cooling for the entire dry period increased milk yield up to 30 wk into lactation compared with all other treatments. Thus, HT at any time during the dry period compromises performance of cows after calving.     

Effects of feeding intensity during the dry period. 1. Feed intake,body weight,and milk production

The objective of this experiment was to study dry matter intake (DMI), body condition, milk yield, and milk composition in cows with different body condition at the time of parturition. Twenty four multiparous cows with genetic merit for high or low milk fat content were assigned to one of three diets during the dry period. The treatments consisted of 6, 9, or 14.5 kg dry matter of a total mixed ration providing 71, 106, or 177 MJ/d of metabolizable energy and are referred to as low (L), medium (M), and high (H) dry period rations, respectively. These diets were introduced when the cows were dried off from the previous lactation, at least 8 wk before expected parturition. After parturition all cows were fed another total mixed ration ad libitum. The dietary treatments generated differences between the groups in body weight as well as in body condition score at parturition. There were no differences in DMI in early lactation, but during wk 6 to 12 DMI was lower among H cows, which was linked to a prolonged negative energy balance in this group. The milk yield was 38.5 +/- 0.8 kg of energy-corrected milk the first 4 wk postpartum and did not differ between treatments or selection lines. Body weight loss mainly occurred in lactation wk 1 to 4 and was greatest in H cows. The mobilization of body tissues was reflected in a higher milk fat content of C18:0 for the H cows during wk 1 to 4. There were no marked effects of treatments on milk fat content or milk protein content, which indicates that cows in early lactation have a potential to compensate for low nutrient intake during the dry period if they are offered a high-quality diet. The observed differences between treatments in DMI wk 6 to 12 could not be explained by differences in milk yield or mobilization of body tissues. Milk fat content was 4.7% in cows with genetic merit for high milk fat content and 4.2% in cows with genetic merit for low milk fat content. There was a tendency for higher body weight in cows with genetic merit for low milk fat content throughout the experiment.