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Unité Mixte de Recherches, Institut National de la Recherche Agronomique (INRA)–Agrocampus Rennes Production du Lait, 33590 Saint-Gilles, France
1 Corresponding author: Jocelyne.Flament{at}agrocampus-rennes.fr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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-amino nitrogen, acetate, ß-hydroxybutyrate, and total glycerol). The efficiency of the mammary gland in synthesizing milk components was estimated by the mammary uptake:milk output ratio. After 7 d of treatment, the decrease in milk yield of 6.1 kg/d between 8- and 24-h milking intervals was associated with a reduction in the uptake of nutrients by the mammary gland, whereas the efficiency of the mammary gland in synthesizing milk components remained relatively unchanged. The mammary uptake decreased by 26% for glucose, 32% for -amino nitrogen, 18% for acetate, 24% for total glycerol, and 24% for ß-hydroxybutyrate, respectively. These reductions in nutrient uptake were due to a decrease in the mammary blood flow (1.23 ± 0.24 L/min). For milk fat precursors (acetate, ß-hydroxybutyrate, and total glycerol), the decrease in mammary blood flow explained the entire reduction in the mammary uptake. For glucose and the milk protein precursors, the reduction in the mammary blood flow explained 60% of the decrease in the mammary uptake, with the other 40% being accounted for by a reduction in the mammary extraction of nutrients. The nutrient uptake was altered as milk yield decreased. These decreases began with the 16-h milking interval and were higher at the 24-h milking interval.
Key Words: dairy cow • milking frequency • mammary blood flow • mammary nutrient uptake
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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-amino nitrogen, BHBA, and total glycerol) declined from 32 to 27% between 8- and 24-h milking intervals. Other studies have suggested that a reduction in MBF may also occur. In cows milked once daily for 7 d, Guinard-Flament and Rulquin (2001) observed a 28% decline in milk yield and a 10% reduction in the MBF. In the goat, a lengthening of the interval between milkings from 26 to 36 h caused a 50% reduction in the MBF (Stelwagen et al., 1994; Farr et al., 2000). On the other hand, the effects of changing to a milking frequency of more than twice daily are less clear. In goats milked 5 times in 12 h, milk secretion increased by 24% and the MBF increased by 44% (Prosser and Davis, 1992). However, in goats milked hourly for 8 h, milk secretion increased by 15%, but there was no change in the MBF (Maltz et al., 1984). The aim of the present study was to determine whether the reduction in milk yield observed in response to a reduced milking frequency was associated with a reduction in the mammary uptake of nutrients, partly due to a reduction in the MBF or in the efficiency of the mammary gland to convert the plasma nutrients into milk components, or both. This study established dose–response curves for the mammary utilization of nutrients as a function of increasing the milking interval from 8 to 24 h with a constant nutrient intake.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Four multiparous Holstein cows (635 ± 30 kg of BW) in their second or third lactation at 72 ± 3 d postpartum at the start of the experiment were used. The cows were surgically prepared to estimate the net mammary balance of nutrients in the left-half udder, according to the method described by Guinard et al. (1994). One month before the beginning of the experiment, 2 permanent catheters were inserted into the left carotid and subcutaneous vein. An ultrasonic flow probe (Probe 20 S, i.d. 20 mm, cable length 2.5 m; Transonic Systems Inc., Ithaca, NY) was implanted around the left external pudic artery, before the S-shaped bend in the artery, to measure the MBF. The flow probe cable was protected with Silastic tubing (Silclear medical grade silicone tubing, i.d. 3 mm, o.d. 6 mm; VWR International SAS, Briare, France). Two rings of Dacron (Mersutures, TS53; Ethicon, Issy-Les-Moulineaux, France) were placed along the cable and at the level of exteriorization to prevent any spread of infection.
The experiment was conducted using a Latin square design with 4 cows and 5 periods. The duration of each period was 2 wk. The first week provided a transition when cows were milked twice daily (0630 and 1830 h). The second week was the experimental week, with cows milked according to the treatments allocated. A fifth period was subsequently added because one ultrasonic flow probe stopped emitting a signal during the first period; this cow was eliminated. During the third period, one cow experienced digestive problems. Hence, during the fifth period, this cow received the treatment planned for the third period. Two other cows were subjected to the 2 extreme treatments, i.e., milking once and 3 times daily. Consequently, the results are for 3 cows.
Measurements, Sampling, and Analyses
MBF.
The MBF was continuously measured throughout the experimental period. The sampling rate of the 2 flow meters (T208D; Transonic Systems Inc.) was fixed at 200 Hz. The MBF and heart rate were averaged every minute and recorded using IOX software (EMKA Technologies, Paris, France). The cows were fitted with a sensor to record their position (standing, lying). Because MBF varies according to the position of the animal, the MBF and animal position were recorded simultaneously to study the variations in the MBF as a function of the position.
Blood.
Analyses were performed as described previously (Delamaire and Guinard-Flament, 2006). Briefly, concurrently with MBF recording, 12 blood samples were collected simultaneously, during the last 24 h of the period, from the artery and vein using heparinized syringes (S-Monovette, 7.5 mL; Sarstedt, Nümbrecht, Germany). Samples were pooled by cow and period. The concentrations of glucose (precursor of lactose), -amino nitrogen, and AA (precursors of milk proteins), acetate, BHBA, NEFA, and total glycerol (precursors of milk fat) were determined from the arterial and venous plasma to analyze the mammary use of blood nutrients. Heparinized plasma was used to determine the levels of glucose, -amino N, BHBA, total glycerol, and NEFA, and deproteinized plasma was used to determine the levels of acetate. Plasma was acidified with 50% sulfosalicylic acid (vol/vol), centrifuged at 3,000 x g for 5 min at 4°C, and then diluted in a buffer solution (vol/vol) to analyze AA concentrations. Samples were pooled by cow and period and analyzed according to the methods described by Moore and Stein (1954) using chromatography on a cation-exchange resin column with a Biotronick LC3000 autoanalyser (Biotronick, Maintal, Germany), and were quantified by reaction with ninhydrin. Oxygen and carbon dioxide concentrations were determined by a gas analyzer (ABL 625, Radiometer Copenhagen, Brønshøj, Denmark) from blood samples collected using special "blood gas" heparinized syringes (S-Monovette, 2 mL, Sarstedt; Nümbrecht, Germany.
Milk.
During the 7 d of treatment, the cows were milked by each half gland. The milk yield was recorded, and the fat and protein contents were determined by infrared analysis (MilkoScan; Foss Electric, Hillerød, Denmark). After 7 d of treatment, the milk lactose and milk fatty acid levels were analyzed as previously described (Delamaire and Guinard-Flament, 2006).
Calculations and Statistical Analyses
The results are given for 3 cows (n = 14). Data concerning milk performance, the MBF, and the net mammary balance of nutrients are given after 7 d of treatment.
The arterial flow is equal to the arterial concentration x mammary plasma flow. The mammary plasma flow is calculated from the MBF (mean of the whole day) corrected for arterial hematocrit values. The mammary uptake is equal to the mammary plasma flow x arterio-venous difference except for oxygen and carbon dioxide analyzed in the blood. Mammary efficiency for use of nutrients for milk synthesis was estimated using the uptake:milk output ratio. De novo synthesis of fatty acid yield was estimated based on the hypothesis that all fatty acids from C4 to C14 were synthesized by the mammary gland, and that only 50% of C16 was synthesized by the mammary gland (Palmquist et al., 1969). For essential AA (EAA), nonessential AA (NEAA), and total AA (TAA), milk protein yield was corrected by 3.5% to take into account proteins issuing from the plasma.
The data were analyzed using the GLM procedure of SAS (SAS Institute, 1990) according to the following statistical model:
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with Yijk being the variable dependent on cow i receiving treatment k for period j, µ being the mean, and 
ijk being the residual error associated with each ijk observation. The linear, quadratic, and cubic effects of treatments were analyzed by orthogonal contrasts. Results were expressed as least squares means with the root mean square error. The threshold of significance was set at P 
0.05 and trends were noted at P 0.10.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The milk protein content remained constant with increasing milking intervals. The milk lactose yield decreased in a curvilinear manner by 213 g/d between 8- and 24-h milking intervals; the content in milk remained unchanged by the treatments. The milk fat yield did not vary significantly in response to treatments; it fell by only 84 g/d between the 16- and 24-h milking intervals. The milk fat content rose linearly by 7.2 g/kg to peak at the 24-h milking interval.
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).
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Plasma Metabolites
The arterial flow of nutrients decreased with increasing milking intervals (Table 3). These reductions were linear for glucose and -amino nitrogen (–2.6 and –3.3 mmol/min, respectively, from 8- to 24-h milking intervals) but resulted in a trend with total glycerol (–40 mmol/min). The arterial flows of acetate and NEFA decreased curvilinearly; they did not vary between 8 and 16 h and were reduced by 1.5 mmol/min and 104 µmol/min between 16 and 24 h, respectively. The arterial flow of BHBA did not change between the 8- and 24-h milking intervals.
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). For glucose, -amino nitrogen, and acetate, these decreases were linear (–1.24, –1.21, and –1.47 mmol/min, respectively, between the 8- and 24-h milking intervals). The mammary uptake of BHBA decreased curvilinearly; it did not vary between 8 and 12 h but decreased by 0.79 mmol/min between the 12- and 24-h milking intervals.
The mammary uptake of AA was not modified when milking intervals were increased (Table 4). The mammary uptakes of the sum of EAA, NEAA, and TAA were not affected by the different treatments.
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). The (acetate + BHBA) uptake:(C4 to C16/2) output ratio decreased; it did not vary between 8- and 12-h milking intervals but decreased by 0.45 points between the 12- and 16-h milking intervals and remained unchanged between the 16- and 24-h milking intervals. For AA, the up-take:output ratio of EAA, NEAA, and TAA remained unchanged when the milking intervals increased.
Blood Gases
The arterial flows of oxygen and carbon dioxide decreased linearly by 6.7 and 36 mmol/min, respectively (Table 5). The mammary uptake of oxygen did not change between the 8- and 24-h milking intervals (P = 0.160). The mammary output of carbon dioxide decreased curvilinearly, falling by 2.9 mmol/min between the 16- and 24-h milking intervals. The respiratory quotient remained unaffected by treatments.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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We observed a decrease in milk yield of 3.7 kg/d when the milking interval increase was accompanied by a reduction in the mammary gland uptake of nutrients and oxidative metabolism on the left-half udder. Our results support those of Fleet and Peaker (1978), but their data were obtained after 48 h of milk accumulation in the udder of the goat and with larger reductions in the mammary uptake of oxygen (–60%), glucose (–75%), and acetate (–65%). In the present study, we found that the mammary uptake of nutrients began to decrease between the 12- and 16-h milking intervals and reduced more between the 16- and 24-h milking intervals. When milking intervals of 8 and 24 h were compared, the uptake of glucose, a precursor of milk lactose, fell by 26%. The mammary uptake of milk protein precursors (-amino nitrogen) was reduced by 32%. With the exception of NEFA, uptake of milk fat precursors by the mammary gland was also reduced. The decrease in the mammary uptake of de novo synthesized fatty acid precursors reached nearly 20% for acetate and 30% for BHBA. The uptake of total glycerol fell by nearly 25% (P = 0.119). The mammary uptake of oxygen and mammary output of carbon dioxide decreased (oxygen: P = 0.160).
In parallel, the efficiency of the mammary gland in converting these nutrients into milk components, estimated by the mammary uptake:milk output ratio, did not appear to be markedly affected after 7 d of treatment. Indeed, the MBF:milk yield ratio remained unchanged. Whatever the milking interval, the quantity of blood necessary to synthesize 1 kg of milk remained the same, 543 ± 54.8 L, which is consistent with the results of Linzell (1974). Although the efficiency of the mammary gland in converting nutrients into milk components was generally preserved, there were some slight deviations, depending on the type of nutrient. Thus, the efficiency of the mammary gland in converting glucose into lactose did not appear to be markedly affected. The ratio between glucose taken up and lactose secreted remained stable no matter what the milking interval. The glucose uptake of the left-half udder decreased by 1.79 mol/d between the 8- and 24-h milking intervals, whereas that of lactose decreased by 0.59 mol/d. Assuming that 2 molecules of glucose are used to synthesize 1 molecule of lactose, the fall in the amount of glucose taken up by the udder and converted into lactose averaged 1.18 mol/d, 66% of the decrease in the glucose taken up.
Amino acid metabolism did not seem to be markedly disturbed by increasing the milking interval: The up-take:output ratio of TAA and NEAA remained constant. Only the uptake:output ratio of EAA appeared to indicate a reduction in the efficiency of the mammary gland to convert EAA into milk protein components for the 24-h treatment (linear effect; P = 0.110). In contrast, the mammary gland seemed more efficient in converting acetate and BHBA into short- and medium-length milk fatty acids when the milking interval was at least 16h long. Indeed, the (acetate + BHBA):(C4 to C16/2) ratio fell as the milking interval increased. This increase in mammary efficiency to convert the acetate and BHBA into short- and medium-length milk fatty acids may contribute to maintaining the synthesis of milk fatty acids and at least partially explain the small reduction in the production of milk fatty acids when compared with that of protein components and milk volume.
Reduction in Nutrient Uptake by the Mammary Gland Is Associated with Reductions in MBF and Nutrient Extraction
Variations in the MBF were consistent with results reported in the literature (Prosser and Davis, 1992; Guinard-Flament and Rulquin, 2001). Thus, neither the MBF (+0.08 L/min) nor the milk yield (+0.6 kg/d) differed with the 8- or 12-h milking intervals. With regard to a difference between the 12- and 16-h intervals, when a reduction in milk yield had been triggered, the MBF fell by 12%. This reduction was amplified between the 12- and 24-h milking intervals (–17%). These results confirm those obtained by Guinard-Flament and Rulquin (2001) in dairy cows milked once daily, in which the MBF fell by –10% and the milk yield by –28%.
The reduction in MBF allowed a decrease in the mammary uptake of nutrients (Table 3). Previously, a reduction in the arteriovenous difference from 10 to 15% for glucose and -amino nitrogen occurred when the milking interval increased from 8 to 24 h (Delamaire and Guinard-Flament, 2006). This reduction in the arteriovenous difference did not account for even half the reductions in the mammary uptake of glucose and -amino nitrogen (which decreased from 25 to 30%; Table 3). Thus, in this study, the drop in MBF explained 60% of the reduction in glucose and -amino nitrogen uptake, with the remaining 40% being due to a reduction in the extraction of these nutrients by the mammary gland. In the case of milk fatty acid precursors, MBF was the only factor responsible for a reduction in the uptake, because the arteriovenous difference was either kept constant for BHBA and total glycerol (Delamaire and Guinard-Flament, 2006) or it depended on the arterial concentration with respect to acetate (Delamaire and Guinard-Flament, 2006).
Reduction in MBF Is Not Explained by an Increase in Time Cows Spend Standing
The reduction in MBF observed following a change from twice daily to once daily milking (Guinard-Flament and Rulquin, 2001), or when the milking interval was extended in the goat (Stelwagen et al., 1994; Farr et al., 2000), may have resulted partly from the increased time the animals spent standing. Indeed, the behavior of animals can be modified when the milking frequency decreases. Thus, Österman and Redbo (2001) observed that animals milked 3 times daily spent less time standing than those milked twice a day (+64 vs. 128 min, 4 h before morning milking). Brulé et al. (2003) suggested that a change to milking once daily caused dairy cows some discomfort, resulting in an increase in the time they spent standing at the beginning of lactation before the morning milking. In the current study, the MBF was higher by 25% when the animal was lying down than when it was in the upright position, according to the results of Rulquin and Caudal (1992), and the time the animal spent standing did not vary significantly with an increase in the milking interval (Table 2). Thus, the decrease in the daily MBF when the milking interval was increased from 8 to 24 h was not due to an increase in the time the animals spent standing.
The MBF fell whether the animal was standing or supine when the milking interval was extended (Table 2), although the drop was more marked when the animal was supine than when it was standing (–1.53 vs. –0.99 L/min). This decline could be of local origin, as suggested by Prosser et al. (1996), and by the lack of disturbance of the heart rate (Table 2). This result is also consistent with the hypothesis of a dual regulatory mechanism, as previously suggested by Guinard-Flament and Rulquin (2001).
The reduction in the MBF could result from lowered metabolic activity of the mammary gland following the drop in milk yield. According to Cant and McBride (1995), MBF would be controlled as a function of mammary requirements, producing sufficient ATP to cover needs. Results obtained in the present study could support this hypothesis. Assuming that energy exported in milk reflects the ATP needs of the mammary gland, the relationship between the MBF and the quantity of net energy exported in milk after 7 d of treatment was linear and positive (Figure 1); the MBF fell when the net quantity of energy exported by milk diminished.
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) decreased as the milking interval increased, falling from 1.23 L/min with an 8-h milking interval to 0.69 L/min with a 24-h milking interval. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication January 27, 2006. Accepted for publication April 7, 2006.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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