Chewing Activity, Saliva Production, and Ruminal pH of Primiparous and Multiparous Lactating Dairy Cows1

J. Dairy Sci. 85:1176–1182  American Dairy Science Association, 2002.

Chewing Activity, Saliva Production, and Ruminal pH of Primiparous and Multiparous Lactating Dairy Cows1 M. Maekawa,* K. A. Beauchemin,† and D. A. Christensen* *Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Canada, S7N 5A8 †Livestock Science Section, Agriculture and Agri-Food Canada, Research Centre, Lethbridge, Alberta, Canada, T1J 4B1

ABSTRACT Four multiparous (MP) and four primiparous (PP) ruminally cannulated lactating Holstein cows were used in a double 4 × 4 Latin square design to study the chewing behavior, saliva production, and ruminal pH of cows in the first or subsequent lactation. Cows were fed one of four diets; three total mixed rations containing 40, 50, or 60% silage (DM basis), and a separate ingredient diet containing 50% concentrate. Dry matter intake was higher for MP cows than for PP cows (19.2 vs. 17.1 kg/d) but not as a percentage of body weight (2.97 ± 0.06%). Multiparous cows spent more time eating than PP cows (260 vs. 213 min/d, respectively), even after adjustment for dry matter intake (13.8 vs. 12.4 min/kg DM). Multiparous cows also spent more time ruminating per day than PP cows (560 vs. 508 min/d, respectively). Eating salivation rate was not affected by parity, but resting salivation rate was higher for MP cows than for PP. Although MP cows spent more time chewing than PP cows, total daily saliva production was only numerically higher for MP cows because the increase in saliva produced during chewing was accompanied by a decrease in saliva produced during resting. Furthermore, pH profiles tended to be lower for MP cows than for PP cows. Multiparous cows may have a greater risk of incurring acidosis than PP cows because increased salivary secretion associated with increased chewing may not sufficiently compensate the increment of fermentation acids produced in the rumen due to high feed intake. (Key words: parity, chewing activity, saliva production)

Abbreviation key: F:C = forage-to-concentrate ratio, MP = multiparous cows, PP = primiparous cows, SI = diet with ingredients allocated separately. INTRODUCTION Multiparous cows (MP) consume more feed per day than do primiparous (PP) cows, and this is usually reflected in higher milk production (Dado and Allen, 1994). Beauchemin and Rode (1994) suggested that compared with MP cows, PP cows require longer eating time and less competition at the manger to maximize feed intake. Primiparous cows reportedly chew feed more thoroughly and more slowly than older cows (Campling and Morgan, 1981; Beauchemin and Rode, 1994). Thus, feed intake and chewing behavior differs by parity (Grant and Albright, 1995). Chewing activity is usually a good indication of rumen health because chewing stimulates saliva secretion. Saliva has been estimated to supply about 70 to 90% of the fluid and buffering capacity entering the rumen (Kay, 1966) and is the major determinant of liquid outflow rate from the rumen (Cassida and Stokes, 1986). Silanikove and Tadmor (1989) reported a linear relationship between feed intake and salivary secretion rate. Despite the importance of salivary secretion in the digestive function of dairy cows, to our knowledge, no studies have been conducted to quantify salivary secretion during mastication as affected by parity. Furthermore, there is limited information in the literature related to differences in chewing activity and ruminal pH of cows as affected by parity. Thus, the objective of this experiment was to determine the effects of parity on chewing activity, saliva production, and ruminal pH of lactating dairy cows. MATERIALS AND METHODS

Received March 5, 2001. Accepted November 23, 2001. Corresponding author: K. A. Beauchemin; e-mail: beauchemin@ em.agr.ca. 1 LRC contribution number 38701014.

Animals and Diets Four MP and four PP ruminally cannulated Holstein cows were used in this experiment. At the start of the

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experiment, the MP cows averaged (± SD) 631 ± 87 kg of BW and 109 ± 67 DIM. The PP group averaged 581 ± 41 kg of BW and 146 ± 69 DIM. The experiment was designed as a double 4 × 4 Latin square, with MP and PP cows assigned to each square, respectively. Cows received one of four diets. Three of the diets were provided as a TMR consisting of different forageto-concentrate (F:C) ratios (40:60, 50:50, and 60:40), whereas for the fourth diet, forage and concentrate were fed separately (SI), and F:C ratio of the diet offered was 50:50 (DM basis). The forage consisted entirely of whole crop barley silage and the concentrates consisted mainly of steam-rolled and ground barley grain. Further information on diet composition and chemical composition of ingredients and diets is given in Maekawa et al. (2002). Measurements for the PP group were collected 7 d apart from the MP group. All cows were cared for in accordance with guidelines of the Canadian Council on Animal Care (Ottawa, ON, Canada). Cows were housed in individual stalls on rubber comfort mats with wood shavings in the Dairy Unit of the Agriculture and AgriFood Canada Lethbridge Research Centre. Cows were milked in their stalls twice daily. The cows were turned outside for 1 or 2 h daily, except during measurements of chewing activity, saliva secretion, and ruminal pH. Cows were weighed at the beginning and end of each period. Feed Intake The TMR was offered twice daily (0800, 1500 h) at 115% of voluntary intake. Forty percent of the daily allocation was provided at the morning feeding, and 60% at the afternoon feeding. For cows fed SI, concentrate was fed at the same time as the TMR diets and the barley silage was fed 1 h later (0900, 1600 h). Equal proportions of concentrate were allocated at each meal, but proportions of silage at each meal were the same as for TMR. Orts for each cow were weighed daily. Samples were taken daily during the last 6 d of the period, composited by period for each cow, and dried at 55°C to a constant weight. Daily DMI was calculated as the difference between the total amount of DM offered and the DM refused, divided by 6 d. Milk Yield and Components Milk production was recorded daily at 0600 and 1630 h during the entire period. Milk samples were taken a.m. and p.m. during the last 4 d of each period. They were preserved with potassium dichromate, stored at 4°C, and sent to the Central Alberta Milk Testing Laboratory (Edmonton, AB, Canada) for fat, protein, and

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lactose determination using an infrared analyzer (Milk0-Scan 605; Foss Electric, Hillerød, Denmark). Chewing Eating and ruminating behaviors were monitored visually for cows in both groups over a 24-h period. Eating and ruminating activities were noted every 5 min, and each activity was assumed to persist for the entire 5min interval. To estimate time spent eating or ruminating per kilogram of DM, NDF, or ADF intake, the average intake for the period was used. A period of rumination was defined as at least 5 min of ruminating activity followed by at least 5 min without ruminating activity. Total time spent chewing was calculated as the total time spent eating and ruminating. Total time spent resting was calculated as 24 h minus total time spent chewing. Salivation The total amount of saliva secreted during eating each day was calculated by multiplying the eating salivation rate (ml/min) by the time spent eating each day (min). Salivation rate during eating was measured by collecting swallowed boluses of ingested feed from the cardia during timed intervals as described in detail by Maekawa et al. (2002). The amount of saliva added to the masticate was calculated as the difference in moisture of the collected bolus and the feed offered as follows: saliva (ml) = weight of bolus (g) – weight of feed as fed (g). The total amount of saliva secreted during rumination each day was calculated similarly; however, the rate of salivation during rumination was assumed to be the same as that during eating (Bailey and Balch, 1961; Seth et al., 1974), because it was not possible to measure this directly. The total amount of saliva secreted during resting each day was calculated by multiplying resting salivation rate (ml/min) by the time (min) spent resting each day. Resting salivation rate was measured by collecting swallowed saliva during timed intervals between feeding times, as described in detail by Maekawa et al. (2002). During this collection phase, feed was removed to prevent eating, and water was turned off to prevent cows from drinking. Volume of saliva was measured immediately after each collection and resting salivation rate was calculated by dividing the total amount of saliva collected by the duration of each collection. Ruminal pH Ruminal pH was measured continuously for 24 h using an indwelling electrode, as described by Maekawa Journal of Dairy Science Vol. 85, No. 5, 2002

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MAEKAWA ET AL. Table 1. Milk production and composition. Item

Multiparous

Primiparous

SE

P-value

DMI, kg BW, kg DMI, % of BW Yield, kg/d Milk 4% FCM Fat Protein Lactose Composition, % Fat Protein Lactose

19.2 639 3.03

17.1 590 2.90

0.4 5 0.06

<0.01 <0.01 0.15

29.33 28.01 1.09 0.95 1.35

25.03 24.21 0.95 0.83 1.19

0.53 0.54 0.26 0.02 0.03

<0.01 <0.01 <0.01 <0.01 <0.01

3.75 3.33 4.56

3.84 3.32 4.76

0.07 0.02 0.02

0.43 0.88 <0.01

et al. (2002). Ruminal pH data were summarized for each cow in each period as daily mean pH, maximum pH, minimum pH, minutes pH was below 5.8, and area between the curve and pH 5.8. The area was calculated by adding the absolute value of negative deviations in pH from pH 5.8 for each 15-min interval. Data from one cow in period 4 was not used due to problems with the pH electrode.

where

Rate of Passage of Rumen Liquid Fraction

Significance was declared at P < 0.05 and trends are discussed at P < 0.15. Because there were no interactions between diet and parity of cows, only the least squared means for parity are presented. Effects of dietary treatments were reported separately (Maekawa et al., 2002).

The rate of passage of the liquid fraction from the rumen was measured using Co-EDTA, as described by Maekawa et al. (2002). The last day of each period, rumen contents were completely removed and weighed. Samples were taken for DM analysis to calculate rumen liquid volume. The digesta from each cow were switched to the cow receiving that diet the next period to minimize the need for a lengthy adaptation period. Statistical Analyses For each period, means for individual cows were calculated for all variables. Data were analyzed as a double 4 x 4 Latin square using the general linear model procedure in SAS (1989) to account for the effect of parity, cow within parity, period within parity, diet, and the interaction between parity and diet. Because there were no interactions between diet and parity of cows, only the effects of parity are reported. The effect of diet is reported separately (Maekawa et al., 2002). To analyze the effect of stage of lactation on DMI and eating saliva, cow and period effects were eliminated from the analyses, and the data were analyzed as a randomized complete block design, using the general linear model procedure of SAS (1989). The effect of block was assumed to be the effect of parity. The model used was: Yij = µ + Si + Ln + (SL)in + ein Journal of Dairy Science Vol. 85, No. 5, 2002

= = = =

overall mean, random effect of parity (i = 1 to 2), effect of stage of lactation (n = 1 to 3), interaction of parity and stage of lactation, and ein = residual.

µ Si Ln (SL)in

RESULTS DMI, BW, Milk Yield, and Composition Dry matter intake was higher for MP cows than for PP cows, but MP cows were heavier than PP cows (Table 1). Higher DMI of MP cows reflected the heavier BW of the MP cows, because there was no difference between groups when DMI was expressed as percentage of BW. Multiparous cows produced 4.3 kg/d (17%) more milk than PP cows (Table 1). Even after correction for fat content, MP cows produced 17% more milk than PP cows. Yield of fat, protein, and lactose were also higher for MP cows compared with PP cows. However, differences in fat and protein yields between parity groups were due to differences in milk yield, not differences in component percentages. Multiparous cows produced milk with lower lactose percentage than PP cows, and, consequently, lactose yield was lower for MP cows. Chewing Activity Multiparous cows spent 47 min/d longer eating than PP cows (Table 2). Multiparous cows also tended (P =

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CHEWING ACTIVITY, SALIVA PRODUCTION, AND PARITY Table 2. Chewing activity of multiparous and primiparous lactating dairy cows. Item Eating Min/d Min/kg of DM Rumination Min/d Min/kg of DM Min/kg of ADF Min/kg of NDF Numbers of periods Periods/kg of DM Length of periods, min Total chewing, min/d Resting Min/d

Multiparous

Primiparous

260 13.8

213 12.4

8 0.5

<0.01 0.07

560 29.3 163 99 14.1 0.75 40.7 821

508 29.6 164 100 15.9 0.94 32.7 720

17 0.9 5 3 0.6 0.03 1.7 19

0.05 0.78 0.81 0.80 0.05 <0.01 0.01 <0.01

619

720

20

<0.01

0.07) to spend 1.4 min more eating per kilogram of DM than PP cows. Rumination time was also greater for MP cows than for PP cows. Multiparous cows spent 52 min/d longer ruminating than PP cows. However, increased rumination time likely reflected the higher DMI of MP cows, because time spent ruminating per kilogram of DM, ADF, or NDF intake was similar between groups. Despite longer rumination time per day, MP cows had fewer rumination periods than PP cows. Rumination periods per kilgram of DM were fewer for MP cows compared with PP cows. However, MP cows ruminated 8 min longer each period compared with PP cows. As a result of longer eating time and longer rumination time, MP cows spent more than 1.5 h longer each day chewing (Table 2). Consequently, MP cows spent less time resting compared with PP cows. Saliva Production Salivation rate during eating was similar for MP and PP cows (Table 3). However, salivation rate during resting tended (P = 0.06) to be higher for MP cows than for PP cows. Daily total secretions of saliva during resting and rumination were similar for both groups. However, MP cows secreted more saliva per day during eating

SE

P-value

than did PP cows. However, because the quantity of saliva secreted during eating represented only approximately 22% of the total daily saliva production, MP cows had only numerically higher total saliva secretion per day than PP cows (P = 0.16).

Ruminal pH The ruminal pH variables measured were similar for both groups, even though mean pH was 0.11 units lower for MP cows than for PP cows (Table 4). Multiparous cows also remained under pH 5.8 about 1.2 h/d longer (P = 0.25) and minimum pH was 0.05 units lower (P = 0.39) than for PP cows. The pattern of diurnal fluctuation of ruminal pH was similar between parities (P = 0.10) (Figure 1). Ruminal pH decreased immediately after the 1500 h meal, then increased during the night and decreased again after the morning meal. Multiparous cows showed greater decline in ruminal pH than PP cows after the afternoon meal, although mean pH from 1500 to 0245 h was similar for both groups (Table 4). However, mean pH calculated for the period of 0300 to 1445 h was lower for MP cows than for PP cows.

Table 3. Saliva production during eating and resting for multiparous and primiparous lactating cows. Item Eating Salivation rate, ml/min Resting Salivation rate, ml/min Saliva output, L/d Eating Resting Ruminating1 Total

Multiparous

Primiparous

SE

P-value

225

226

11

0.93

114

88

9

0.06

56 70 125 252

49 63 115 227

2 6 8 12

0.02 0.36 0.44 0.16

1

Rate of salivary secretion was assumed to be the same during ruminating as during eating. Journal of Dairy Science Vol. 85, No. 5, 2002

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MAEKAWA ET AL. Table 4. Ruminal pH of multiparous and primiparous lactating dairy cows. Item

Multiparous

Primiparous

SE

P-value

Mean pH Minimum pH Maximum pH Area under pH 5.8, pH × h/d Time under pH 5.8, h/d Percent of time under pH 5.8, % Mean pH from 15:00 to 02:45 Mean pH from 03:00 to 14:45

5.84 5.25 6.54 3.54 11.42 47.63 5.74 5.94

5.95 5.30 6.56 3.20 8.93 37.18 5.75 6.15

0.06 0.04 0.05 0.57 1.33 5.59 0.07 0.06

0.25 0.39 0.77 0.71 0.25 0.25 0.97 0.05

Rumen Liquid Turnover The MP cows had higher rumen digesta weight than the PP cows, due to the higher liquid content because the amount of DM in the rumen was similar for both groups (Table 5). The MP cows had higher liquid outflow rate than PP cows and they also had higher liquid turnover rate. Higher liquid content in the rumen of MP cows compared with PP cows was associated with larger rumen volume due to body size; as the relation between rumen liquid volume and BW was similar for both groups. DISCUSSION Dry matter intake was positively correlated with BW, thus MP, larger cows ate more than PP, smaller cows as reported previously (Kertz et al., 1991; Beauchemin and Rode, 1994; Dado and Allen, 1994). Milk production was also positively correlated with DMI, accounting for the higher milk and FCM yield of MP cows than PP cows, as reported previously by others (Colenbrander et al., 1991; Dado and Allen, 1994; Beauchemin et al., 1997). However, effects of parity on milk composition are less clear. Colenbrander et al. (1991) fed cows alfalfa silage and corn grain and observed no difference in milk fat percentage between MP and PP cows, as was observed in the present study. However, when cows were fed alfalfa hay and barley concentrate, Beauchemin and Rode (1994) found that milk of MP cows contained similar fat content but less protein and less lactose than the milk of PP cows. In another experiment, Beauchemin and Rode (1997) fed diets consisting of

three percentages of NDF from barley silage combined with concentrates based on either barley or corn, and found that MP cows yielded more milk, of lower fat content, than did PP cows. This, milk composition is likely influenced more by factors such as stage of lactation of the cows, physical characteristics of the diet, and proportion of fiber in the diet (Robinson, 1989; Tessman et al., 1991; Coulon et al., 1994) than by parity. Chewing behavior was affected by parity, with higher eating and ruminating activity for MP cows than for PP cows. Previously, Beauchemin et al. (1997) found that MP cows spent more time eating than PP cows, although rumination time was similar for both groups. In contrast, Beauchemin and Rode (1997) reported that MP cows spent less time eating and more time ruminating than did PP cows, although MP cows spent less time eating and ruminating per kilogram of DM or NDF. The relationship between parity and time spent eating and ruminating may depend on the type of diet. With high fiber diets, cattle with a greater intake capacity (MP cows) tend to chew feed more efficiently during eating and ruminating, requiring less chewing time per unit of feed DM consumed, because the relationship between rumination capacity and body size is near unity (De Boever et al., 1990). This was not the case in the present study; MP cows did not chew more efficiently than PP cows. Beauchemin (1991) reported that the amount of saliva secreted daily was proportional to feed intake. In the present experiment, there was also a significant relationship between DMI and the total amount of saliva produced during eating (r = 0.58, P = < 0.01; Figure

Table 5. Passage rate of the rumen liquid fraction for multiparous and primiparous lactating dairy cows. Item

Multiparous

Primiparous

SE

P-value

Rumen digesta weight, kg Rumen content, kg of DM RLV1, L Liquid outflow, L/h Liquid turnover rate, %/h RLV/BW, L/kg

93 13.3 79 9.52 12.04 0.125

86 13.2 73 7.99 11.13 0.124

2 0.4 2 0.30 0.22 0.002

0.05 0.71 0.03 <0.01 0.01 0.85

RLV = Rumen liquid volume.

1

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CHEWING ACTIVITY, SALIVA PRODUCTION, AND PARITY

Figure 1. Diurnal fluctuation in ruminal pH of multiparous (dashed) and primiparous (solid line) lactating dairy cows. Each line represents the mean of 16 observations. Measurements were collected every 15 s and averaged for each 15-min interval. Arrows indicate feeding time.

2). Thus, higher salivary secretion during eating by MP cows compared with that of PP cows was partly in response to higher DMI. Cassida and Stokes (1986) reported that stage of lactation influenced saliva production, with reduced saliva production for cows in early lactation. However, stage of lactation also influences DMI. In the present study, stage of lactation influenced salivary secretion during eating to a lesser extent than DMI (Figure 3). The decrease in DMI for cows in late lactation was not accompanied by a decrease in salivary secretion during eating. Although salivation rate during resting was higher for MP cows, there was no affect of parity on total amount of saliva secreted during resting because MP cows spent less time resting than PP cows. The higher salivation rate during resting for MP cows may be an involuntary response to reduced time spent resting and the necessity to buffer the high level of fermentation acids. Although MP cows ruminated 52 min/d longer than PP cows, the net increase in saliva production was only about 5 L/d. This is because the 10-L increase in saliva output during rumination was accompanied by a 5-L decrease in saliva output during resting (Table 3). Even though only numerical differences between MP and PP cows were observed for the ruminal pH variables measured, these differences are likely to be of biological significance in terms of lowering fiber digestion. A larger study with a greater number of cows is needed to confirm the trend of lower ruminal pH for MP cows. Higher rumen liquid volume of MP cows compared with PP cows could be the result of higher production of saliva and higher water intake. Although in our experiment water intake was not measured, Dado and Allen (1994) found that MP cows drank 30% more water than PP cows. Increased liquid outflow rates, as was observed for MP cows compared with PP cows is thought to be beneficial, in terms of increased delivery of metab-

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Figure 2. Relationship between DMI and saliva secreted during eating. Eating saliva (L/d) = −0.51 + 2.59 ( DMI (kg/d), r = 0.58, P = < 0.01.

olizable protein to the lower gut (Russell and Hespell, 1981). CONCLUSION Salivation rate during eating was not affected by parity, but the greater eating time of MP cows resulted in greater saliva output during eating. However, saliva produced during eating only accounted for 22% of the total daily saliva output. Salivation rate during resting was higher for MP cows than for PP, which may be an involuntary response to reduced time spent resting and the necessity to buffer the rumen environment. Although MP cows ruminated more than PP cows, the net increase in saliva production was small, because of the accompanying decrease in resting saliva. Thus, daily saliva production was only numerically higher for MP cows. Furthermore, production of fermentation acids in the rumen was likely higher for MP cows due to higher feed intake, thus pH profiles of MP cows tended to be lower than for PP cows. Multiparous cows may have a greater risk of incurring acidosis than PP

Figure 3. Relationship between saliva secreted during eating (䊐)(SE = 4.66, 2.57, and 3.20, for early, medium, and late lactation, respectively) and DMI (SE = 1.01, 0.56, and 0.69, for early, medium and late lactation, respectively) and stage of lactation of dairy cows. Each point represents the mean of 5, 16, and 11 observations, for early, medium, and late lactation, respectively. Journal of Dairy Science Vol. 85, No. 5, 2002

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cows because the increased salivary secretion associated with increased chewing may not sufficiently compensate for the higher production of fermentation acids in the rumen due to higher feed intake. ACKNOWLEDGMENTS The authors wish to thank the staff of the Lethbridge Research Centre Dairy Unit for caring for the animals and G. Bowman, S. Eivemark, B. I. Farr, J. Bovinec, and J. Chang for their technical assistance during experimentation. The authors also thank to W. Yang and K. Koenig for reviewing this manuscript. Agriculture and Agri-Food Canada and the Canada/Alberta Livestock Research Trust Inc. funded the project. REFERENCES Bailey, C. B., and C. C. Balch. 1961. Saliva secretion and its relation to feeding in cattle. II. The composition and rate of secretion of mixed saliva in the cow during rest. Br. J. Nutr. 15:371–382. Beauchemin, K. A. 1991. Ingestion and mastication of feed by dairy cattle. Vet. Clin. North Am. Food Anim. Pract. 7:439–463. Beauchemin, K. A., and L. M. Rode. 1994. Compressed baled alfalfa hay for primiparous and multiparous dairy cows. J. Dairy Sci. 77:1003–1012. Beauchemin, K. A., and L. M. Rode. 1997. Minimum versus optimum concentrations of fiber in dairy cow diets based on barley silage and concentrates of barley or corn. J. Dairy Sci. 80:1629–1639. Beauchemin, K. A., L. M. Rode, and W. Z. Yang. 1997. Effects of nonstructural carbohydrates and source of cereal grain in high concentrate diets of dairy cows. J. Dairy Sci. 80:1640–1650. Campling, R. C., and C. A. Morgan. 1981. Eating behavior of housed dairy cows, a review. Commonwealth Bureau of Dairy Sci. and Tech. 43:57–63. Cassida, K. A., and M. R. Stokes. 1986. Eating and resting salivation in early lactation dairy cows. J. Dairy Sci. 69:1282–1292.

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Colenbrander, V. F., C. H. Noller, and R. J. Grant. 1991. Effect of fiber content and particle size of alfalfa silage on performance and chewing behavior. J. Dairy Sci. 74:2681–2690. Coulon, J. B., C. Ababriel, G. Brunswig, C. Muller, and B. Boinaiti. 1994. Effects of feeding practices on milk fat concentration for dairy cows. J. Dairy Sci. 77:2614–2620. Dado, R. G., and M. S. Allen. 1994. Variation in and relationships among feeding, chewing, and drinking variables for lactating dairy cows. J. Dairy Sci. 77:132–144. De Boever, J. L., J. I. Andries, D. L. De Brabander, B. G. Cottyn, and F. X. Buysse. 1990. Chewing activity of ruminants as a measure of physical structure. A review of factors affecting it. Anim. Feed Sci. Technol. 27:281–291. Grant, R. J., and J. L. Albright. 1995. Feeding behavior and management factors during the transition period in dairy cattle. J. Anim. Sci. 73:2791–2803. Kay, R. N. 1966. The influence of saliva on digestion in ruminants. World Rev. Nutr. Diet. 6:292–325. Kertz, A. F., L. F. Reutzel, and G. M. Thomson. 1991. Dry matter intake from parturition to midlactation. J. Dairy Sci. 74:2290– 2295. Maekawa, M., K. A. Beauchemin, and D. A. Christensen. 2002. Effect of concentrate level and feeding management on chewing activities, saliva production, ruminal pH of lactating dairy cows. J. Dairy Sci. 85:1176–1182. Robinson, P. H. 1989. Dynamic aspects of feeding management for dairy cows. J. Dairy Sci. 72:1197–1209. Russell, J. B., and R. B. Hespell. 1981. Microbial rumen fermentation. J. Dairy Sci. 64:1153–69. SAS User’s Guide: Statistics, Version 6 Edition. 1989. SAS Inst., Inc., Cary, NC. Seth, O. N., G. S. Rai, P. C. Yadav, M. D. Pandey, and J. S. Rawat. 1974. Effect of diet and rumination on the rate of secretion and chemical composition of parotid saliva of Bubalus bubalis and Bos indicus. Indian J. Anim. Sci. 44:717–724. Silanikove, N., and A. Tadmor. 1989. Rumen volume, saliva flow rate, and systemic fluid homeostasis in dehydrated cattle. Am. J. Physiol. 256:R809–R815. Tessman, N. J., H. D. Radloff, J. Kleinmans, T. R. Dhiman, and L. D. Satter. 1991. Milk production response to dietary forage:grain ratio. J. Dairy Sci. 74:2696–2707.