Physiological and Production Responses to Feeding Schedule in Lactating Dairy Cows Exposed to Short-Term, Moderate Heat Stress
K. H. Ominski,* A. D. Kennedy,† K. M. Wittenberg,†and S. A. Moshtaghi Nia†

*Manitoba Agriculture and Food, Teulon, Manitoba, Canada R0C 3B0
†Department of Animal Science, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2

ABSTRACT

The objective of this research was to characterize the production responses of lactating dairy cows during and after short-term, moderate heat exposure, and to determine whether evening (p.m.) feeding would alleviate the associated production losses. In a two-period, crossover design, eight mature lactating cows were fed a total mixed ration at either 0830 or 2030 h. Each 15-d period consisted of a 5-d thermoneutral phase, a 5-d heat stress phase and a 5-d thermoneutral recovery phase. Mean daily vaginal temperature and respiration rate increased by 0.6 ± 0.04°C and 27 ± 1.3 breaths/ min, respectively, during short-term heat exposure. Daily dry matter intake, milk yield and solids-not-fat were depressed by 1.4 ± 0.13 kg, 1.7 ± 0.32 kg and 0.07 ± 0.023%, respectively, during heat exposure. During the recovery phase, dry matter intake remained depressed, milk protein declined by 0.05 ± 0.020%, and daily milk yield exhibited a further decline of 1.2 ± 0.32 kg. Time of feeding had no effect on vaginal temperature, respiration rate, dry matter intake, water intake, milk yield, fat-corrected milk, protein percent, solidsnon- fat percent or somatic cell count during heat exposure or during the recovery period that followed. Fat percent was, however, significantly lower in p.m.-fed animals. These data indicate that short-term, moderate heat stress, which occurs during the spring and summer months in Canada and the Northern United States, will significantly decrease production in the lactating cow. Shifting from morning to evening feeding did not alleviate production losses associated with this type of heat stress.

(Key words: heat stress, time of feeding, vaginal temperature, milk yield)

Abbreviation key: HS = heat stress, RR = respiration rate, THI = temperature-humidity index, TN = ther moneutral, TN-R = thermoneutral recovery, Tv = vaginal temperature.

INTRODUCTION

Over the past several decades, an abundance of research has been conducted to characterize and alleviate the detrimental effects of heat stress on milk production, reproduction, and health in lactating dairy cows. Much of this research has been conducted under conditions of moderate to severe heat stress, as defined by Armstrong (1994), for periods of 4 to 5 mo. These conditions, which are typical in climates such as those found in Florida, Arizona, Texas, Georgia, and Israel, differ dramatically from the mild to moderate stress that occurs in temperate climates for periods of 1 d to 1 wk during the spring and summer months. Although production losses associated with mild to moderate heat stress have been recognized as problematic in temperate climates such as Canada and the Northern United States, there is little, if any, information available regarding animal response during and after short-term exposure to mild or moderate heat. It has been speculated that the episodic thermal stress that occurs in temperate regions may pose serious problems because animals have not adapted physiologically to the heat stress conditions (Beede and Collier, 1986; Beede and Shearer, 1996). This is not unexpected because the upper critical temperature for lactating cows is in the range of 24 to 27°C (Fuquay, 1981; Berman et al., 1985) or when the THI exceeds 72 (Armstrong, 1994). In the midprairie region of Canada, where the relative humidity is usually between 35 and 55%, a THI of 72 will be reached when the temperature is in the mid 20’s. Based on Environment Canada meteorological summaries, dairy cows in Western Canada would experience heat stress (temperature > 25°C) on 40% of summer days.

Several feeding management strategies to alleviate heat stress have been researched and utilized successfully in subtropical climates. The value of such strategies in temperate climates remains unknown and may be questionable, given the differences that exist in the duration and intensity of the heat.

Evidence suggesting that cows may respond differently when in different environments has been provided by Higginbotham et al. (1989b), who showed that cows subjected to hot environmental temperatures (weekly mean ambient maximum and minimum temperatures were 35.1 and 18.2°C, respectively) yielded less milk on a high protein, high degradability diet compared with those on a high protein, medium degradability diet, or medium protein diets with high or low degradability. Milk yields were not affected by percent protein or by protein degradability at moderate temperatures (weekly mean ambient maximum and minimum temperatures were 26.8°C and 9.1°C, respectively) (Higginbotham et al., 1989a).

Feeding during the cooler hours of the day or at night is one technique that has been recommended by several researchers and nutritionists (Beede and Shearer, 1991, 1996; Dildey, 1996; Hutjens, 1998) and has been utilized by Florida dairy producers (Beede and Shearer, 1996) to alleviate the detrimental effects of heat stress. In doing so, the heat of fermentation associated with eating should occur during the cooler night hours rather than during the more intense midday heat. To the authors’ knowledge, no dairy trials in which feed was delivered at night have been conducted to confirm these recommendations.

The objective of this research was to characterize animal response to short-term, moderate heat exposure by monitoring daily body temperature, respiration rate and production, as well as to determine whether evening feeding would influence diurnal body temperature and thus alleviate the decline in production resulting from exposure to these conditions.

MATERIALS AND METHODS

Eight mature, lactating Holstein cows averaging 132 ± 14.4 DIM, producing 37.4 ± 0.64 kg of milk were assigned to one of two feeding regimens on the basis of milk yield. The experiment was a two-period, crossover design, with four cows assigned to each feeding regimen in each period. Feeding regimens were once a day feeding of aTMR(Table 1) at either 0830 or 2030 h. Each 15- d period consisted of a 5-d thermoneutral (TN) phase, a 5-d heat stress (HS) phase, and a 5-d thermoneutral recovery phase (TN-R), as depicted in Figure 1. Period 1 was followed by a 4-d rest period during which the animals were acclimated to the change in time of feeding. During the TN and TN-R phases ambient temperature was set at 24°C from 0700 to 1800 h and at 20°C for the remainder of the day. During the HS phase, ambient temperature was set to rise gradually to 32°C between 0700 to 1000 h, remained at 32°C until 1800, and was lowered to 20°C for the remainder of the day.

	% of DM
Ingredient
Alfalfa hay	43.3
Steam rolled barley	27.6
Steam rolled corn	5.2
Wheat shorts	5.7
Canola meal	5.5
Distillers dried grain	3.3
Soybean meal	2.1
Meat meal	1.0
Blood meal	0.5
Fish meal	0.3
Wheat	1.0
Tallow	2.0
Beet molasses	0.1
Mold inhibitor	0.1
Cobalt-iodized salt	0.4
Potassium and magnesium sulfate	0.4
Limestone	0.3
Dicalcium phosphate	0.4
Sodium bicarbonate	0.2
Vitamin-muneral premix2	0.5
	%of Dm
Nutrient content
DM(89.7%)	100
CP	18.2
ADF	18.3
NDF	29.7
Ca	1.08
P	0.42
Mg	0.27
K	1.35
Na	0.19
	Mcal/kg DM
NE L2	1.75

1 Provides (per kg of concentrate): vitamin A (13,050 IU), vitamin D (2300 IU), vitamin E (46 IU), copper (46 mg), manganese (120 mg),zinc (152 mg) and selenium (41 mg).2Estimated value (NRC, 1989).

Temperature and relative humidity were recorded at 20-min intervals using an HMP35C temperature and RH probe (Campbell Scientific, Edmonton, Alberta, Canada) attached to a PC208W datalogger (Campbell Scientific). Temperature-humidity indices (THI) were calculated according to West (1994) where:

THI = td − (0.55 − 0.55RH)(td − 58)

with td = dry bulb temperature (°F, where °F = (°C + 9/5) + 32), and RH = relative humidity expressed as a decimal. Cows were maintained in raised metabolic crates in a well-ventilated room in the Animal Science Research Unit located at the University of Manitoba, Winnipeg, Manitoba, Canada, and were handled in accordance with the guidelines established by the Canadian Council on Animal Care. Artificial light was the sole source of illumination from 0530 to 2200 h.

Figure 1. Ambient temperature-humidity index (THI) achieved during the thermoneutral (TN), heat stress (HS) and recovery (TNR) phases of each 15-d period.

Diets were formulated to meet NRC requirements (National Research Council, 1989) for a 625-kg cow producing 40 L milk/d and provided 18.2% CP and 1.75 Mcal of NEL kg-1 DM (Table 1). The forage-to-concentrate ratio was 43:57 (DM basis). The TMR was fed to individual cows on an ad libitum basis; feed offered was adjusted daily to ensure approximately 10% orts. The forage and concentrate components of the diet were sampled twice during the TN and HS phases of each period. Samples were composited on the basis of phase for each period, dried by forced-air oven (60°C), ground with a Wiley mill to pass through a 1-mm screen and analyzed for ADF and NDF according to Goering and Van Soest (1970) and CP using Kjeldahl N according to the Association of Official Analytical Chemists, method no. 984.13 (AOAC, 1990). Calcium and phosphorus were determined after dry ashing at 550°C for 12 h, followed by flame atomic spectroscopy (AA/E spectrophotometric model 551, Instruments Laboratory Inc., Willmington, MA). Orts were removed on a daily basis before the next feeding, subsampled, and analyzed for DM using forced air at 60°C for 48 h. Intake patterns of individual cows were recorded for two 24-h intervals during the TN phase and the HS phase of both periods. Intake was recorded at 0.5, 1.5, 3.5, 7.5, 11.5, 15.5, 19.5, and 23.5 h postfeeding.

Cows were milked at 0600 and 1600 h daily using a Porta-Milker milking system (The Coburn Company, Inc., Whitewater, WI). At each milking, milk weights were recorded and representative samples were collected and submitted to an accredited lab (#125) ISO Guide 25, for fat, protein, and SNF analysis (Milk-OScan 303AB, Foss Electric, Hiller?d, Denmark) and SCC (Fossomatic 300, Foss Electric).

Vaginal temperature (Tv) was recorded using a temperature radiotelemetric system, as described in Redden et al. (1993). The radiotransmitters were inserted into the vagina 3 d before the start of the trial. Vaginal temperatures were recorded automatically at approximately 4-min intervals. Respiration rate (RR) was measured visually at 3-h intervals from 1000 to 2200 h during the TN and HS phases and was recorded as number of breaths per minute.

Water intake was recorded twice daily (0900 and 2100 h) for each cow using individual water meters (Sensus SR11, 5/8″, Uniontown, PA).

Data for mean hourly Tv, as well as mean daily RR, DMI, water intake, milk yield and milk composition were analyzed as a crossover design using the analysis of variance procedure of SAS (Cary, NC), with four animals assigned to each of two feeding regimens in each of two periods. Effect of cow, feeding regimen, and period were tested against the cow ? feeding regimen ? period error term. Phase and feeding regimen ? phase were tested against the cow ? feeding regimen ? period ? phase error term. Mean comparisons were performed using Bonferroni’s mean separation test when P < 0.05. Trends were determined at P < 0.10 unless otherwise indicated. The model for the 24-h intake pattern also included feeding regimen ? hour and hour ? period in the model. These latter terms were tested against the residual error term. Mean hourly Tv data collected during the TN, HS, and TN-R phases was also compared for three time intervals over the course of a 24-h period (1800 to 200 h, 200 to 1000 h, and 1000 to 1800 h). Effect of cow, feeding regimen, and period were tested against cow ? feeding regimen ? period interaction to determine differences during these time periods.

RESULTS

The mean daily THI achieved for the TN, HS, and TN-R phases during the two periods were 69.3 ± 0.5, 74.1 ± 0.3, and 69.5 ± 0.9 for period 1 and 68.3 ± 0.2, 72.8 ± 0.1, and 67.8 ± 0.1 for period 2, respectively. Hourly mean THI for periods 1 and 2 are given in Figure 1. Mean 5-d Tv were different during the TN, HS, and TN-R phases (P < 0.001) as indicated in Table 2. Exposure to HS conditions resulted in a 0.6°C rise in mean

Table 2. Mean 5-d (n = 8 ) vaginal temperature (Tv) and respiration rate of a.m.- and p.m.-fed cows during thermoneutral (TN), heat stress (HS), and recovery (TN-R) phases.

		Time of feeding							Significance
		AM			PM
	TN	HS	TN-R	TN	HS	TN-R	SE	Time of feeding	Phase	Time of feeding *phase
Tv (°C)	37.9	38.6	37.9	38.0	38.6	37.9	0.05	NS	p<0.001	NS
RR(breaths/min)	60	87	-	61	81	-	1.88	NS	p<0.001	NS

daily Tv. Upon return to thermoneutral conditions, Tv was 0.1°C below that in the initial TN phase. Time of feeding had no effect on mean daily Tv nor was the time of feeding ? phase interaction significant.

Response of Tv to heat exposure in a 24-h period is depicted in Figure 2. Ambient THI was at a maximum from 1100 h until 1800 h, and Tv continued to rise sharply through this time. At 1800 h, Tv declined concurrently with ambient THI.

Further analysis of the data during specific intervals of the HS phase (Table 3) indicated that the Tv of p.m.- fed animals was 0.2°C higher from 0200 to 1000 compared with the a.m.-fed animals (P = 0.10) and 0.2°C lower from 1800 to 0200 h (P = 0.07).

Respiration rate increased (P < 0.001) from 60 breaths per minute during the TN phase to 87 breaths per minute as a result of exposure to HS (Table 2). Time of feeding, however, had no effect on RR, nor was the interaction with phase significant.

Differences in DMI were observed during TN, HS, and TN-R phases (P < 0.001, Table 4). Heat exposure resulted in a 6.5% decrease in DMI, which remained significantly depressed during the TN-R phase. Time of feeding had no effect on overall DMI; however, the interaction of time of feeding ? phase was significant (P < 0.05, Table 4). During the TN phase, DMI of p.m.- fed cows was 22.0 kg/d compared with 21.0 kg/d in the a.m.-fed animals. A 1.9-kg depression in DMI was observed as a result of exposure to HS in p.m.-fed cows. This depression in intake was still apparent for p.m.- fed cows during the TN-R phase, as intake remained at 20.1 kg/d during the 5-d recovery phase. There were no differences in DMI in a.m.-fed cows during TN, HS, and TN-R phases.

Table 3. Mean vaginal temperature (Tv, °C) of a.m.- and p.m.-fed cows (n = 8 ) during three intervals of the 5-d heat phase.

Time of feeding
AM PM SE Significance
1800 to 200 h	38.6	38.4	0.05	p=0.07
200 to 1000 h	38.1	38.3	0.06	p=0.10
1000 to 1800 h	38.9	39.0	0.04	NS

The DMI pattern over a 24-h period was affected by time of feeding at several points in time during the TN (P < 0.05) and HS (P < 0.001) phases (Figure 3). During the TN phase, a.m.-fed animals consumed lessDMthan p.m.-fed animals at 1.5 (P < 0.05), 3.5 (P < 0.001), 7.5 (P < 0.05), and 11.5 h (P < 0.05) postfeeding. This same trend was still apparent but more pronounced during the HS phase, as a.m.-fed animals consumed less DM than p.m.-fed animals at 1.5 (P < 0.05), 3.5 (P < 0.001), 7.5 (P < 0.001), and 11.5 (P < 0.001) postfeeding. Between 7.5 and 15.5 h postfeeding during the HS phase; however, a.m.-fed animals exhibited a sharp increase in DMI and by 15.5 h consumed more feed than p.m.- fed animals (P < 0.05). Accumulated 24-h DMI in a.m.- fed animals remained greater than that of p.m.-fed animals at 19.5 (P < 0.01) and 23.5 h (P = 0.08).

Time of feeding did not affect 24-h water intake; however, differences in water intake were observed during the TN, HS, and TN-R phases (P < 0.05, Table 4). Specifically, total daily water intake was similar during the TN and HS phases and reduced during the TN-R phase. Further analysis of water intake indicated that a.m.-fed animals consumed more water during the daytime hours (P < 0.001) and less during the nighttime hours (P < 0.001) compared with p.m.-fed animals. Daytime intake differences were also observed during the TN, HS, and TN-R phases (P < 0.001), as water intake was less during the TN-R phase compared with that consumed during the TN and HS phases (Table 4). Water intake during the night tended to be higher for the TN-R phase (P = 0.07), relative to the TN and HS phases.

In addition to the responses observed above, several production responses to heat stress were found. Differences (P < 0.001) in milk yield were observed when cows were exposed to the TN, HS, and TN-R phases (Table 5). Milk production decreased by 4.8% when animals were exposed to HS compared with that produced during the TN phase. Production then declined an additional 3.6% during the TN-R phase. As such, milk production during the TN-R phase was 8.2% lower than that of the initial TN phase. Time of feeding, however, had no effect on milk production, nor was there a time of feeding ? phase interaction.

Table 4. Dry matter intake and water intake of a.m.- and p.m.-fed cows (n = 8 ) during thermoneutral (TN), heat stress (HS) and recovery (TN-R) phases.

Times of feeding Singnificance
AM PM
TN HS TN-R TN HS TN-R SE Times of feeding Phase Time of feeding * Phase
DMI(kg/day)	21.0	20.1	20.7	22.0	20.1	20.1	0.3	NS	P<0.001	P<0.05
Water intake
24 h, L/d	105.1	106.6	99.9	107.6	111.9	102.1	2.64	NS	P<0.05	NS
900-2100 h,L	61.9	61.4	52.3	47.9	49.8	39.6	1.83	P<0.001	P<0.001	NS
2100-900 h,L	43.3	45.2	47.6	59.7	62.1	62.4	1.46	P<0.001	P=0.07	NS

a,bMeans in the same row with different letters differ, P < 0.05.

A similar trend was also apparent for FCM. Neither time of feeding nor time of feeding ? phase affected FCM (Table 5), but differences were observed during the TN, HS and TN-R phases (P < 0.01). Fat-corrected milk production was lower during the TN-R phase relative to the TN phase, but neither differed from that observed during the HS phase.

Component analysis of the milk (Table 5) indicated that although milk fat percent was lowered (P < 0.05) by p.m.-feeding, it was not significantly different during the TN, HS, and TN-R phases. Conversely, milk protein percent was not different between a.m.- and p.m.-fed animals, but was impacted by phase (P < 0.05). Although, milk protein (%) was not different during the TN and HS phases, a 2.5% decline was observed in the TN-R phase compared with that produced in the TN phase. Phase had an effect on SNF (P < 0.05), as it declined during the HS and TN-R phases. Solids-notfat also tended to be higher for the p.m.-fed relative to the a.m.-fed animals (P = 0.08). Milk SCC (counts ? 103/ ml of milk) did not differ between a.m.- and p.m.-fed animals nor among TN, HS, and TN-R phases. Time of feeding ? phase interactions were not significant for any of the components or for SCC.

A significant period effect was observed for several variables including RR (P < 0.05), Tv (P < 0.01), water intake during the day (P < 0.01), and milk fat (%) (P < 0.05) which were higher in period 1 relative to period

2. Conversely, 24-h DMI during the TN phase (P < 0.01) and SNF (%) were higher in period 2 (P < 0.05). The authors are unable to determine if the observed period effects were due to differences in ambient THI during periods 1 and 2 or due to animal acclimation to the heat stress conditions.

DISCUSSION

The observed increase in Tv during the HS phase indicates that the ambient THI imposed in this trial was sufficient to cause heat stress in the cows. The lower Tv observed during the TN-R phase (37.9°C) compared with that of the TN phase (38.0°C) during period 1 might be related to a slightly lower ambient THI during the TN-R phase relative to the TN phase (Figure 1). Time of feeding did not appear to prevent the increased Tv associated with exposure to heat stress as there were no differences between the a.m.- and p.m.- fed animals in either average 24-h Tv over the 15-d trial or during the hours of highest THI in the HS phase (1000 to 1800 h). The p.m.-fed cows had twice the decline in Tv compared with that of a.m.-fed cows (−0.6°C vs. −0.3°C, respectively) in the 8 h immediately following exposure to high temperatures during the HS phase demonstrating that p.m.-fed animals were able to cool down more quickly than a.m.-fed cows following exposure to heat (1800 to 0200 h). Morning-fed animals,

Time of feeding Significant
AM PM
	TN	HS	TN-R	TN	HS	TN-R	SE	Time of feeding	phase	Time of feeding * phase
Milk yield(kg)	34.9	34.0	32.3	35.9	33.5	32.7	0.45	NS	p<0.001	NS
FCM(kg)	34.7	33.9	32.7	31.9	30.3	29.4	0.62	NS	p<0.01	NS
Fat(%)	3.74	3.78	3.79	3.45	3.56	3.61	0.10	p<0.05	NS	NS
Protein(%)	3.11	3.11	3.05	3.19	3.14	3.09	0.03	NS	p<0.05	NS
SNF(%)	8.44	8.37	8.35	8.54	8.48	8.41	0.03	p<0.08	p<0.05	NS
SCC(counts * 103/mL)	730	1029	447	535	864	531	98	NS	NS	NS

(Figure 2. Ambient temperature-humidity index (THI, —) and mean vaginal temperature (Tv, —, n = 8 ) during the 5-d HS phase mean of periods 1 and 2). M = Milking time.)

however, still tended to achieve the lowest average Tv. Examination of intake pattern (Figure 3) shows that a.m.-fed cows consumed 6 kg of feed within the first 1.5 h after feeding and then decreased their rate of intake

(Figure 3. Cumulative 24-h DMI (kg/d) for a.m.- (.) and p.m.- (▼) fed cows (n = 8 ) during the thermoneutral (T.N.) and heat (H.S.) phases (mean of periods 1 and 2, TN SE = 0.31, HS SE = 0.25). Asteriks (*) denote significant differences (P < 0.05) between a.m.- and p.m.- fed animals. Time of imposed heat was 1.5 to 9.5 h postfeeding for a.m.-fed animals and 13.5 to 21.5 h postfeeding for p.m.- fed animals.)

during the hot part of the day. A sharp increase in intake occurred as the temperatures cooled. In the 8 hperiod following the imposed heat, a.m.-fed animals were recovering from heat stress, while also generating additional heat of fermentation from the increased rate of intake. Evening-fed animals, which were fed 2 h after temperatures had declined, consumed a large portion of their feed within the first 4 h postfeeding. As such, the heat of fermentation was probably manifested from 0200 to 1000 h the following day and thus accounted for the higher vaginal temperature in the p.m.-fed cows during this time.

The increase in RR observed from the TN phase to the HS phase is characteristic of heat stressed animals that thermoregulate by increasing evaporative heat loss from the upper respiratory passages (Thatcher and Collier, 1982). It appears that evening feeding offered little advantage in terms of the amount of heat required to be dissipated through increased respiration, as RR did not differ between a.m.- and p.m.-fed animals.

A negative response in DMI of p.m.-fed cows was observed not only during the HS phase, but also during the TN-R phase (Table 5). This apparent disadvantage in DMI experienced by p.m.-fed cows may be attributed to the Tv disadvantage experienced by these animals from 0200 to 1000 h, as described above. Other researchers have demonstrated that for each 0.55°C increase in rectal temperature, intake of TDN declined 1.4 kg in cows exposed to various combinations of temperature and humidity above 18°C (Johnson et al., 1963). In the current study, feeding 2 to 3 h after temperatures had declined did not allow the p.m.-fed cows to achieve the same minimum core body temperature as that achieved with morning feeding. Several researchers have suggested that under hot conditions, the minimum body temperature in cattle may have a greater influence on subsequent intake than maximum body temperature (Hahn and Mader, 1997; Mader et al., 1997). This same phenomenon has been reported for daily milk yield whereby a decline in milk production was minimized with the occurrence of nighttime cooling (Igono et al., 1992). The continued depression in DMI that occurred in p.m.-fed animals during the TN-R phase suggests that although Tv had been restored, recovery was not immediate.

The observation of DMI over a 24-h period indicates that differences in pattern of consumption existed between a.m.- and p.m.-fed animals during the TN phase (Figure 3). These differences are more apparent during the HS phase. The decline in rate of intake in a.m.-fed animals coincided with the onset of increased ambient THI. Rate of intake remained depressed, while THI was elevated and increased as ambient THI declined. Conversely, rate of intake in p.m.-fed animals remained relatively uniform. Although both groups consumed most of their rations within 7.5 h after feeding, it was evident that a proportion of DMI of a.m.-fed cows was shifted toward cooler evening hours. Nighttime compensatory eating has been observed by several other researchers. Mallone?e et al. (1985) observed that cows in Florida that did not have access to shade ate 19% more during the nighttime than did cows with access to shade. Total feed intake, however, was still 13% less in nonshaded compared with shaded cows. Maust et al. (1972), Guthrie et al. (1968), and Rainey et al. (1967) observed nominal changes in DMI during heat stress, attributing this lack of response to increased intake during cooler nighttime hours. It is important to note that in above-mentioned trials, feed was offered at the same time for both shaded and nonshaded animals. To the authors’ knowledge, the current trial is unique in that the feed was delivered exclusively at night or in the morning.

Observed changes in 24-h water intake during the HS and TN-R phases can be explained by changes in DMI. Water intake during the HS phase did not differ from that consumed during the TN phase, but was reduced during the TN-R phase. However, when expressed as intake/kg of DM, water intake increased from 4.95 L/kg during the TN phase to 5.43 L/kg during the HS phase and returned to 4.95 L/kg during the TN-R phase. Increases in water intake resulting from exposure toHS conditions have been reported in several other studies (McDowell et al., 1969; NRC, 1981). Observed differences in daytime and nighttime intake between a.m.- and p.m.-fed cows can also be explained by DMI patterns. Daytime intake was significantly higher in a.m.-fed animals and nighttime intake was signifi- cantly higher in p.m.-fed animals.

Corresponding to the observed changes in DMI, the data presented here demonstrate that short-term exposure to moderate heat stress can cause a considerable reduction in milk production. Furthermore, milk production continued to decline although thermoneutral conditions had been restored in the TN-R phase. Therefore, moderate and episodic heat may cause greater losses in production than previously anticipated, as the negative effects may persist for several days following exposure to elevated temperatures. Unfortunately, evening feeding was unable to ameliorate these production losses. Production losses that occurred can be attributed to the imposed heat and not due to changes in lactation persistency as milk production levels eventually recovered and were similar during the TN phases of both periods 1 and 2 (data not shown).

Milk fat percentage was the only parameter in this trial for which significant differences between a.m.- and p.m.-fed animals were observed. Because total DMI, and therefore total energy intake, was not affected by evening feeding, the authors are unable to explain the shift in rumen energy metabolism or endocrine changes that may have led to the reduction in milk fat (%) observed in p.m.-fed animals. Furthermore, milk fat (%) is also the only variable in this trial that was not affected by phase. Although several researchers have observed a depression in milk fat (%) as a result of exposure to heat stress (Laben et al., 1963; Moody et al., 1967; Rodriquez et al., 1985), others have shown no effect on milk fat percent in changing environments (Maust et al., 1972; Mallone?e et al., 1985). Depressions in milk fat as a result of exposure to heat have been associated with a decline in forage intake and a subsequent shift in the acetate-to-propionate ratio (Collier, 1985). In the current study, feeding a TMR may have prevented a shift in the forage-to-concentrate ratio.

The reduction in milk protein (%) observed during the TN-R phase can be attributed to lower dietary energy and protein intake, which is a consequence of decreased feed intake (Collier, 1985). There appeared to be a lag in protein response as DMI decreased during the HS phase, but milk protein (%) did not decrease until the TN-R phase. In contrast, changes in SNF (%) resulting from exposure to heat stress more closely followed the observed decline in DMI. Other researchers have also observed a decrease in SNF (%) with increasing temperature (Cobble et al., 1951).

Neither feeding regimen nor phase had an effect on SCC. Other studies conducted in environmental chambers (Paape et al., 1973; Wegner et al., 1976) concur with these findings in that heat stress did not result in an elevated SCC.

CONCLUSION

These data indicate that short-term, moderate heat stress, such as that which occurs in temperate climates, can adversely affect production in the lactating dairy cow. Although shifting from morning to evening feeding has been utilized in subtropical climates, it offered no advantage in terms of alleviating production losses associated with moderate heat stress. Although evening feeding may have decreased fat (%) and increased SNF (%), it had a prolonged negative effect on DMI and milk production during the 5-d recovery period that followed heat stress conditions. As such, additional research is required to determine whether other nutritional strategies, which are successfully utilized in subtropical climates, are effective in temperate climates such as the Northern United States and Canada.

ACKNOWLEDGMENTS

The authors would like to thank the Dairy Farmers of Canada and the Natural Sciences and Engineering Research Council of Canada for providing financial support, MFC Testing and Research Inc for performing the component analysis of the milk samples, Deanne Fulawka for performing the statistical analysis, David Puff for providing technical assistance, and Manitoba Agriculture and Food for their cooperation with this project.

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Source: Journal of Dairy Science
Author: Ominski, Kennedy