Effects of Ruminally Inert Fat and Evaporative Cooling on Dairy Cows in Hot Environmental Temperatures

Effects of Ruminally Inert Fat and Evaporative Cooling on Dairy Cows in Hot Environmental Temperatures S. C. CHAN,1 J. T. HUBER,2 K. H. CHEN,3 J. M. SIMAS,4 and Z. WU5 Department of Animal Sciences, University of Arizona, Tucson 85721

ABSTRACT Under hot summer conditions of Tucson, Arizona, 24 Holstein cows ( X = 80 d of lactation) were assigned for 56 d to four treatments in a randomized block design with a 2 × 2 factorial arrangement of treatments. Factors were 1 ) medium [4.6% of dry matter (DM)] versus high (7.4% of DM) amounts of dietary fat and 2 ) corral shade only versus shade equipped with evaporative cooling. The high fat diet contained 3% prilled fatty acids. The efficiency of the conversion of feed to milk tended to be better for cows fed prilled fat than for cows fed medium dietary fat, but other lactation measurements were unaffected. Cows with access to evaporative cooling had greater milk yields than did cows with access to shade only. Prilled fatty acids did not depress the percentage of milk protein, but reduced short- and medium-chain fatty acids ( C 6:0 to C14:0) in milk fat and increased palmitic acid. Digestibilities of DM, organic matter, crude protein, acid detergent fiber, neutral detergent fiber, and starch were unaffected by amount of fat or by cooling method, but prilled fatty acids tended to decrease apparent digestibility of fatty acids. No differences were observed among treatments in respiration rates or rectal temperatures. When rectal temperatures were determined, cows were crowded, which probably negated detection of an effect of evaporative cooling. Evaporative cooling increased milk yield of cows in hot weather, but the addition of 3% fatty acids did not increase yield, and no interactions were observed. ( Key words: fat, heat stress, cooling, dairy cows)

Received September 12, 1995. Accepted November 15, 1996. 1Present address: Department of Animal Sciences, Chinese Culture University, Taipei, Taiwan 111. 2Reprint requests. 3Present address: Nutrena Feeds, 10365 Iona Avenue, Hanford, CA 93230. 4Present address: Department of Animal Sciences, Escola Superior Agricultura Luiz de Quieroz, Universidade de Sao Paulo, Piraciba, S.P., Brazil 13418-900. 5USDA Forage Research Laboratory, Madison, WI 53706. 1997 J Dairy Sci 80:1172–1178

Abbreviation key: BCS = body condition score, EC = S plus evaporative cooling, FA = fatty acids, HF = high fat, MF = medium fat, PFA = prilled FA, RR = respiration rates, RT = rectal temperatures, S = shade, THI = temperature-humidity index, WCS = whole cottonseed. INTRODUCTION Heat stress adversely affects lactational performance of dairy cows because of thermoregulatory responses resulting in increased respiration rates ( RR) , increased heart rates, reduced feed intake, lower nutrient absorption, and redirection of blood flow from internal to peripheral tissues in an effort to balance heat loads (20). Fat is high in energy density and low in heat increment; therefore, ruminally inert fat in the diets of cows under heat stress should result in higher energy concentrations without increased body temperature or adverse effects on ruminal fermentation, allowing for increased milk yield (13, 16) and greater lactational persistency (11). However, lower intakes resulting from supplemental fat may negate its beneficial effects (16). Supplementation of prilled fatty acids ( PFA) tended to increase milk yields of cows under moderate temperatures (21), and under heat stress ( 7 ) . Chen et al. ( 8 ) reported lower rectal temperatures ( RT) and RR, but higher DMI and milk yields (2.5 kg/d) for cows receiving shade ( S) plus evaporative cooling ( EC) than for cows under S only. The benefits of EC are consistent with results of other research (2, 3, 4, 23). However, studies are limited on the effects of supplemental fat in cows under heat stress, and interactions of added fat with EC are unknown. The objective of this study was to determine the effects of adding a ruminally inert fat (PFA) to the diets of cows receiving EC or S only. MATERIALS AND METHODS Twenty-four Holstein cows ( X = 80 ± 50 d of lactation) were assigned to one of four treatments in a

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completely randomized block design with a 2 × 2 factorial arrangement of treatments. Factors were 1 ) amount of dietary fat, either medium fat ( MF; 4.6% of total diet) or high fat ( HF; 7.4% of total diet), and 2 ) cooling method ( S vs. EC). Cows were fed the regular herd diet during the pretreatment period (14 d ) and were assigned randomly to four treatments based on milk yield during pretreatment. The treatment period was for 56 d during the hot summer months (June to August) of Tucson, Arizona. Treatments were 1 ) MF + S, 2 ) HF + S, 3 ) MF + EC, and 4 ) HF + EC. Treatment groups were balanced for parity and days of lactation. Diets were typical of those used in the region and contained alfalfa hay, whole cottonseed ( WCS) , soybean meal, cottonseed hulls, and steam-flaked sorghum grain (Table 1). The PFA was added to the HF diets at 3% of DM. Steam-flaking was accomplished by placing whole grain in a steam chamber for 40 to 50 min, which increased moisture to 16 to 20%, and then passing the grain through large rollers adjusted to achieve a flake density of 360 g/L. Cows were housed in open drylots equipped with S and had free access to water and trace-mineralized salt blocks. Cows were fed individually once daily in mangers equipped with electronically controlled gates (American Calan, Inc., Northwood, NH). Diets were fed as a total mixed diet. Total mixed diets were prepared by placing chopped alfalfa ( ∼10 cm stem length) into a truck with load cells and mixing augers. Other ingredients were added while augers were engaged. Mixing of the complete diet continued for about 5 min.

TABLE 1. Ingredient composition of the diets. Diet1 Ingredient

MF

Alfalfa hay Whole cottonseed Cottonseed hulls Soybean meal Steam-flaked sorghum grain2 Molasses, dehydrated Prilled fatty acids3 Minerals and vitamins4 Buffer5

39.0 7.0 8.5 7.3 33.3 1.5 . . . 2.0 1.4

1MF

HF ( % of DM) 38.1 7.0 8.5 8.5 30.0 1.5 3.0 2.0 1.4

= Medium fat; HF = high fat. g/L. 3Energy Booster-100 (Milk Specialties Co., Dundee, IL). 4Composition: 0.66% CaCO , 0.78% dicalcium phosphate, 0.35% 3 trace-mineralized salt, and 0.21% vitamins A (120,000 IU/kg) and D (15,000 IU/kg). 51% NaHCO and 0.4% MgO. 3 2360

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The amount of feed offered was adjusted daily to allow for 10% orts, which were weighed back daily. Cows were weighed on 2 consecutive d at the beginning and end of the trial. Cows were evaluated weekly for body condition scores ( BCS) on a fivepoint scale where 1 = thin to 5 = fat (26). One individual, who had no knowledge of which treatments cows received, was responsible for all BCS evaluations. Ambient temperatures and humidities were recorded hourly at the Arizona Meteorological Network weather station ( ∼1.5 km from the experimental site). The temperature-humidity index ( THI) was calculated from these data using a psychrometric chart ( 6 ) . The RR and RT of cows were measured weekly near the hottest time of the day. Cows were milked twice daily at 0700 and 1900 h, and milk yields were recorded daily. Individual milk samples were collected weekly from two consecutive milkings (a.m. and p.m.). Composite samples were analyzed at the Arizona DHIA Laboratory (Phoenix) for fat, protein, lactose, and total solids by infrared procedures (Foss 360; Foss Technology, Eden Prairie, MN); for SCC by a white blood cell counting method (Multispec 2; Foss Technology); and for SNF by calculating the difference between the percentage of total solids and fat. On wk 6, individual milk samples also were collected and frozen at –10°C until thawed and analyzed for fatty acids ( FA) . The FA were analyzed according to procedures for analysis of methyl esters as described by Sukhija and Palmquist ( 2 2 ) using GLC (VA 3300; Varian Associates, Inc., Walnut Creek, CA). During the last 14 d of the lactation trial, all diets were mixed with 0.1% chromium oxide, an indigestible marker, to estimate digestibility of nutrients. For the last 5 d, total mixed diets were sampled daily, all orts were collected, and fecal grab samples were taken twice daily immediately after milking and were frozen at –10°C. Fecal samples were composited for each cow as collected. A representative portion of total mixed diets and orts sampled during the last 5 d of the trial were composited for each cow and dried at 100°C for 24 h to determine DM. The remaining portion was dried at 50°C for 48 h and ground in a Wiley mill (Arthur H. Thomas, Philadelphia, PA) through a 2-mm screen. Subsamples were then ground in a Cyclone mill (Udy Co., Fort Collins, CO) to pass a 1-mm screen. The composited fecal sample from each cow was thawed and thoroughly mixed, and a subsample was dried at 50°C in a forced-air oven for 72 h. Dried fecal samples also were ground through the Wiley and Cyclone mills for the analysis of nutrients. Samples of total mixed diets, orts, and feces were analyzed for DM and OM according to AOAC ( 5 ) procedures; for CP by Kjeldahl digestion ( 5 ) and by Journal of Dairy Science Vol. 80, No. 6, 1997

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the N autoanalyzer (Bran and Luebbe, Analyzing Technologies, Elmsford, NY) according to AOAC ( 5 ) procedures; for ADF and NDF according to Robertson and Van Soest (18); and for starch by glucose determination following enzymatic hydrolysis using the starch autoanalyzer (YSI 2700 Select; Yellow Springs Instrument Co., Yellow Springs, OH) according to the procedures of Poore et al. (17). Fatty acids were converted to methyl esters and analyzed according to Sukhija and Palmquist ( 2 2 ) using GLC (VA 3300), and Cr was determined using the atomic absorption spectrophotometer (Hitachi Ltd., Tokyo, Japan) according to the procedure of Fenton and Fenton (10). Total tract digestibilities for the previously mentioned nutrients were calculated by the chromium oxide ratio technique ( 9 ) . Intake of DM, efficiency of conversion of feed to milk (FCM/DMI), BW, milk yield, 3.5% FCM, as well as protein, fat, lactose, and SNF in milk were adjusted for covariate effects using data from the 14-d pretreatment period and analyzed by the general linear models procedure of SAS ( 1 9 ) using the following statistical model: Yijkl = m + Bi + Fj + Ck + FCjk + Covl + Eijkl where m = overall mean, Bi = blocking effect (for pretreatment milk yield), Fj = effect of supplemental fat, Ck = cooling effect, FCjk = interaction between Fj and Ck, Covl = covariate effect, and Eijkl = random error. All other data were also analyzed by the general linear models procedures of SAS ( 1 9 ) using a similar model, but the covariate effect was excluded. Treatment means were determined to be significant at P < 0.10, and trends were indicated at P < 0.15. RESULTS AND DISCUSSION The nutrient composition of diets is presented in Table 2. Diets contained 17.8% CP with similar RUP (37% of CP). All nutrients met NRC ( 1 5 ) requirements for lactating cows yielding 35 kg of milk/d with a BW of approximately 650 kg. Values for NEL and RUP were calculated from NRC ( 1 5 ) recommendations; however, an NEL value for steam-flaked sorghum was not available from NRC ( 1 5 ) recommendations, so the NEL value for steam-flaked sorghum (2.2 Mcal/kg) of Theurer et al. ( 2 4 ) was used. The Journal of Dairy Science Vol. 80, No. 6, 1997

TABLE 2. Nutrient composition of the diets. Diet1 Ingredient

MF

NEL,2

1.72 1.80 ( % of DM) 90.6 90.4 17.8 17.8 37.2 36.8 23.2 22.8 32.6 31.8 23.2 20.9 4.6 7.4

Mcal/kg of DM

OM CP RUP,2 % of CP ADF NDF Starch Fatty acid

HF

1MF = Medium fat (7% whole cottonseed); HF = high fat (7% whole cottonseed + 3% prilled fatty acid). 2Estimated from NRC ( 1 5 ) recommendations, except for NE of L steam-flaked sorghum (2.2 Mcal/kg), which was estimated from the report of Theurer et al. (24).

HF diets contained 5% more estimated energy than did the MF diets (1.80 vs. 1.72 Mcal/kg). Because PFA was substituted principally for steam-flaked sorghum, the HF diets contained a lower starch content than did the MF diets (20.9% vs. 23.2%). The FA composition of WCS, PFA, and of the total mixed diets is presented in Table 3. Compared with WCS, PFA contained large amounts of C16:0 and C18:0 but small amounts of C18:2, which reflects the higher C16:0 and C18:0 and lower C18:2 in the HF diets than in the MF diets. Dry matter intake, milk yield, and 3.5% FCM yield were not different among cows fed the HF and MF diets (Table 4). The numerically lower DMI for cows fed PFA might have contributed to the lack of response in milk yield compared with results of a previous study in which PFA were fed to cows under heat stress ( 7 ) . Calculated NEL intake was equal (44.5 Mcal/d) for the two diets in this study. High FA content (7.4%) also might have contributed to lower

TABLE 3. Fatty acid ( F A ) composition of whole cottonseed (WCS), prilled FA (PFA), 1 and experimental diets.2 FA

WCS

C14:0 C16:0 C18:0 C18:1 C18:2 C18:3 Other

0.9 25.9 2.9 17.9 47.1 0.2 5.1

PFA

MF

(g/100 g of methyl esters) 3.0 1.1 42.4 26.9 44.3 2.8 5.5 21.7 0.7 31.0 . . . 7.4 4.1 12.3

HF 1.6 33.2 12.6 21.2 19.8 3.6 7.8

Booster-100 (Milk Specialties Co., Dundee, IL). = Medium fat (7% whole cottonseed); HF = high fat (7% whole cottonseed + 3% prilled fatty acid). 1Energy

2MF

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DMI for cows fed the HF diets (27). Compared with a diet containing 12% WCS and no added fat (4.7% FA), Wu et al. ( 2 8 ) showed that the addition of 2.2% PFA to increase FA to 6.9% did not affect DMI (27.2 vs. 26.8 kg/d); however, DMI was decreased by supplementation of 4.4% PFA, which increased FA to 9.1%. Wu et al. ( 2 9 ) also reported that DMI did not decrease for cows fed 7.2% WCS and 2.5% PFA (6.2% FA) compared with cows fed a diet of 7.2% WCS (3.7% FA). The efficiency of conversion of feed to milk (FCM/ DMI) tended to improve ( P < 0.11) for cows fed the HF diets because of the numerically lower DMI and slightly higher milk yield. However, when treatment means for DMI, milk yield, milk fat percentage, and BW changes were used to calculate efficiency of NEL utilization (NE L output/intake) with standard values (15), the HF diets were slightly more efficient than the MF diets in NEL conversion to milk and tissue energy (0.47 vs. 0.52). The advantage of EC versus S was considerably greater (0.43 vs. 0.55). The higher BW changes of the cows in EC than for cows in S only were partially responsible for the greater overall efficiency in energy utilization of cows in the EC treatment. Percentages and yields of protein, fat, lactose, SNF content of milk and of SCC in milk were not affected by the addition of fat to the diets.

Compared with S, EC increased milk yield and FCM, which is consistent with results of other studies (8, 23). Evaporative cooling tended to increase milk fat yield because of higher milk yield, but decreased percentages of lactose and SNF in milk, which contradicts the observations of Chen et al. ( 8 ) and Taylor et al. (23). Those researchers reported no effect of cooling on percentages of lactose or SNF in milk. The reason for the decreased percentage of lactose and SNF in milk of cows that received EC treatment is not known. The lack of decrease in milk protein percentage of cows fed added dietary fat contrasts with results of previous experiments reviewed by Wu and Huber (27); however, few of those studies were conducted under heat stress conditions. In a previous study conducted in hot weather ( 7 ) , however, added fat did not decrease milk protein content. Addition of PFA decreased the proportion of shortand medium-chain FA ( C 4 to C14) in milk fat by 12%; effects were significant for all FA except C4. The data suggest that supplementation of PFA reduced de novo synthesis of FA in the mammary gland but increased palmitic acid (Table 5), which is in agreement with data of Wu et al. (28, 29), who noted similar changes in milk FA in cows fed PFA. Cooling did not affect FA

TABLE 4. Effect of supplemental fat and evaporative cooling on performance of cows. Treatment1 Item

MF + EC

Effect2

MF + S

HF + S

HF + EC

26.2 29.5 27.6 1.06

24.7 30.0 27.7 1.18

25.5 31.0 29.8 1.15

24.6 31.7 29.6 1.21

2.93 0.89

3.23 0.94

3.25 1.00

2.99 0.89

2.95 0.88

5.06 1.51 8.61 140 0.15 –0.10

4.97 1.47 8.52 121 –0.03 –0.17

SEM

F × C

F

C

1.2 0.8 1.1 0.05

NS3 NS NS 0.11

NS 0.08 0.06 NS

NS NS NS NS

3.13 0.98

0.12 0.05

NS NS

NS 0.12

NS NS

2.90 0.89

2.94 0.92

0.04 0.03

NS NS

NS NS

NS NS

4.91 1.53 8.41 119 0.30 –0.12

4.92 1.54 8.45 86 1.17 –0.07

0.04 0.06 0.06 38 0.20 0.07

NS NS NS NS NS NS

0.03 NS 0.02 NS 0.05 NS

NS NS NS NS NS NS

P DMI, kg/d Milk, kg/d 3.5% FCM, kg/d FCM/DMI Milk fat % kg/d Milk protein % kg/d Milk lactose % kg/d Milk SNF, % Milk SCC, ×103/ml BW Change,4 kg/d BCS5 Change

1MF = Medium fat diet [7% whole cottonseed (WCS)], HF = high fat diet (7% WCS + 3% prilled fatty acid), S = shade, and EC = S plus evaporative cooling. 2F = Fat effect, C = cooling effect, and F × C = interaction between F and C. P < 0.15 indicates a trend. 3P > 0.10. 4Mean daily changes in BW calculated from BW determined at the beginning and the end of treatment. 5Body condition score.

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TABLE 5. Effect of supplemental fat and evaporative cooling on fatty acid composition of milk. Treatment1

Effect2

Fatty acid

MF + S

HF + S

MF + EC

C4:0 C6:0 C8:0 C10:0 C12:0 C14:0 C16:0 C18:0 C18:1 C18:2 C18:3 Other

3.87 2.41 1.40 3.09 3.61 11.26 30.63 10.81 25.80 5.21 1.02 0.90

(g/100 g of methyl esters) 3.97 3.90 2.18 2.38 1.17 1.32 2.40 3.06 2.90 3.68 9.69 11.43 33.37 31.27 11.43 11.30 25.70 24.56 5.12 5.31 1.08 0.99 1.00 0.80

HF + EC

SEM

F

C

F × C

3.90 2.18 1.17 2.64 3.00 9.75 33.69 11.95 24.59 5.21 0.95 0.99

0.14 0.10 0.05 0.27 0.26 0.51 1.11 0.69 0.74 0.28 0.05 . . .

NS3 0.05 0.01 0.06 0.02 0.01 0.03 NS NS NS NS . . .

P NS NS NS NS NS NS NS NS NS NS 0.12 . . .

NS NS NS NS NS NS NS NS NS NS NS . . .

1MF = Medium fat diet [7% whole cottonseed (WCS)], HF = high fat diet (7% WCS + 3% prilled fatty acid), S = shade, and EC = S plus evaporative cooling. 2F = Fat effect, C = cooling effect, and F × C = interaction between F and C. P < 0.15 indicates a trend. 3P > 0.10.

composition in milk of cows under heat stress, except that C18:3 tended to be lower in cows receiving EC than in those receiving S only; the reason for this result is unclear. The BCS of cows were not affected by treatments even though cows receiving EC gained slightly more BW than did cows under S only (Table 4). Mean maximum ambient temperatures, THI, and hours of varying THI are presented in Table 6. At a THI above 72, heat stress was sufficient to lower milk yield (1, 12), and yields were even more depressed when THI exceeded 76 (12). In this study, the uncooled cows were probably adversely affected by heat stress because THI exceeded 72 for a mean of 18.6

h/d. The THI was determined at a meteorological station located approximately 1.5 km from the experimental site, which was used to determine conditions representative of the University of Arizona Campbell Avenue Farm Complex. Temperature and humidity were not measured under evaporative coolers, but a previous study ( 2 3 ) that determined climatic conditions in treatment pens showed that the EC system reduced maximum daily temperature 5.6°C and reduced average THI 2.3 units. These changes were not indicative of the total cooling effect of the EC system because of evaporative heat loss from cow bodies that was attributed to wetting and strong air movement ( 1 ) .

TABLE 6. Mean maximum temperatures, temperature-humidity indexes (THI), and hours of various THI.1

Pretreatment wk 1 wk 2 wk 3 wk 4 wk 5 wk 6 wk 7 wk 8 Mean wk 1 through 8

THI

Maximum Maximum temperature THI

<72

72 to 76

( °C ) 37.0 37.0 35.9 35.6 38.4 34.8 33.3 35.8 36.6 35.9

6.8 3.1 1.3 3.4 1.1 1.9 10.1 11.5 9.7 5.3

(mean daily h ) 8.6 7.9 9.3 6.7 9.3 9.6 8.8 7.0 7.4 7.4 11.1 6.1 6.4 7.3 4.5 8.0 5.7 8.3 7.8 7.6

80.4 82.8 83.3 84.1 84.1 82.9 79.7 79.6 80.9 82.2

77 to 81

>82 0.8 4.9 3.9 4.9 8.0 4.9 0.1 0 0.3 3.4

1Data were collected at the Arizona Meteorological Network weather station ( ∼1.5 km from the experimental site).

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TABLE 7. Effect of supplemental fat and evaporative cooling on rectal temperatures ( R T ) and respiration rates ( R R ) of cows. Treatment1

Effect2

Item

MF + S

HF + S

MF + EC

HF + EC

SEM

F

C

RT, °C RR, breaths/min

39.6 81.1

39.8 85.7

39.4 81.2

39.7 84.4

0.3 5.2

NS3 NS

NS NS

F × C P NS NS

1MF = Medium fat diet [7% whole cottonseed (WCS)], HF = high fat diet (7% WCS + 3% prilled fatty acid), S = shade, and EC = S plus evaporative cooling. 2F = Fat effect, C = cooling effect, and F × C = interaction between F and C. 3P > 0.10.

Rectal temperatures and RR of cows did not differ because of fat supplementation (Table 7). These results agree with those of our earlier experiment ( 7 ) and with those of Moody et al. (14), who showed that supplemental fat did not affect RT and RR of cows that were subjected to heat stress. West ( 2 5 ) indicated that effects of high environmental temperatures on the performance of cows were mediated through changes in body temperature, and Johnson et al. ( 1 2 ) showed that milk yield decreased 1.8 kg for each increase of 0.56°C in body temperatures above 38.6°C. The lack of response in milk yield to added fat and the small response to cooling might have been because the high body temperatures determined for all groups ( X = 39.6°C ) masked the effects of treatments. In the present study, cooling did not change RT or RR of cows. The lack of detectable effects of cooling on RR and RT might be attributed to moving cows (approximately a 2-min walk) from pens to a crowded lane to obtain RT. Measurements of RR were made approximately 10 min after cows were returned to pens. Because measurements were made during the hottest part of the day (about 1400 h), movement

and crowding of the cows might have negated differences caused by EC and S treatments. The study by Armstrong et al. ( 4 ) showed that, under similar conditions, the EC system lowered RR and RT of cows more than did the S system; however, in that study cows were not moved from pens, and RR and RT measurements were made in lock-up stanchions. These data support one trial of Taylor et al. ( 2 3 ) in which milk yield increased, but RT was unaffected by EC even though RR were lower in cows receiving EC. In another trial of Taylor et al. (23), both RT and RR decreased in cows receiving EC, but milk yield was unaffected. Chen et al. ( 8 ) showed that EC decreased RT (39.1 vs. 38.6°C ) and RR (82 vs. 64 breaths/min) and increased milk yield. However, the magnitude of increased milk yield by cows receiving EC in our study was lower (5.3%) than that observed by Chen et al. [9.1%; (8)]. Differences between the studies might be explained by the higher ambient THI in the present study (82.0) than in the study of Chen et al. ( 8 ) in which THI averaged 76.6; differences might also be explained by a lower efficiency of cooling in our study.

TABLE 8. Effect of supplemental fat and evaporative cooling on apparent digestibilities of nutrients. Treatment1 Item

MF + S

HF + S

MF + EC

Effect2 HF + EC

SEM

F

F × C

C P

DM OM CP ADF NDF Starch Fatty acid

64.3 65.6 69.9 43.1 53.0 98.4 76.4

65.0 65.8 69.9 46.6 57.1 98.3 74.1

59.6 60.7 65.4 34.4 47.3 98.8 76.6

61.6 62.7 67.9 42.6 52.8 98.5 75.0

3.2 3.2 2.9 5.5 4.9 0.2 1.1

NS3 NS NS NS NS NS 0.11

NS NS NS NS NS NS NS

NS NS NS NS NS NS NS

1MF = Medium fat diet [7% whole cottonseed (WCS)], HF = high fat diet (7% WCS + 3% prilled fatty acid), S = shade, and EC = S plus evaporative cooling. 2F = Fat effect, C = cooling effect, and F × C = interaction between F and C. P < 0.15 indicates a trend. 3P > 0.10.

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Digestibilities of DM, OM, CP, ADF, NDF, and starch were unaffected by the supplementation of PFA and cooling method (Table 8). Chen et al. ( 8 ) also showed that digestibilities of DM and CP were unaffected by EC. Supplemental fat tended to decrease FA digestibility ( P < 0.11). The addition of PFA to diets increased dietary FA to 6.2 or 9.1% [(28, 29), respectively], and decreased FA digestibilities, suggesting that 6 to 7% dietary FA is the upper limit that can be fed to dairy cows without adversely affecting FA digestibility. The reason for the decrease in FA digestibility as FA in the diet increased is not clear but may be related to enzymic or micellar saturation. The high starch digestibilities (>98%) and the lack of difference in digestibilities of ADF and NDF or in the percentage of milk fat for cows fed either the HF or MF diet support a previous study ( 7 ) that showed that supplemental fat did not adversely affect ruminal fermentation. CONCLUSIONS As dietary FA increased from 4.6 to 7.4% by the addition of 3% PFA, lactational response was not affected for cows under heat stress and yielding 32 kg/d of milk, but efficiency of conversion of feed to milk tended to increase as supplemental fat increased. Evaporative cooling positively affected milk yield, 3.5% FCM, and milk fat yield, suggesting some relief of heat stress by EC. No effect was observed on RT or RR, probably because of the method of measurement. REFERENCES 1 Armstrong, D. V. 1994. Heat stress interaction with shade and cooling. J. Dairy Sci. 77:2044. 2 Armstrong, D. V., S. K. DeNise, F. J. Delfino, E. J. Hayes, P. H. Grundy, S. Montgomery, and A. Correa. 1993. Comparing three different dairy cattle cooling systems during high environmental temperatures. J. Dairy Sci. 76(Suppl. 1):240.(Abstr.) 3 Armstrong, D. V., F. Wiersma, T. J. Fuhrmann, J. M. Tappan, and S. M. Cramer. 1985. Effect of evaporative cooling under corral shade on reproductive and milk production in a hot arid climate. J. Dairy Sci. 68(Suppl. 1):167.(Abstr.) 4 Armstrong, D. V., F. Wiersma, M. E. Wise, J. T. Huber, K. M. Marcus, and D. S. Ammon. 1986. Effect of refrigerated air conditioning and evaporative cooling on milk production. J. Dairy Sci. 69(Suppl. 1):161.(Abstr.) 5 Association of Official Analytical Chemists. 1990. Official Methods of Analysis. Vol. I, 15th ed. AOAC, Arlington, VA. 6 Chambers, A. B. 1970. A psychrometric chart for physiological research. J. Appl. Physiol. 29:406. 7 Chan, S. C., J. T. Huber, Z. Wu, K. H. Chen, and J. Simas. 1992. Effect of fat supplementation and protein source on performance of dairy cows in hot environmental temperatures. J. Dairy Sci. 75(Suppl. 1):175.(Abstr.) 8 Chen, K. H., J. T. Huber, C. B. Theurer, D. V. Armstrong, R. C. Wanderley, J. M. Simas, S. C. Chan, and J. L. Sullivan. 1993. Effect of protein quality and evaporative cooling on lactational Journal of Dairy Science Vol. 80, No. 6, 1997

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