Animal Sciences Research and Reviews

Special Circular 156

Site of Digestion and Milk Production by Cows Fed Fats Differing in Saturation, Esterification, and Chain Length

J. Pantoja, J.L. Firkins, and M.L. Eastridge
Department of Animal Sciences

Summary

The effects of supplemental dietary fatty acids from roasted soybeans combined with tallow or partially hydrogenated fats varying in esterification and fatty acid chain length on nutrient digestion and lactation performance were studied using four cannulated cows in a 4 X 4 Latin square design. Diets were formulated to contain (DM basis) 48% forage in addition to 1.5% fatty acids from tallow, partially hydrogenated tallow triglycerides, partially hydrogenated tallow fatty acids, or a blend of 30% tallow and 70% hydrogenated fatty acids that were rich in palmitic acid. Supplemental fat as fatty acids compared with triglyercides increased digestibilities of total fatty acids and C_18:1 in the small intestine, perhaps indicating that lipolysis was rate limiting. Fatty acids also increased milk fat percentage and efficiency of 4% FCM production. Digestibility of fat in the small intestine was not improved by feeding the fat source rich in palmitic acid. Dry matter intake, 4% FCM, and milk protein percentage were similar among treatments.

Introduction

Feeding fats to high-producing dairy cows has become a common practice to increase energy density of the diet. Two characteristics of fats that influence their use in dairy diets are saturation and esterification of fatty acids (FA). Unsaturated FA tend to be more inhibitory to ruminal cellulolytic microbes than saturated FA and also have a higher digestibility in the small intestine. Partial hydrogenation of FA effectively renders them ruminally inert but often decreases their digestibility in the small intestine. Some researchers have reported higher digestion of FA in cows fed a saturated fat source rich in palmitic acid (C_16:0) than from a fat source rich in stearic acid (C_18:0). In most studies, only one supplemental fat source was added per treatment, but, in practice, many dairy farmers feed a ruminally inert fat source (e.g., partially hydrogenated fat) only after supplementing a conventional fat source. Clemson workers noted that combinations of canola oil with partially hydrogenated tallow (PHT) had higher FA digestibilities than predicted based on FA digestibilities of diets with either fat fed alone. Therefore, partially hydrogenated fat sources of different degrees of saturation, ratio of palmitic to stearic acids, and whether fed as triglycerides (TG) or FA (effects of esterification) may have different effects when fed in combination with an unsaturated fat source. The objectives of this study were to evaluate the effects of dietary FA from roasted soybeans combined with tallow or with partially hydrogenated TG, FA, or FA rich in palmitic acid on site of fiber digestion, digestibility of FA in the small intestine, microbial N flow to the duodenum, and lactation performance by dairy cows.

Materials and Methods

Four cows in midlactation fitted with ruminal and simple T-type duodenal and ileal cannulas were used in a 4 X 4 Latin square design to evaluate the effects of FA source on the site of nutrient digestion and lactation performance. All diets (Table 1) were formulated to contain 1.5% FA from roasted soybeans and 2.5% FA from either tallow, prills of PHT, partially hydrogenated tallow FA (PHTFA), or a blend of 30% tallow and 70% palmitic-rich FA (TPFA). For diet formulation, TG in tallow and PHT were assumed to contain 90% FA.

Each experimental period consisted of 21 days; 14 days were for diet adaptation, and 7 days were for data collection. Diets were mixed daily as TMR and fed twice daily. Samples taken during days 14 to 17 for feed offered and during days 15 to 18 for feed refused were composited for the collection period.

Chromic oxide was mixed with wheat middlings and formed into a pellet, the pellet was dosed into the rumen at 0700, 1500, and 2300 hours (75 g/dose), and Cr₂O₃ was used as a digesta marker. Samples from the rumen, duodenum, ileum, and feces were taken and processed using standard techniques.

Cows were milked twice daily; milk weights were recorded from days 15 to 21 of each period and averaged. Milk samples from days 15 to 17 were analyzed for milk fat and protein concentrations by infrared spectroscopy (DHI Cooperative Inc., Powell, OH). Body weights were recorded on days 20 and 21 of each period.

Data were analyzed using analysis of variance for a 4 X 4 Latin square design. Treatment comparisons were effect of fat saturation (tallow vs PHT), effect of partially hydrogenated TG vs FA (PHT vs PHTFA), and effect of concentration of C₁₆ vs C₁₈ (PHTFA vs TPFA).

Table 1. Ingredient amd chemical composition of experimental diets.
	Diets1
	Tallow	PHT	PHTFA	TPFA
Ingred composition	(% of DM)
Corn silage	24.00	24.00	24.00	24.00
Alfalfa silage	24.00	24.00	24.00	24.00
Roasted soybeans	8.33	8.33	8.33	8.33
Soybean meal, 44% CP	6.89	6.89	6.89	6.89
Blood meal	2.00	2.00	2.00	2.00
Tallow	2.78	. . .	. . .	. . .
PHT	. . .	2.78	. . .	. . .
PHTFA	. . .	. . .	2.50	. . .
TPFA	. . .	. . .	. . .	2.58
Corn, ground shelled	14.69	14.69	14.97	14.89
Soyhulls	15.00	15.00	15.00	15.00
Other	2.31	2.31	2.31	2.31
Chemical composition
Crude protein, %	17.9	17.9	18.1	18.3
NDF, %	38.0	38.0	37.7	38.1
Fatty acids, %	4.9	5.1	5.1	5.2
NEL , Mcal/kg	1.71	1.72	1.72	1.72
NEL2, Mcal/kg	1.71	1.68	1.71	1.72
Ca, %	1.06	1.07	1.04	1.09
Mg, %	0.38	0.41	0.41	0.40
K, %	1.45	1.45	1.46	1.45
P, %	0.43	0.42	0.43	0.43
1 Diets were tallow, partially hydrogenated tallow (PHT), partially hydrogenated tallow fatty acids (PHTFA), or a blend of 30% tallow and 70% hydrogenated FA rich in palmitic acid (TPFA). 2 The equation was modified by standardizing total fatty acid digestibilities in the total tract of PHT and PHTFA relative to that of tallow.

Results and Discussion

Concentrations of FA (Table 1) were lower than would be expected based on NRC values, probably because of lower concentrations of FA than ether extract in the forages used (alfalfa haylage and corn silage were analyzed to contain 1.5 and 2.2% FA, respectively, but ether extract from NRC were at least 3.0 and 3.1%, respectively). The FA composition of fat sources is in Table 2. The calculated iodine value for tallow was lower than typical values. The FA composition of PHT was in good agreement with previous reports.

Table 2. Fatty acid (FA) composition of tallow and partially hydrogenated fat sources.
	Fat sources1
Fatty acid2		PHT	PHTFA	TPFA
	(g/100 g of FA)
14:0	2.24	1.61	1.17	1.27
14:1	0.32	. . .	0.07	0.09
16:0	22.33	22.92	29.61	40.14
trans 16:1	0.72	0.44	0.13	0.13
cis 16:1	2.45	0.42	0.80	0.70
18:0	22.34	50.39	48.03	40.00
trans 18:1	3.63	10.26	1.37	1.95
cis 18:1	32.33	5.78	10.43	8.49
18:2	1.28	. . .	1.42	1.33
18:3	0.41	0.33	. . .	0.06
Other	11.95	7.85	6.97	5.84
Iodine value	38.4	15.8	14.6	13.0
Mass retained on sieve (%)
2.36 mm	. . .	26.3	1.2	0.7
1.18 mm	. . .	44.4	33.6	28.0
0.6 mm	. . .	22.8	55.4	61.8
Mean particle size	. . .	1.61	1.00	0.96
1 Fat sources were tallow, partially hydrogenated tallow (PHT), partially hydrogenated tallow fatty acids (PHTFA), or a blend of 30% tallow and 70% fully hydrogenated FA rich in palmitic acid (TPFA). 2 Number of carbons:number of double bonds.

Ruminal NDF digestion was relatively high for all diets (Table 3), probably because of the high concentration of potentially digestible NDF from soyhulls that were included at 15% of the dietary DM. We are not able to explain the tendency for lower ruminal NDF digestion of cows fed TG compared with those fed FA. Although no effects of saturation were detected, inhibitory effects of fats on ruminal fiber digestion generally are observed when highly unsaturated fat sources are fed but not when saturated fats are fed.

Intakes of total FA were similar among diets (Table 3), and intakes of individual FA were generally affected by treatments as planned. Because concentrations of total as well as C₁₆ or C₁₈ FA in harvested bacteria were similar among experimental diets (data not shown), differences in duodenal flows were caused by ruminally undegraded feed FA.

The lower degree of saturation of tallow resulted in a higher total FA digestibility in the small intestine (percentage of duodenal flow) compared with PHT. Similar results have been reported in our previous work. Supplemental dietary FA (PHTFA) versus supplemental TG (PHT) increased digestion of total, C₁₆, and C₁₈ FA in the small intestine. These results are in agreement with results of some researchers, but other researchers reported similar digestibilities for PHT fed as FA or TG. The higher digestibility of FA than of TG of equal iodine value is an indication that the rate-limiting step in FA digestion in the small intestine probably is lipolysis of FA from TG. This difference may be due to the lower solubility of TG than of FA in the small intestine or to the reduced access of lipases to saturated TG. In support of this conclusion, digestibility of C_18:1 also was lower in cows fed PHT than PHTFA. Ruminal biohydrogenation only occurs with nonesterified FA; the lack of difference in biohydrogenation between cows fed PHT than PHTFA or PHT appeared to indicate that lipolysis of PHT occurred readily in the rumen. More work is needed to explain why saturated TG are less digestible than saturated FA in the small intestine. No differences were observed in apparent digestibilities of total, C₁₆, or C₁₈ FA in the small intestine from TPFA compared with PHTFA. In a regression analysis of available data, we reported that the C₁₆:C₁₈ ratio of partially hydrogenated fat sources was found to interact with iodine value and the percentage of dietary fat, which may explain lack of results in the current study in which only 2.5% FA from partially hydrogenated fat sources were fed. Intake of C₁₆ FA was about 23% higher in cows fed TPFA than PHTFA, but this increase accounted for only about 5.1% of total FA intake. In typical feeding situations with ranges in C₁₆:C₁₈ and amounts of fat similar to ours, C₁₆:C₁₈ ratio probably would not be as important as esterification (FA vs TG) for FA digestibility.

Particle size of hydrogenated fats appeared to play a role in FA digestion in the small intestine, but data are conflicting. In our study, the mean particle size (Table 2) of the source of TG (PHT) was 60% higher than that of the FA sources (PHTFA and TPFA). We previously fed prilled or flaked PHT (iodine value = 14) to lactating dairy cows and noted total tract FA digestibilities of 45.7 and 36.7%, respectively. Mean particle size of the prilled and flaked PHT was 0.53 and 1.36 mm, respectively.

Lactation performance generally was similar among experimental diets (Table 4). Milk fat percentage tended to be lower for cows fed PHT than for those fed tallow and was lower for cows fed PHT than for those fed PHTFA. Perhaps this reflects the effect of saturation on FA digestion in the small intestine and ruminal NDF digestibility (Table 3). Theoretically, lower FA digestibility of the PHT than of the tallow or PHTFA diets should have decreased concentration of long-chain FA in milk for cows fed PHT; however, percentages of total C₁₆ and total C₁₈ FA in milk FA were not affected by treatment (ranges were 31.7 to 33.9% and 37.7 to 38.8%, respectively; data not shown). The higher fat percentage in milk from cows fed FA compared with that in milk from cows fed TG is supported by data from other studies.

The efficiency of 4% FCM production, expressed as a percentage of DMI, tended to be higher for cows fed PHTFA than for those fed PHT, probably reflecting the higher FA digestion in the small intestine and in the total tract with PHTFA (Table 3). We observed a similar tendency previously when partially hydrogenated FA or TG were fed at 2 or 5% of dietary DM. The numeri-cally higher 4% FCM of cows fed PHTFA was sufficient to result in a tendency for a higher effi-ciency of production when expressed as 4% FCM/ megacalorie of NE_L intake in cows fed fat as FA. Incorporation of preformed dietary FA into milk fat increases the efficiency of milk fat secretion.

Conclusions

Partial hydrogenation causes fats to be inert in the rumen but may decrease FA digestibility in the small intestine. Our data gave no support to the theory of a higher FA digestion of a fat source rich in palmitic acid in relation to a fat source rich in stearic acid, perhaps because of a relatively small change in concentration of this FA in total dietary FA. In agreement with previous reports, partially hydrogenated fat sources fed as FA than as TG had higher digestibility of total FA and C_18:1 in the small intestine. However, the cost of producing PHTFA versus PHT should be reflected in higher milk production resulting from improved FA digestibility; this needs to be verified in a long-term lactation trial.

Table 3. Means of fiber and fatty acid (FA) digestion by cows fed diets with tallow or partially hydrogenated fat sources.
	Diets1					Contrasts2
Item	Tallow	PHT	PHTFA	TPFA	SE	Saturation	TG vs FA	C16 vs C18
NDF digestion, % of intake
Rumen	61.2	58.3	62.9	63.7	1.5	0.22	0.08	0.73
Total tract	70.4	67.5	67.9	72.8	1.8	0.28	0.88	0.09
Ruminal NDF digestion, % of total digestion	86.7	86.8	92.8	87.5	2.3	0.99	0.12	0.16
FA Intake, g/day
Total	987	1046	1029	1037	40	0.34	0.78	0.97
C16	203	189	226	279	11	0.41	0.05	0.01
C18	603	748	702	632	32	0.11	0.35	0.17
FA flow to the duodenum, g/day
Total	949	1024	935	859	28	0.11	0.07	0.10
Total C16	191	173	200	202	9	0.22	0.07	0.10
Total C18	683	752	671	583	21	0.06	0.03	0.03
Biohydrogenation, %	65.2	63.3	60.4	67.4	1.1	0.27	0.11	0.01
FA digestion in the small intestine, % of duodenal flow
Total	72.9	59.8	65.6	64.8	1.3	0.01	0.02	0.66
Total C16	79.0	49.3	63.7	59.5	2.9	0.01	0.01	0.34
Total C18	71.6	60.6	66.1	65.8	1.3	0.01	0.02	0.84
C18:0	67.4	58.6	62.8	62.4	1.6	0.01	0.11	0.84
C18:1	81.9	62.4	74.3	77.1	1.3	0.01	0.01	0.18
C18:2	77.8	68.6	71.7	72.2	2.4	0.05	0.40	0.88
FA digestion in the total tract, % of intake
Total	72.2	62.7	69.8	72.5	1.5	0.01	0.01	0.25
Total C16	76.4	56.5	69.1	72.9	1.7	0.01	0.01	0.17
Total C18	69.9	63.6	70.0	71.2	1.2	0.01	0.01	0.51
1 Diets were tallow, partially hydrogenated tallow (PHT), partially hydrogenated tallow fatty acids (PHTFA), or a blend of 30% tallow and 70% hydrogenated FA rich in palmitic acid (TPFA). 2 Contrasts: effect of saturation (tallow vs PHT); effect of partially hydrogenated triglycerides (TG) vs fatty acids (FA; PHT vs PHTFA); and concentration of C16 vs C18 FA (PHTFA vs TPFA).

Table 4. Means for lactation performance by cows fed diets with tallow or partially hydrogenated fat sources.
	Diets1					Contrasts2
Item3	Tallow	PHT	PHTFA	TPFA	SE	Saturation	TG vs FA	C16 vs C18
DMI, kg/day BW, kg DMI, % of BW Milk, kg/day Milk protein, % Milk protein, kg/day Milk fat, % Milk fat, kg/day 4% FCM, kg/day 4% FCM/DMI 4% FCM/NEL intake4	20.7 557 3.73 26.8 3.07 0.818 3.92 1.040 26.3 1.27 0.744	21.1 561 3.80 28.5 3.08 0.870 3.60 1.029 26.8 1.27 0.755	20.8 558 3.72 28.0 3.18 0.889 4.15 1.166 28.6 1.39 0.817	20.4 557 3.67 28.7 3.06 0.877 3.97 1.140 28.6 1.42 0.827	0.8 5 0.14 1.1 0.07 0.045 0.11 0.045 1.1 0.04 0.019	0.69 0.62 0.76 0.34 0.91 0.34 0.08 0.86 0.78 0.95 0.69	0.76 0.75 0.70 0.78 0.36 0.78 0.01 0.07 0.26 0.05 0.07	0.72 0.90 0.82 0.66 0.27 0.86 0.27 0.69 0.96 0.63 0.74
1 Diets were tallow, partially hydrogenated tallow (PHT), partially hydrogenated tallow fatty acids (PHTFA), or a blend of 30% tallow and 70% hydrogenated FA rich in palmitic acid (TPFA). 2 Contrasts: effect of saturation (tallow vs PHT); effect of partially hydrogenated triglycerides (TG) vs fatty acids (FA; PHT vs PHTFA); and concentration of C16 vs C18 FA (PHTFA vs TPFA). 3 DMI = dry matter intake, BW = body weight, and FCM = fat-corrected milk. 4 NEL was modified to standardize apparent total tract FA digestibilities by cows fed PHT or PHTFA relative to those fed tallow.