Yang, H.J., S.L. Li, X. Jin & R. Wang
State Key Laboratory of Animal Nutrition, College of Animal Science and Technology, China Agricultural University, Beijing 100193. P.R. China
Abstract
Milk performance of dairy cows in most middle-to-small scale dairy farm in China is mainly maintained by incorporating high ratio of concentrate to forage in the total mixed rations, which resulted in low milk fat and a relatively high milk protein. Ryegrass hay with limited yield in China and alfalfa hay increasingly imported from USA are still regarded as luxury forage options in dairy feeding in China. Under high ratio of concentrate to forage, different corn-stover based forage combinations in the present study were compared with hay based forage combination in terms of their effects on milk yield, milk composition, rumen degradability and milk fat and protein precursors generated during the rumen fermentation.
The results obtained in the present study suggested that milk yield and milk protein content was greater in dairy cows fed total mixed rations with forage combination of corn silage together with ryegrass hay and/or alfalfa hay than those in the rations with corn stover based forage combination, and this could be caused by relative high VFA production in the rumen and greater ruminal MCP flowed into small intestine in the ration with corn-silage based forage combination. The diurnal ratio of milk fat precursors (acetate plus butyrate) to lactose precursor (propionate) was contrary to diurnal VFA variation at different sampling time in a day in which dominant production of milk fat precursors mainly occurred before feeding, and dominant milk lactose production occurred mainly after feeding, especially in the rations with corn stover based forage combinations, and this could explained why milk fat content was greater in the rations with corn stover-based forage combination than the rations with corn silage based forage combination. Further phenolic acid profiles showed that ADL content in the corn stover based forage combinations was correlated with proportions of vanillic, ferulic and p-coumaric acids in total phenolic acids and the ferulic: p-coumaric acid ratio and in situ rumen degradation kinetics confirmed that ferulic and p-coumaric acids, especially in the rations with corn stover based forage combination would be the most important factors to inhibit fibre degradation in the rumen of dairy cows.
Introduction
In the last decades, the increase of milk production in China mainly relied on the increase of cow numbers, but the quality of raw milk as indicated by milk fat and protein contents has not been improved quite a lot as customer expected. Milk fat and protein contents in most middle-to-small scale dairy farms are still quite lower than those values in developed countries. In order to maintain a relative high milk yield, increasing concentrate to forage ratio up to 60:40 in the rations of dairy cows has been well accepted by these farms in China.
The forage ingredients mainly include agricultural crop residues, such as grain bran, corn
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stover and corn silage, etc. Ryegrass hay with limited yield in China and alfalfa hay increasingly imported from USA are still regarded as luxury forage options in dairy feeding in China. The ambitious objective of the present study is firstly to determine if there are any forage combinations without high-quality hay inclusion to maintain milk yield and milk composition, if not, what would be main limiting factor in corn stover based forage compared with hay included forage from a fibre degradation angle; the another objective attempt to explore milk fat and protein precursors generated in the rumen and their associations with different forage combinations based on a preliminary experiment trial finished recently.
Materials and Methods
Five nonlactating, ruminally cannulated Holstein cows (543 ± 25 kg) were used in a 5 × 5 Latin square design, balanced for residual effects, to investigate the effects of feeding cow five region-representative total mixed diets (concentrate: forage=60: 40) differing in forage combinations: 1) 100% corn stover (ration I: CS); 2) 50% corn stover plus 50% ensiled corn stover (ration II: CS+ECS); 3) 50% ensiled corn stover plus 50% Chinese wild ryegrass hay (ration III: ECS+CW); 4) 50% Chinese wild ryegrass hay plus 50% whole corn silage (ration IV: CW+WCS); and 5) Chinese wild ryegrass hay plus whole corn silage plus alfalfa hay in approximate ratio of 1:1:1 (Ration V: CW+WCS+AH). The surgical preparation of rumen fistulated cows was done 2 months before the start of the study at the College of Animal Science and Technology, China Agricultural University. Cows were confined by means of stanchions in 5 individual metabolism stalls in a monitor room with rubber-padded concrete flooring. The experiment was conducted under the approval of China Agricultural University Animal Care and Use Committee. During the experimental period, daily milk performance, milk composition, feed intake, urine volume and feces amounts of each cow were individually measured and recorded, rumen fluids and blood samples were collected for later analysis of rumen fermentation indexes and blood metabolites. The ratio of NFC to NDF in the five rations was calculated to indicate dietary carbohydrate structure. To determine microbial degradation characteristics of plant cell wall in the rumen and their relationships with phenolic acid profiles in fibrous feeds, bran and hull (corn bran, wheat bran, rice bran and rice hull), straw (corn stover, wheat straw and rice straw) and forage (alfalfa hay, reed hay, ryegrass hay, sudangrass hay) were chosen as target feeds and investigated.
Results
Effect of total mixed rations with different forage combinations on milk performance and milk composition
During whole experimental period, average milk production varied between 14.5-22.8 kg/d.
Compared with the ration I, daily milk yield in rations II, III, IV and V were increased by 1.8%, 0.8%, 6.25%, 10.2%; Milk fat contents in ration I, II, III without any inclusion of corn silage and high quality hay were all higher than 3.5% above the 973 projects target value, and milk fat contents in both rations IV and V were above 3.4%, slightly lower than the rations I- IV, but no significant difference was observed among these ration groups; The highest milk protein content occurred in the ration V, followed by ration IV, both higher than the 3.1% of 973 projects target value. The ration II with inclusion of ensiled corn stover presented the lowest milk protein content of 3.04%. Milk fat and milk protein content in milk showed a significant negative correlation with milk yield. In the case of milk fat composition, the corn stover plus ensiled corn stover ration II significantly increased the content of the C20 and
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C22: 1 long-chain fatty acids in milk fat, while the ensiled corn stover plus ryegrass hay ration significantly decreased the contents of C20 and C22: 1 long-chain fatty acids in milk fat. Different rations did not exhibited differences for C8 to C18 long-chain fatty acids in milk fat.
Effect of total mixed rations with different forage combinations on rumen fermentation characteristics
Under the high concentrate ratio (60%) in rations, total mixed rations with difference forage combinations did not alter average pH value in the rumen, and pH in all ration treatment was above 6.3. However, significant differences of rumen pH were observed among different sampling times within a day, and pH value peaked in the morning (7:00 am) and afternoon (7:00 pm) before feeding. After the feed, rumen pH decreased gradually to pH 6.1 in the afternoon (11:00-13:00) and pH 5.9 at night (23:00-01:00). The real-time Q-PCR population of three main cellulolytic bacteria including Fibrobacter succinogenes, Ruminococcus albus, and Butyrivibrio fibrisolvens in the rumen were decreased in the rations IV and V with inclusion of corn silage and ryegrass hay and alfalfa hay compared with corn stover based rations.
Total milk fat precursors, indicated as total volatile fatty acids (VFA) yielded in the rumen, did not significantly differ among five total mixed rations with different forage combinations. Diurnal mean VFA concentration in five rations were 146.8, 150.2, 149.1, 154.2 and 152.9 mM, respectively, in which the lowest VFA occurred in the ration I with single forage of corn stover inclusion, and the high VFA production was observed in the ration IV-V with the forage combination of corn silage together with ryegrass hay and/or alfalfa hay. Rumen VFA difference were also observed among the different sampling time within a day before and after feeding in the morning and afternoon. VFA production peaked 4 hrs after the morning feeding (11:00, 164.0 mM) and afternoon feeding (23:00, 163.0 mM), and it gradually decreased at 19:00in the afternoon (131.3 mM) and at 7:00 in the morning (136.6 mM). The diurnal ratio of milk fat precursors (acetate plus butyrate) to lactose precursor (propionate) was contrary to diurnal VFA variation at different sampling time in a day in which dominant production of milk fat precursors mainly occurred before feed, and dominant milk lactose production occurred mainly after feeding especially in the rations with corn stover based forage combinations. Rumen fluids were sample daily at final diurnal interval of 2 h, and microbial protein (MCP) concentration in rumen fluids did not differ among different time intervals (P>0.05), but it differ among five rations (P<0.05). The highest MCP concentration (1.217 mg/mL) occurred in ration III, and it was greater than those of rations I and V (P<0.05), but did not differ from those of ration II and IV (P>0.05).
Allantoin, uric acid, total purine derivative (PD) in urine and microbial nitrogen (MN) and MCP flowed into small intestine increased significantly from rations I-V (P<0.05). Allantoin and uric acid in total PD excretion ranged from 83% ~ 92% and 8% ~ 17%, respectively.
Allantoin (y=50.44x +35.58, R² = 0.733), uric acid (y=27.22x-28.77, R² = 0.734), and total PD (y=77.67x +6.811, R² = 0.734) excretion were linearly correlated with NFC/NDF value in the rations. Consequently, PD excretions in urine and MCP flowed into small intestine were greater in the ration with a roughage combination of Chinese wildrye hay + Whole corn silage+ Alfalfa hay than the other rations.
Effect of total mixed rations with different forage combinations on blood metabolites and hormone levels in the rumen
Triglyceride (P = 0.36) and insulin (P = 0.20) in the blood serum of ration III was the highest and lowest blood glucose concentration occurred in ration III, but the difference was not
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significant among five rations, BHBA, NEFA, plasma urea nitrogen between each treatment group showed a significant difference (P <0.01).And each treatment group BHBA, plasma urea nitrogen increased after feeding, feeding 4-6 hours after the highest, and then reduce;
each treatment group, NEFA fluctuated over time points, the basic 4-6 hours after feeding to a minimum.
Progesterone in the rumen among treatments showed significantly difference (P
<0.01), but progesterone and gastrin content changes in the rumen showed a contrary order among treatments, estrogen and testosterone in ration V had the highest expression level, in others treated group, the content did not indicated the same regulation. In hormone variation at different time points, estrogen in the feed 1-2h reached a maximum value and then decreased to a minimum on 4-6h, but the variation of the pregnancy hormone, gastrin, and testosterone in each treatment were different.
Effect of phenolic acid profiles of plant cell wall in fibrous feeds and their relationships with microbial degradation characteristics in the rumen
Neutral detergent fibre disappearance rate with nylon bag technique decreased with lower dietary NFC to NDF ratio, but no significant difference was observed among five rations with different forage combinations. To deeply investigate microbial degradation characteristics of plant cell wall in the rumen and their relationships with phenolic acid profiles in fibrous feeds, corn bran, wheat bran, rice bran, rice hull, corn stover, wheat straw, rice straw, alfalfa hay, reed hay, ryegrass hay, sudangrass hay were chosen as target feeds for in situ rumen degradation. A rapid and sensitive reversed-phase method for the simultaneous quantification of protocatechuic (PRA), vanillic (VA), ferulic (FA) and p-coumaric (PCA) acids was developed and PRA, VA, FA, PCA and ADL of 13 feeds were determined. The result showed that ADL content was correlated with proportions of VA (r=0.88, P<0.05) (except tamato pomace), FA (r=-0.95，P<0.05) and PCA (r=0.90，P<0.05) in the summed phenolic acids and the PCA: FA ratio (r=0.91，P<0.01). It was assumed that VA，PCA and PCA/FA may be the most important factors to inhibit feed degradation.
There was no difference in degradation rate of cellulose among four bran/hull feeds, and the highest degradation rate of hemicellulose was rice hull (c=0.063). Effective degradability (ED) of DM、NDF、cellulose and hemicellulose in corn bran and wheat bran were all higher than rice bran and rice hull. PRA, VA, PCA and PCA/FA were the factors that may limit plant cell wall degradation in bran hull feeds, but FA did not show its limitation to feeds. Degradation rate of FA in corn and wheat bran were higher than PCA while lower than PCA in rice bran and rice hull. Among four phenolic acids, FA was the most easily degradable one but PCA was the most difficult degradable one. And there were no uniform degradation rule of PRA and VA in bran hull feeds.
Among crop stover or straw feeds, the highest degradation rate of cellulose was rice straw (c=0.043) while corn straw and wheat straw had no difference. And there was no difference in degradation rate among three straw feeds. ED of cellulose in wheat straw was significantly lower than corn straw and rice straw, while ED of hemicellulose in corn straw was significantly higher than wheat straw and rice straw. PCA had the significant positive relationship with ED of NDF and hemicellulose, but it showed depression on quickly degradable fraction (a) of cellulose. And FA showed significant positive relationship with the degradation of DM. PCA was most difficult to degrade in four phenolic acids. The degradation rate ranked: PRA (0.060) >VA (0.040) >FA (0.034) >PCA (0.018).
Among hay feeds, degradation rate of cellulose in sudangrass hay was highest (c=0.062) and there was no difference among alfalfa hay, ryegrass hay and reed hay. There was no difference in degradation rate of hemicellulose and ED of cellulose among four straw
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feeds. And reed hay had the highest ED of hemicellulose and alfalfa hay had the lowest one.
PCA was the most important factor to inhibit feed degradation. The degradation model of phenolic acids in alfalfa was most special and the digestibility of four phenolic acids in alfalfa were much higher than other three feeds. In hay feeds, the degradation rate of FA was higher than PCA. Meanwhile, FA was more easily degradable than PCA.
Conclusions
Under the high inclusion (60%) of concentrate in the rations, the results obtained in the present study suggested that milk yield and milk protein content was greater in dairy cows fed total mixed rations with forage combination of corn silage together with ryegrass hay and/or alfalfa hay than those in the rations with corn stover based forage combination, and this could be caused by relative high VFA production in the rumen and greater ruminal MCP flowed into small intestine in the ration with corn-silage based forage combination. The diurnal ratio of milk fat precursors (acetate plus butyrate) to lactose precursor (propionate) was contrary to diurnal VFA variation at different sampling time in a day in which dominant production of milk fat precursors mainly occurred before feeding, and dominant milk lactose production occurred mainly after feeding, especially in the rations with corn stover based forage combinations, and this could explained why milk fat content was greater in the rations with corn stover-based forage combination than the rations with corn silage based forage combination. Further phenolic acid profiles showed that ADL content was correlated with proportions of VA (r=0.88), FA (r=-0.95) and PCA (r=0.90，P<0.05) in total phenolic acids and the PCA: FA ratio (r=0.91，P<0.01), and degradation kinetics confirmed that VA，PCA and PCA/FA, especially in the rations with corn stover based forage combination would be the most important factors to inhibit fibre degradation in the rumen of dairy cows.
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