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The effects of heat stress on the welfare of intensively managed cattle have been considered. However, there is very little discussion in the scientific literature regarding the interrelationships between heat stress and PENDF of feedstuff. Foraging is usually the major source of fibre in dairy ration. Optimal utilization of diets by dairy cows is influenced by the chemical composition and physical characteristics of the ration. Differences in the amount and physical properties of fibre can affect the utilization of the diet and performance of the animal. When too much fibre is included in the ration, energy density is low, intake is reduced, and productivity is decreased. When too little fibre is included in the ration a variety of symptoms can occur including altered fermentation and mild or borderline acidosis, which affects ruminal digestive efficiency, intake and metabolism, milk fat production, and the long-term health of the animal may have the greatest economic impact on dairy production (11).
The physical and chemical characteristics of fibre vary among feedstuffs and affect cow responses, such as chewing activity and milk fat percentage. Chewing stimulates secretion of salivary buffers (5) that neutralize fermentation acids. The ability of fibre to stimulate chewing has been termed physical effectiveness (PE) because chewing response by the cow is highly related to the physical properties of fibre, such as particle length (1). Because Sugar beet pulp (SBP) is commonly used to replace forage in the diets of highly producing dairy cows it is important to know the effectiveness of SBP in stimulating chewing, which is an index of the ability of SBP to neutralize fermentation acids in the rumen. However, standard forages have not been used in these studies. Because PE is affected by particle size, PE values calculated for non-forage fibre sources are expected to vary depending on the PE of the forage used in the experiments. The objective of this study was to determine the PE of different levels of SBP NDF relative to alfalfa NDF.
MATERIALS AND METHODS
Twelve Holstein cows in mid lactation with 130±50 days (milk and 482.6±40.6 kg) body weight are used in complete block design study. Dietary treatments were 0, 7, 14, and 21 percentage of SBP substitution of alfalfa on an equivalent NDF basis. Experimental treatment periods were 30 d (9 d for dietary adjustment followed by 21 d for data collection). Experimental diets were composed of alfalfa, the described SBP percentage, barley, soybean meal, wheat bran, salt, sodium bicarbonate, calcium carbonate, urea and vitamin and trace mineral supplements. The diets were balanced to 35% NDF and 17% crude protein. Diets were isofibrous (NDF) and isonitrogenous, but not isocaloric. Diets balanced with Cornell University software CNCPS 5.0 were: first diet (0, 50), second diet (7, 43), third diet (14, 36) and forth diet (21, 29) percent of sugar beet pulp and alfalfa in whole diet, respectively.
Diets were mixed daily and offered twice each day after cows were milked, in amounts adequate to allow refusal of 10% of the feed offered. Daily dry matter intake (DMI) was measured and averaged. Dietary ingredients (0.5 kg), experimental diets (0.5 kg), and individual cow orts (12.5%, wet weight basis) were collected daily, frozen, and combined at the end of the experiment. At the end of collection period, the composites were dried at 105 °C for 24 h and ground through a Wiley mill (1-mm screen; Arthur H. Thomas, Philadelphia, PA).
Cows were milked twice daily in parlor at 0400 and 1600 h. Milk production was recorded at week of end of experiment. Milk samples were taken at milking on d 25, 27, and 29 of period and analyzed for fat, protein, lactose, and SNF concentrations with MILCO Scan 133B. Body weights were measured after the morning milking. The evaluations were made independently by three scorers on the 1st d and at the end of the experiment.
Chewing activity was monitored continuously for 24 h on d 25 of experimental with chewing, rumination and resting recorded. Each cow was observed once every 5 min. Cows were observed during milking and for differences in intake on the day of measurement. There was one recorded at any moment throughout the 24 h. Rumen pH was determined directly using a glass electrode instrument on d 27 of experimental. Fecal pH was determined on 3 consecutive d starting on d 27 of experimental. Fifty grams of rectum fecal material obtained was mixed with 50 ml of deionized water with pH recorded using a glass electrode instrument. Total tract apparent digestibility was detected with method of Vankeulen and young (18). Cows were housed in tie stalls shade shelter and were exposed to continuous light throughout the experiment. Ambient temperature and humidity were controlled (controlled say with a heater or recorded) using a thermometer every hour experimental days. PENDF of SBP calculated with method of Mooney and Allen (12). Data were analyzed by a randomized complete block design according to the GLM procedure of SAS (16).
RESULTS AND DISCUSSION
Little attention has been paid to the interaction between heat stress and the PENDF of feedstuff. The thermal environment is a major factor that can negatively affect milk production and DMI of dairy cows (9). Heat stress can have an affect on rumination activity, reticulo-rumen motility and passage rate resulting in a change in the PENDF of feedstuff. A rise in ambient temperature above the thermal neutral zone (about 5 to 20 0C) will decrease milk production because of reduced DMI. A decrease in DMI up to 55 percent of that eaten in the thermal neutral zone along with an increase of 7 to 25 percent in maintenance requirement has been reported for cows subjected to heat stress (13). In this experiment the mean ambient temperature was 33 0C and minimum and maximum of that were 25 0 C and 47 0C respectively. Mean relative humidity was 72% and minimum and maximum of that were 57% and 86% respectively.
As beet pulp increasingly substituted for alfalfa, DMI, increased (Table 3). Intake can be affected by numerable variables, such as ruminal fill, moisture and fat content of the diet, metabolic fuels, weather, feeding management, eating habits, cow behavior, ruminal patterns of fermentation and pH (14). According of NRC 2001 the DMI increased linearly (p< 0.01) with increasing concentrate in diets, regardless of forage type (concentrate have high passage rate from rumen). DMI was not different between diets in this study (table 3). Other factors that increase DMI of diets containing beet pulp are related to the high palatability and high NDF digestibility due to its fast clearance from the rumen and passage through digestive tract (19). Coppock et al., 1987; Harison et al., 1995; Debrabander et al., 1999; Voelker, and Allen. 2003a; Iphrraguerre and Clarck, 2003; and Mansfield et al., 1994; showed DMI increased when the non forage fibre source (NFFS) substituted for forage had low fill effect.
Body weight increased from the beginning of the experiment but did not differ significantly between dietary treatments. Milk production, milk protein percentage, milk fat (percentage and yield), fecal pH, yield of FCM, and SNF did not differ between treatments, but milk protein yield and ruminal pH did differ significant between treatment (p< 0.05). Dry matter intake, milk production, milk fat percentage and yield, milk protein percentage and yield tended to increase with increasing SBP replacement of forage DM. Increasing DMI can be related with high NDF digestibility, low heat increment and lower particle size of SBP. Increasing milk production can be related to the high energy and readily digestible nutrients of SBP. Synchronization of release of carbohydrate (energy) and protein are very important for high microbial protein yield. Research indicates that total milk production and milk protein (percentage and yield) can be increased by improving the amino acid profiles of in microbial proteins by increasing synchronization of ruminal protein and carbohydrate digestion allowing greater synthesis of microbial proteins. However this increase in microbial synthesis affects has many outcomes in dairy cows, such as increased DMI, milk production and a better profile of intestinal amino acids in that tend to result in a greater milk protein production. In this experiment it is probable that diets containing more SBP than alfalfa had better synchronization between energy and protein. In this study we found an increase in milk fat (yield and percentage) in cows on a diets contain SBP was associated with high ruminal pH and increased NDF digestibility. Increases in milk fat generally have been observed in association with increases in milk protein (20)
In the present study, the decrease in rumination time, eating time and consequently in total chewing time for diets containing SBP apparently resulted from the lower particle size of SBP (table 4). Chewing activities are related to a multitude of factors including DMI, particle size, particle shape, fragility, moisture and type of preservation. As intake increases, the amount of chewing per unit of DM decreases (11). Fibre is needed in the diet of ruminants to prevent acute acidosis and death, founder, erosion of the ruminal lining, abscessed livers, milk fat depression, metabolic changes that induce fattening, borderline acidosis causing ruminal parakeratosis and chronic laminitis, altered ruminal fermentation, and reductions in both energy intake and FCM production. However low fibre in the diet may be detrimental to the animal without a significant decrease in milk fat percentage suggesting that factors other than milk fat percentage, such as ruminal acetate: propionate, ruminal pH, or chewing activity should be monitored.
Sudweeks et al. (17) proposed that rations should contain ingredients that result in approximately 30 min of chewing activity per kilogram of DM in order to maintain milk fat percentage. Norgard (15) proposed the same requirement to maintain optimal function and milk fat percentage. However the data of Woodford and Murphy (22) indicated that as little as 24 min of chewing activity per kilogram of DM was adequate to maintain milk fat percentage.
In this study increased eating and rumination time per kg of NDF, forage NDF and the buffering capacity of SBP may play a role in stabilizing ruminal pH. Increasing saliva secretion due to increased eating time and rumination time may increase total salivary output per day, neutralizing the acid of rumen resulting in a higher pH. Ruminal contents of cows receiving diet SBP possibly had decreased retention time due to the small NDF particles of this feedstuff. Cows fed with SBP probably had more rapid rumen turnover and may have had decreased rumen VFA, increased rumen pH, increasing the flow of fermentable NDF to the lower tract, resulting in lower pH of fecal. However significant differences in fecal pH were observed between treatments (table 4).
Coefficients of physical effectiveness (PE) of NDF based on total chewing time (minutes per day) were calculated with method of Mooney and Allen (12). Table 5 show that in this experiment with decrease alfalfa in diets (decreasing NDF with long particle size), cows chewing forage NDF very effectively.
Chewing activity was highly correlated to the forage NDF intake of the diets regardless of the percentage of SBP NDF intake in diets. However correlations were observed between chewing time and NDF intake, forage and SBP NDF intake (P< 0.01), indicating that the forage portion of the diet was a major component in promoting chewing activity. Positive correlation between eating time and rumen pH was probably due to increased saliva secretion during eating (2, 6).