Effect of dietary fiber on endogenous nitrogen flows in lactating dairy cows.

The effect of dietary fiber on endogenous N secretion was studied using a 15N isotope dilution technique in four fistulated Holstein cows. Two isonitrogenous diets differing only in fiber (NDF and ADF) content were used in a crossover design. One diet (HF) contained 37.4% NDF, while the other (LF) contained 23.3%. A new model was developed to estimate endogenous N secretions and losses for the preintestinal, intestinal, and the total sections of the gastrointestinal tract. Three precursor pools: TCA-soluble fraction of plasma, intestinal mucosa, and milk were compared. Although endogenous losses estimated with the model were numerically different for each precursor pool selected (TCA-soluble fraction > mucosa > milk), treatment effects were similar. As intestinal mucosa is probably closest to the precursor pool, these data are discussed. Non-urea N endogenous secretions contributed 13% of the duodenal N flow but were not affected by the fiber content of the diet. The nonurea N endogenous flow at the duodenum was comprised of approximately equal inputs from endogenous N direct, and that incorporated into the microbial biomass. Total endogenous N flows at the duodenum exceeded, by nearly twofold, estimated inputs of urea-N to microbial biomass. Metabolic fecal output averaged 17% of fecal N and was not affected by level of dietary fiber, but net losses from secretions occurring in the small intestine were higher with the low fiber diet. Overall, endogenous N secretions represented 30% of total digestive tract protein synthesis.

Effects of buffers on milk fatty acids and mammary arteriovenous differences in dairy cows fed Ca salts of fatty acids.

Ten Holstein cows in early lactation were used in a replicated 5 x 5 Latin square design to study the effects of MgO and three buffers added to diets containing Ca salts of canola oil fatty acids. Treatments were 1) control (basal diet; no buffer). 2) 1.1% NaHCO3 plus 1.1% KHCO3, 3) 1.9% NaHCO3, 4) 0.5% MgO, and 5) 2.0% Na sesquicarbonate (percentage of dry matter). The control diet contained 53% grass silage, 43% concentrate, and 4% Ca salts. Body weight, intake, milk yield, and percentages of milk fat, protein, and lactose were unaffected by treatments. Buffers and MgO tended to increase triacylglycerol extraction by the mammary gland and changed the proportions of some fatty acids in milk. Arterial concentrations of acetate and triacylglycerol were correlated with their respective arteriovenous differences. Extraction by the mammary gland was high for acetate (approximately equal to 58.2%), triacylglycerol (approximately equal to 47.3%) propionate (approximately equal to 34.6%), and glucose (approximately equal to 24.3%). Extraction of free fatty acids, phospholipids, or cholesterol was negligible. Mammary triacylglycerol arteriovenous difference tended to be higher than when MgO was fed than when NaHCO3 was fed. Sodium sesquicarbonate, NaHCO3, and the blend of bicarbonate buffers increased C18:2 in milk fat when compared with the control treatment. The concentration of C18:2 in milk fat decreased when MgO was fed, but the ratio of cis-C18:1 to trans-C18:1 increased compared with effects of dietary NaHCO3. Medium-chain fatty acids in milk fat tended to be higher than Na sesquicarbonate than with NaHCO3. Buffers and MgO modified the profiles of fatty acids in milk.

Utilization of grass silage by growing cattle: effect of a supplement containing ruminally protected amino acids.

Sixteen crossbred steers (278 +/- 4.9 kg) were used to determine the influence of supplemental ruminally protected lysine and methionine on performance of growing cattle fed grass silage. During the 154-d experiment, all steers were allowed ad libitum consumption of a good-quality grass silage during the first 70 d and of a lesser-quality silage during the remaining 84 d of the trial. The steers received a supplement of .5 kg/d of barley with or without a mixture of ruminally protected amino acid (RPAA) containing 8.2 g of lysine and 2.6 g of methionine. Compared with controls, steers supplemented with RPAA showed 16.3% improved (P less than .03) ADG (.92 vs 1.07 kg/d). Dry matter intake was not affected (P greater than .50) by treatment and averaged 2.03% BW across treatments. Supplementation with RPAA improved feed/gain by 13.6% (7.88 vs 6.81 for control and RPAA treatments, respectively, P less than .01). Plasma levels of methionine, lysine, arginine, and glutamic acid were higher (P less than .05) and of histidine were lower (P less than .001) when RPAA were fed. These results indicate that feeding RPAA can improve the performance of growing steers fed grass silages of varying qualities.

The nutritive value for sheep of quackgrass and timothy hays harvested at two stages of growth.

Thirty-two wethers were used to compare the nutritive value of Climax timothy (Phleum pratense L.) and quackgrass (Agropyron repens L. Beauv.) harvested at two stages of maturity, joint and early heading, and fed as hay. Crude protein was higher for quackgrass than for timothy (P less than .01). As maturity advanced, CP decreased (P less than .01), but NDF and ADL increased (P less than .01). Dry matter intake was similar between species but decreased by 8% with increasing maturity (P less than .01). Intake of NDF (g/kg.75) was similar for all hays. With increasing plant maturity, apparent digestibility decreased, the effect being more pronounced for quackgrass. Apparent digestibilities of DM and energy were slightly higher (P less than .06) for timothy, and those of CP and hemicellulose were higher for quackgrass (P less than .01). Apparent digestibilities of NDF, ADF, ADL, crude fiber and cellulose were similar between species. Dry matter intake and digestibility were correlated negatively with the ADL/ADF ratio of the hay (r = -.99, P less than .01), whereas CP intake and CP digestibility were correlated with CP of the hay (r = .99, P less than .01). The Lucas test estimated true protein digestibility at 88.1% and metabolic fecal protein at 29.4 g per kg of DM intake. During the growth trial, DM intake was similar between hays, but ADG of sheep was lower (P less than .01) for those fed hays at early heading vs those receiving hays at the joint stage of maturity. The feed to gain ratio was slightly lower for hays at joint (P less than .08). Under the climatic conditions of the 1988 growing season, the nutritive value of quackgrass was similar to that of Climax timothy.

Effects of intrahypothalamic injections of GABA, muscimol, pentobarbital, and L-glutamic acid on feed intake of satiated sheep.

Five wethers were surgically prepared with cranial implants to study the role of gabaminergic neural pathways on the hypothalamic control of feeding behaviour in ruminants. In the first experiment, the animals were injected (1 microL) with a physiological Tyrode (0.95%) solution, muscimol (0.5 and 1.0 nmol), GABA (0.5 and 1.0 nmol), and L-glutamic acid (0.5 and 1.0 nmol). Feed intake following injections of muscimol (1.0 nmol) and L-glutamic acid (0.5 and 1.0 nmol) was twice as large as that following the Tyrode solution, at 60-min postinjections. These results, however, were not statistically significant (p = 0.12-0.15). In the second experiment, the animals were injected (1 microL) with saline, muscimol (0.8 nmol), L-glutamic acid (0.8 nmol), and pentobarbital (0.26 mumol). Fifteen minutes after the injections, pentobarbital had induced a significant feeding response when compared with control values (p less than 0.01), whereas the effect of L-glutamic acid was not significant. However, 30 min after the injections, feed intake of sheep having received L-glutamic acid was higher than that obtained with the control injections (p less than 0.01). The response to pentobarbital was stronger than that to either muscimol or L-glutamic acid. Histological analyses of brain tissue indicated that injections were performed in the ventromedial hypothalamus of four sheep and in the dorsomedial hypothalamus of the other. The data indicate that L-glutamic acid stimulates feed intake by acting either as a precursor of GABA or by a direct stimulation of glutaminergic neural pathways involved in the control of feed intake.

Comparison between pentobarbital- and muscimol-induced feeding in satiated sheep.

Twenty sheep were used to study the mechanisms by which the intracerebral administration of pentobarbital and of muscimol induces feeding in ruminants. Injections of 1 mumol calcium induced a weak feeding response at 1 h postinjection compared with control values (108 vs. 63 g, p less than 0.05). Injections of 78 mumol pentobarbital and of 100 nmol muscimol elicited strong feeding responses (p less than 0.01). A preinjection of 1 mumol calcium reduced the response to pentobarbital by about 40% but did not affect the response to muscimol. Administration of 1.1 mmol sodium chloride reduced the effect to pentobarbital by about 60% but only partially decreased the effect to muscimol. Administration of picrotoxin, a GABA antagonist, slightly decreased the feeding response to pentobarbital and to muscimol. Administration of gamma-vinyl GABA, an inhibitor of the enzyme GABA transaminase, did not affect feeding behavior of sheep at any of the doses tested (0-10 mumol). Injections of gamma-vinyl GABA followed by equimolar injections of GABA failed to provoke any feeding response. The data suggest that pentobarbital and muscimol may induce feeding by acting on a similar hypothalamic receptor complex but by different mechanisms. The lack of effect of GABA itself remains unexplained.

Topographic studies of the effects of microinjections of muscimol on the hypothalamic control of feed intake in sheep.

Ten sheep were used to define the anatomical basis for the feeding systems sensitive to gamma-aminobutyric acid, by using intrahypothalamic microinjections of the gamma-aminobutyric acid agonist, muscimol. In satiated sheep, 1 microL of muscimol (0.5 nmol/microL) elicited feeding when injected into paraventricular, ventromedial, and anterior hypothalamic areas. Similar injections into 39 sites tested in 6-h fasted sheep failed to decrease feed intake. The data suggest that neurons sensitive to gamma-aminobutyric acid in medial hypothalamus may be involved in the initiation of feeding.

Studies of the role of gamma-aminobutyric acid in the hypothalamic control of feed intake in sheep.

Fourteen sheep were used to study the role of gamma-aminobutyric acid (GABA) on the hypothalamic control of feed intake. Injections (1 microL) of pentobarbital (262 nmol) into preoptic and paraventricular areas induced feeding in satiated sheep. Injections of GABA into the same loci gave variable results, probably because the neuronal and glial uptake of GABA limits its effects. Muscimol, a GABA agonist with a higher affinity for postsynaptic GABA receptors than GABA, injected at doses from 0 to 0.750 nmol, gave a cubic dose-response curve; the highest feed intake was measured at 0.5 nmol. The response induced by muscimol was blocked by preinjections of two GABA antagonists, picrotoxin and bicuculline, with picrotoxin being more effective than bicuculline. Muscimol responsive loci were identified mainly in the preoptic, paraventricular, and anterior hypothalamus. The data suggests that neurons sensitive to gamma-aminobutyric acid may be implicated in the control of feed intake in sheep.

Effects of intraventricular injections of gamma-aminobutyric acid and related substances on feeding behavior in satiated sheep.

Feed intake was measured following injections of gamma-aminobutyric acid (GABA), muscimol (a GABA agonist), and picrotoxin (a GABA antagonist) into the lateral ventricles of satiated sheep. Doses ranging from 0.20 to 3200 nmol of GABA did not affect feeding behavior at 15, 30, 60, and 120 min postinjection. A dose of 160 nmol of muscimol induced a marked increase in feeding, comparable to that provoked by an injection of 78 mumol of pentobarbital. Muscimol-induced feeding was blocked effectively by a preinjection of picrotoxin. These observations implicate that neurons sensitive to gamma-aminobutyric acid may be involved in the control of feeding behavior in ruminants.

Absorption of colostral immunoglobulin G in the newborn dairy calf.

Five groups of eight newborn calves were used to study absorption of colostral immunoglobulin G. One feeding of 2 liters of pooled colostrum was given at one of 6, 12, 24, 36, or 48 h after birth. Concentrations of immunoglobulin G in blood plasma and feces were measured by an immunodiffusion technique. Plasma volume and fecal excretion also were measured. When colostrum was given 6 h after birth, 65.8% of the ingested immunoglobulin G appeared in the plasma. This percentage declined rapidly to reach 46.9%, 11.5%, 6.7%, and 6.0% when colostrum was given at the ages of 12, 24, 36, and 48 h. Total fecal immunoglobulin G increased linearly with age. The quantities not recovered from plasma and feces reached a maximum when colostrum was given at 24 or 36 h after birth. Immunoglobulin G can be "lost" to a great extent via routes other than plasma and feces during this time. Quantities of immunoglobulin G measured in plasma represent apparent rather than true absorption.

Effects of intraventricular injections of Ca2+ and(or) Mg2+ on eating behavior, water intake and cardiac rhythm in sheep.

Feeding behavior, water intake and cardiac rhythm were measured following injections of Ca2+ and(or) Mg2+ into the lateral ventricles of sheep. Injections of 20 mumoles of either Ca2+ or Mg2+ elicited an eating response approximately three to four times greater than normal. The effect of Ca2+, however, appeared to be more powerful than that of equimolar doses of Mg2+. When both ions were injected in an equimolar mixture (10 mumoles of each), they did not antagonize each other, but instead elicited an eating response that was additive and equal to that elicited by 20 mumoles of Ca2+. Water intake varied markedly and was not related to the experimental treatments. Cardiac rate was decreased by the injections of 20 mumoles of Ca2+, with the T-P interval of the electrocardiogram affected most. The other experimental treatments did not affect cardiac rate, although the intervals between the waves QRS, T and P of the electrocardiogram were slightly modified. It is concluded that lateral ventricular injections of Ca2+ and Mg2+ can affect the excitability of neurons controlling feeding behavior without necessarily affecting other autonomic centers in sheep, unless the ions reach the vicinity of the fourth ventricle at relatively high concentrations.

Ionic composition (Na+, K+, Ca+2, and Mg2+) of cerebrospinal fluid and feeding behavior in sheep.

Concentrations of Na+, K+, Ca2+, and Mg2+ were measured in the cerebrospinal fluid from the lateral ventricles of hungry and satiated sheep. The concentrations of those ions and the ratios between them varied little before and after a meal. Concentrations were, in milliequivalents per litre: Na+, 156.6 +/- 1.5; K+, 2.94 +/- 0.02; Ca2+, 2.37 +/- 0.04; and Mg2+, 2.18 +/- 0.03.

Feeding and temperature changes in sheep following injections of barbiturates, Ca-++, or Mg-++ into the lateral, third, or fourth ventricle or cerebral aqueduct.

Eleven sheep were prepared with cannula guides directed toward four areas within the ventricular system to determine effective sites of action of chemicals which when injected into the cerebrospinal fluid produce changes in feeding behavior and temperature regulation. Pentobarbital, barbital, calcium chloride, and magnesium chloride elicited feeding in sheep when injected into the third ventricle or into the cerebral aqueduct; however, feeding response was less after injections into the latter. Pentobarbital and magnesium chloride elicited an increase in body temperature when injected into the third ventricle but not when injected into the cerebral aqueduct. Perfusions (push-pull) of the lateral and third ventricles with calcium chloride and magnesium chloride solutions (50 mM) resulted in feeding while similar perfusions of the fourth ventricle resulted in no response. Responses to lateral and third ventricular injections presumably involved effects on both anterior and posterior hypothalamic areas while injections into the cerebral aqueduct, due to the caudal flux of the cerebrospinal fluid, may have affected primarily only the posterior hypothalamus and more caudal structures. The feeding response probably resulted from depression of neural fibers which inhibit feeding.

Feeding following intrahypothalamic injections of calcium and magnesium ions in sheep.

To study ionic effects on hypothalamic control of feeding, seven sheep were each prepared with six bilateral medial-hypothalamic guides. Hypothalamic loci were identified where feeding resulted following 1 mu 1 injections of calcium chloride and/or magnesium chloride. Doses of 1 and 2 mumol were most effective to induce feeding, and potassium chloride was not effective. Injections of phentolamine, LB-46, carbachol, atropine, or prostaglandin E1 did not inhibit the feeding responses following magnesium chloride injections into magnesium ion responsive loci. Feeding following calcium chloride injections into calcium ion responsive loci was blocked by atropine and reduced by phentolamine. Carbachol and prostaglandin E1 elicited feeding when injected into magnesium but not calcium ion responsive loci. Injections of carbachol followed by magnesium chloride resulted in greater feeding than when either chemical was injected alone. Calcium and/or magnesium ion bound feeding loci were in the anterior, ventromedial, dorsomedial, posterior and lateral hypothalamus, and the mammillary bodies. These ions elicited feeding in sheep perhaps by decreasing excitability of feeding-inhibitory neural pathways in the hypothalamus.