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Understanding the particularities of pasture inclusion on cows' diets and their subsequent impact on the digestive processes, together with possible differences in the nutrient utilization of cows managed under grazing conditions, are challenges that must be elucidated in order to design strategies to improve the cows' productive performance. The objectives of this dissertation were: 1) to review the existing literature on ration formulation for dairy cows in pasture-based systems, identifying limitations and potential improvement areas, 2) to evaluate the adequacy of the Molly model predictions of ruminal fermentation, nutrient digestion, and performance from cows consuming fresh ryegrass-based diets, identifying mechanisms that could be used to direct further model improvements, 3) to evaluate the model predictions of milk, fat and protein production using data from Holstein-Friesian and Jersey cows from Chile managed under pasture-based systems, identifying mechanisms that could be used to direct further model improvements, 4) to identify the different dairy breeds and their crosses that are used in the Chilean dairy population, determining if there are specific breed and heterosis effects on productive and some functional traits, and 5) to characterize the ruminal degradation dynamics of crude protein (CP) and individual amino acids (AA) from ryegrass (Lolium perenne) at vegetative stage, generating information to improve the ration formulation in dairy cows in pasture-based systems. From the literature revision in objective 1, different factors that could help to create a more specific classification of dairy pasture-based systems were identified. Maximizing grass inclusion seems to leads to an oversimplification of diets, potentially limiting the performance of medium and high producing cows. Despite the low N use efficiencies reached in pasture-based systems, there is a lack of information in studies that delve into the ruminal degradation of CP and AA, as well as their intestinal absorption, and subsequent post-absorptive utilization. Advances in the understanding of these areas could help to create strategies to face this problem. There is evidence suggesting that cows can perform different under different productive systems, but various breeds, strains, and crossbreds are used in pasture-based systems, being needed first a better characterization of them. To achieve the second objective, a total of 25 studies (n = 115 treatments) including dairy cows consuming ryegrass predominant diets, published from 1970 through 2020, were collected from the literature and used to assess the model accuracy and precision based on root mean squared errors and concordance correlation coefficients (CCC). Predictions of protein and fiber digestion and fiber and organic matter fecal excretion were improved after model reparameterization, while body weight and body condition score predictions were improved after model modifications and reparameterization. Although digestion of nutrient was better represented by the new set of parameters derived, the accuracies of milk, protein, and fat production remained low (CCC of 0.13, 0.12, and 018, respectively), with moderate slope bias. To achieve the third objective, a dataset including 180 group cows' diets (153 from Holstein Friesian and 27 from Jersey cows) from 18 commercial dairy farms with different inclusion of ryegrass was compiled. Chemical composition of feeds ingredients, group intakes and animal performance records, were provided by the Chilean cooperative Colun. Separately model mammary reparameterization of Holstein Friesian and Jersey cows' data, improved both milk production (CCC values of 0.69 and 0.90, respectively) and milk solids predictions (CCC values of 0.68 and 0.92 for milk protein and 0.65 and 0.80 for milk fat, for Holstein Friesian and Jersey cows, respectively). This indicates that there seems to be some differences in mammary cells and cells activity between breeds evaluated, translated into different lactation curves and milk solids synthesis patters. To achieve the fourth objective, a dataset considering 1,429,132 records from 586,624 cows that calved between 1998 and 2018 was compiled. Pedigree information, milk, milk protein, and milk fat, as well as somatic cell score and calving interval records were provided by the Chilean cooperative COOPRINSEM. The proportion of each breed according to the different country origin (strain) was calculated for all the animals, being identified eight genetic strains with enough information to perform separated genetic evaluations: Chilean Friesian, French Holstein-Friesian, US-Holstein, US-Jersey, French Montbeliarde, New Zealander Holstein-Friesian, Swedish Red and White, and British Friesian. Four different genetic models, increasing in complexity (considering breed or strain classification, crossbreeding proportion, and heterosis effects) were tested. Specific effects for some genetic strains, as well as heterosis effects between some strains with Chilean Friesian were identified, so these effects should be considered when performing genetic evaluations in the Chilean dairy population. Also, they can be used to direct future selection programs. To achieve the last objective, an in situ study using three cannulated cows was carried out at the Agricultural Research Station of the Austral University of Chile (Valdivia, Chile). Samples from a perennial ryegrass pasture were collected during winter, spring and summer at vegetative stage. Duplicate bags were ruminally incubated for 4, 8, 12, 24, and 96 h. CP and AA profile of original samples and from incubation residues were determined. Ryegrass samples presented a high crude protein content (averaging 25.8%), however, the RUP content was ~15 to 20% of CP. Significant differences were observed for ryegrass CP and individual AA ruminal degradation kinetics, this seems to be associated to some specific AA. / Doctor of Philosophy / The used of pasture-based systems in dairy production are an important alternative in countries were local environmental and climatological conditions, as well as soil properties are suitable for the growth of high-quality grasslands. The main advantages attributed to the use of pasture-based system are the reduction in the production costs, and potentially improving the nutrition quality of dairy products as well as the cows' welfare. However, they have some important limitations as the natural variation in the quantity and quality of available herbage. Also, there is a lack of information on fundamental nutrition of pasture nutrients digestion and utilization by the cows manage and selected under these environmental conditions. The first two objectives of this dissertation consisted in using the Molly nutritional model, which represents the biology of the whole cow, in order to identify key aspects to better understand the cows manage under pasture-based systems and develop strategies to precise their nutrition. Particularly, topics such as prediction of ruminal fermentation and digestive processes, mobilization of body reserves, mammary glands metabolism, and potential differences between Holstein-Friesian and Jersey cows were analyzed. Overall, work on these objectives resulted in improvements to the Molly model for making predictions of diets that include grass. The diversity of breeds and their country of origin, as well as different crosses in the Chilean dairy population were explored. A more comprehensive genetic model was proposed in this work to precise the calculation of breeding values in the country. Furthermore, our findings could be used as inputs to direct future selection programs. The last objective explored the ruminal degradation of crude protein and each individual amino acid in ryegrass. We demonstrate that indeed there are differences in the degradation parameters for some amino acids. Due to the high degradability of fresh ryegrass, there could be some potential limitations in the supply of some essential amino acids, especially in high producing cows consuming predominant ryegrass diets.

Successful implementation of dietary P management strategies demand improved understanding of P digestion dynamics in ruminants and this is not possible without a reliable and accurate phytate (Pp) quantification method. The objective of the first study was to develop a robust, accurate, and sensitive method to extract and quantify phytate in feeds, ruminant digesta and feces. Clean-up procedures were developed for acid and alkaline extracts of feed, ruminant digesta and feces and clarified extracts were analyzed for Pp using high performance ion chromatography (HPIC). The quantified Pp in acid and alkaline extracts was comparable for feed but alkaline extraction yielded greater estimates of Pp content for digesta and feces than did acid extraction. Extract clean-up procedures successfully removed sample matrix interferences making alkaline extraction compatible with HPIC. The developed method was applied to investigate the disappearance of Pp from the large intestine of dairy heifers. Eight ruminally- and ileally-cannulated crossbred dairy heifers were used and each heifer was infused ileally with 0, 5, 15, or 25 g/d Pp and total fecal collection was conducted. On average 15% of total Pp entering the large intestine was degraded but the amount of infused Pp did not influence the degradability of Pp. Net absorption of P from the large intestine was observed. A feeding trial was conducted to investigate the effect of dietary Pp supply on ruminal and post-ruminal Pp digestion. Six ruminally-and ileally-cannulated crossbred lactating cows were used and dietary treatments were low (0.10% Pp), medium (0.18% Pp), and high (0.29% Pp) Pp, and a high inorganic P (Pi; 0.11% Pp; same total P content as high Pp). Ruminal Pp digestibility increased linearly with dietary Pp. As in the infusion study, net disappearance of Pp from the large intestine was only 16% of total Pp entering the large intestine and not influenced by dietary Pp. Fecal P excretion increased linearly with increasing dietary Pp but was not affected by form of dietary P. In lactating cows Pp digestibility was not affected by dietary Pp and fecal P excretion was regulated by total dietary P rather than by form of dietary P. / Ph. D.

Pasture-based beef production is well-suited for the Appalachian region of the United States. This research investigated pasture, beef cattle, and economics components within the cow-calf sector of pasture beef production, and presents implications of their interplay for the vitality of the whole system. Samples of forage DM mass and CP, ADF, NDF, and ash contents in each paddock of a rotational stocking system were collected monthly for 4 grazing seasons. Effects of month, stockpiling, hay feeding, temperature, precipitation, and durations of paddock grazing and rest on forage mass and quality measurements were investigated. The system was complex and dynamic; precipitation and rest days in particular showed clear interactions with both month and stockpiling when predicting forage mass and quality. Available DM, TDN, and CP were compared to nutrient density requirements for beef cows to conclude that the system met or exceeded requirements. Six years of production data from a spring-calving cow-calf enterprise that utilized rotational stocking and fall stockpiling were analyzed. Comparisons among 2 cow frame size and 2 calf creep system treatments for production efficiency (total weaning weight per land area), and net returns to the enterprise, were made. Pastures with medium frame cows and designated creep systems had the greatest production efficiency, but also had the highest costs and netted the least returns. Greatest net returns were achieved in large frame, forward creep systems, which had the lowest production efficiency but also the lowest costs. Provided the quality of calves produced is suitable for other phases of the production stream, we conclude that minimizing costs rather than maximizing productive outputs is a better focus for cow-calf enterprises faced with similar decisions among frame size and creep system treatments. / Ph. D.

The beef cattle industry has placed increased focus on mature cow size as a result of its influence on production efficiency and profitability. The objectives of this study were to evaluate relationships among lifetime measures of body weight (BW) and frame score (FS) in commercial beef females, and to assess the value of immature measures as predictors of mature cow size. Measurements of BW, hip height (HH), body condition score (BCS), and calculated FS were recorded at weaning (WN), breeding at 13 mo age (BR), and 8 subsequent periods, ceasing at approximately 5 yr of age for 232 Angus-cross females born 2004 through 2008. Correlation analysis revealed significant (P < 0.001) relationships among BW taken at WN and BR with BW measurements taken at 2.5, 3.8, and 4.8 yr of age (WN r = 0.70, 0.51, 0.61; BR r = 0.65, 0.57, 0.64, respectively). Significant relationships (P < 0.001) existed between FS collected at WN and BR, and FS at 2.5 and 3.8 yr (WN= 0.70, 0.72; BR= 0.79, 0.82, respectively). Repeatability of lifetime FS measures was 0.73. BCS was a significant (P < 0.001) source of variation in mature BW, with a unit change in BCS accounting for 41 kg BW change at 4.8 yr (P < 0.001). BW and FS were moderately to strongly related (P < 0.001) at WN, BR, 2.5, 3.8, and 4.8 yr (r = 0.62, 0.49, 0.62, 0.62, and 0.47 respectively). Prediction models for BW at 4.8 yr were similar using weaning BW alone, or with inclusion of both weaning BW and HH (R2 = 0.57 and 0.56). Similarly, breeding BW and HH were non-additive for prediction of 4.8 yr BW (R2 = 0.68, 0.58, and 0.68 for BW, HH, and BW +HH respectively). Performance at immature ages proves to be a satisfactory indicator of mature size, supporting continued incorporation of immature BW and HH and/or FS measurements into selection practices. / Master of Science

Two studies were conducted to evaluate the effects of diet on phosphorus (P) digestion in dairy cattle. The objective of the first study was to evaluate the effects of forage and non-fiber carbohydrate (NFC) content on total P (TP) and inositol phosphates-P (IPs-P) digestion. Samples of feed, duodenal digesta and feces from a previously conducted study were analyzed for TP and IPs-P. In this study, eight lactating Holstein cows were fed diets containing either 60 or 35% forage and either 30 or 40% NFC in a 2 &#215; 2 factorial with replicated 4 &#215; 4 Latin square design. Dietary TP content (% DM) was 0.35, 0.36, 0.36, 0.36 and dietary IPs-P content (%DM) was 0.08, 0.13, 0.06, 0.11 for diets with forage: NFC ratio 35:30, 35:40, 60:30, and 60:40, respectively. Increasing dietary forage content decreased IPs-P and TP intake, fecal TP excretion, and total tract IPs-P digestibility (72.4 vs. 61.4%). Fecal IPs-P excretion tended to decrease as increasing forage content. Duodenal IPs-P and TP flow and apparent TP digestibility were unaffected by forage content. Increasing dietary NFC content increased IPs-P and TP intake, duodenal IPs-P flow, fecal IPs-P excretion, total tract IPs-P digestibility (61.4 vs. 72.4%), and apparent TP digestibility (32.8 vs. 41.6%). Dietary forage and NFC content affected IPs-P and TP digestion. The second study was to evaluate the effects of increasing dietary beet pulp (BP) content to replace high moisture corn (HMC) on ruminal and post-ruminal digestion of TP and IPs-P. Eight lactating Holstein cows were fed diets containing 0, 6.1, 12.1 or 24.3% BP in a replicated 4 &#215; 4 Latin square design. Samples of rumen contents, duodenal digesta, and feces from this previously conducted study were analyzed for TP and IPs-P content. Linear and quadratic effects of BP content were analyzed using Proc Mixed of SAS. Dietary TP and IPs-P content were reduced linearly with increasing BP (0.59, 0.58, 0.57, 0.56% TP and 0.15, 0.14, 0.13, 0.11% IPs-P). Intake, ruminal content, and rumen pool size of TP decreased with increasing BP content. Digestion of TP and duodenal flow and fecal excretion of IPs-P and TP were not affected. With increasing dietary BP content, IPs-P intake was reduced, ruminal IPs-P pool size was reduced, and rumen turnover time (h) of IPs-P was increased. Apparent ruminal IPs-P digestibility (36.5, 31.8, 24.6, 13.6 %) and apparent total tract IPs-P digestibility (85.3, 82.7, 82.1, 79.1%) decreased linearly with increasing BP. Fecal excretion of IPs-P averaged 5.2 g/d. Replacing HMC with BP reduced digestion of IPs-P. The majority of IPs-P disappearance occurred post-ruminally. In conclusion, dietary BP, forage, and NFC content affected IPs-P digestion in dairy cows. / Master of Science

Two studies were conducted to evaluate the effects of 5 or 7 d post-AI GnRH on first service PR, plasma P4, and CL volume in lactating dairy cows synchronized using 72 h Cosynch. All cows were synchronized and randomly assigned to one of three treatment groups: Control – no additional GnRH; 5 d – GnRH 5 d after TAI; 7 d – GnRH 7 d after TAI. In the first study, P4 concentrations were evaluated in samples collected at five separate times and CL volume and number were recorded at 30 d pregnancy examination for Holstein (n = 77) and Jersey (n = 33) cows. GnRH treatment did not affect PR (Control - 47.2%, 5 d GnRH - 40.5%, 7 d GnRH – 44.7%) or P4, but increased TCLV compared to controls (Control – 7.33 cm3, 5 and 7 d GnRH – 10.77 cm3). Incidence of accessory CL increased PR (94.7 vs. 60.6%), P4 (6.95 vs. 5.88 ng/mL), and TCLV (15.51 vs. 6.78 cm3) compared to cows with a spontaneous CL. Cows classified as cycling based on P4 evaluation had significantly higher PR than acyclic cows (54.4 vs. 16.1%). In the second study, Holstein cows (n = 1055) were submitted to the same experimental protocol and evaluated for first service PR. Post-AI GnRH treatment did not significantly affect PR. Primiparous cows (32.8%) tended to have higher PR than multiparous cows (27.6%), but GnRH treatment had no influence on this relationship. In conclusion, GnRH post-AI did not affect PR. Further evaluation of accessory CL incidence is warranted as it significantly affected PR. (Abbreviations: AI – artificial insemination, CL – corpus luteum, PR – conception rate, P4 – progesterone, TCLV – total corpus luteum volume) / Master of Science

The objectives of this study were to determine: 1) ammonia emission rates from stored scraped and flushed manure from dairy cows fed either normal or low N diet; and 2) seasonal effects on ammonia emission rates from stored scraped and flushed dairy manure. Four pilot-scale tanks were used for manure storage with different treatments - scraped manure for normal diet (NS), flushed manure for normal diet (NF), scraped manure for low N diet (LS), and flushed manure for low N diet (LF). The first part of the study lasted for 1 month and four treatments were all investigated; the second part of the study lasted for 12 months and two tanks with treatments NS and NF were investigated. Dynamic flux chambers and a photoacoustic gas analyzer were used to measure ammonia emission rates. There was no significant change of the N content of manure as the dietary N content is reduced (from 17.8% to 15.9% crude protein). However, ammonia emission rates from manure storage tanks were reduced by 33% (from 27.4 ± 38.1 to 18.4 ± 21.9 mg m⁻²h⁻¹; P<0.0001 based on paired t-test). Flushing manure reduced emission rates by 72% compared to scraping manure (from 35.6 ± 39.6 to 10.1 ± 8.2 mg m⁻²h⁻¹; P<0.0001 based on paired t-test). Ammonia emission rates for NS, NF, LS and LF were 43.9 ± 48.0, 10.9 ± 8.7, 27.4 ± 27.3, and 9.3 ± 7.8 mg m-2 h-1, respectively. The chamber headspace temperature for NS, NF, LS and LF were 26.0 ± 6.9, 25.8 ± 6.8, 26.6 ± 6.5, and 27.2 ± 6.7 °C, respectively. The manure pH for NS, NF, LS, and LF were 6.3 ± 0.1, 6.4 ± 0.3, 6.4 ± 0.1, and 6.1 ± 0.1, respectively. Both dietary N reduction and manure flushing are recommended to reduce ammonia emission rates from dairy manure storage tanks. Ammonia emission rates were higher in summer and fall, due to higher air temperature and higher manure pH. The pH of scraped manure was 7.2 ± 0.6, 6.7 ± 0.2, 6.5 ± 0.3 and 7.0 ± 0.3 for fall, winter, spring and summer, respectively. The pH of flushed manure was 6.8 ± 0.4, 6.7 ± 0.4, 6.4 ± 0.3 and 6.8 ± 0.4 for fall, winter, spring and summer, respectively. Ammonia emission rates from scraped manure for fall, winter, spring, and summer were 7.4 ± 8.6, -0.5 ± 1.2, 1.1 ± 1.9, and 5.8 ± 2.7 mg m⁻²h⁻¹, respectively. Ammonia emission rates from flushed manure for fall, winter, spring, and summer were 3.9 ± 4.2, -0.5 ± 0.9, 0.8 ± 1.4, and 4.4 ± 1.2 mg m⁻²h⁻¹, respectively. Seasonal changes of air temperature and manure pH were key factors affecting ammonia emissions from manure storage in this study. Seasonal climate conditions including precipitations (rainstorms and snows) and icing can cause reduction of ammonia emissions from manure storage in open air. More attention should be paid to reduce ammonia emissions in warmer seasons, e.g., by covering the storage facilities. / Master of Science

Heat stress (HS), a stress response in homeotherms mainly due to elevated ambient temperature and failure of effective heat dissipation, causes a substantial negative economic impact to livestock industry worldwide. Reduced feed intake, a typical phenomenon observed during HS, was thought to be the primary driver for the milk production loss. However, accumulating evidence indicates that HS influences animal metabolism and endocrine profiles independent of reduced feed intake. Previous studies comparing heat-stressed lactating cows with control group pair-fed (PF) to the intake of HS group but housed in thermoneutral conditions, in order to eliminate the confounding factors result from differentiated feed intakes, showed that HS increased circulating insulin and decreased plasma non-esterified fatty acid (NEFA) in lactating cow, the opposite responses typical of PF cohorts. Therefore, the present studies were performed in order to elucidate the mechanism(s) underlying these counterintuitive changes. In response to a glucose tolerance test (GTT), both HS and PF decreased whole body glucose disposal rate, a sign of insulin resistance. Only PF decreased skeletal muscle insulin sensitivity in terms of reduced protein kinase B (PKB/AKT) phosphorylation, a downstream protein of insulin receptor (IR), while HS group maintained similar intact insulin responsiveness in the liver and skeletal muscle as thermoneutral conditions. There was a global reduction in gene expression of the enzymes related to lipid metabolism in adipose tissue of heat-stressed cows. Similarly, β-adrenergic signaling, a major stimulator of lipid mobilization, was suppressed in terms of NEFA release response during a chronic epinephrine challenge in HS group. After the challenge, phosphorylations of cAMP-response element binding protein (CREB) and hormone sensitive lipase, both located downstream of β-adrenergic receptor, were decreased in HS, but not in thermoneutral conditions, another indicator of impaired adrenergic signaling. In contrast, IR and AKT phosphorylation were increased in HS conditions indicating insulin signaling may be elevated during HS in adipose. Collectively, HS reduces lipid mobilization and appears to favor glucose utilization via alterations of lipid metabolism and hormones signaling pathways. These unique alterations in HS might shed some light on developing counter-HS approaches in the future. / Ph. D.

The largest agricultural enterprise in the Appalachia region of Virginia is livestock production, particularly beef cow calf operations. However; topography and land holding patterns have resulted in a majority of small farms operating part time on less than 150 acres. These farms, while abundant, are not necessarily profitable. Management intensive grazing has been suggested as an alternative to traditional production practices to increase profitability. A profitability analysis was conducted by comparing a traditional style of cow-calf management where hay was produced on farm with a full machinery complement to a management intensive grazing farm where forages are stockpiled and all hay was purchased on farm, requiring minimal machinery investment. Four farms were simulated using Finpack Farm Management software by utilizing production data from the Virginia Tech Shenandoah Valley Agriculture Research and Extension Center and secondary financial data. The first two farms were listed as having a traditional style of management with hay production and two different stocking rates, 1.75 acres per cow-calf unit and 2.25 acres per cow-calf unit, respectively. Farm 3 and Farm 4 were simulated utilizing management intensive grazing and the two socking rates. It was found that while none of the farms actually show a profit the management intensive farms did outperform the traditional style farms. Farm 3 with the 1.75 acre per cow-calf unit stocking rate was the best performing farm financially. With these findings, beef cow producers will be able to make better management decisions and explore more profitable alternatives. / Master of Science