<p>Urea kao deo neproteinske frakcije azota mleka, predstavlja krajnji produkt metabolizma proteina u predželucima preživara. Portalnim krvotokom toksični amonijak, nastao mikrobiolo&scaron;kom razgradnjom proteina u rumenu dolazi do jetre gde se transformi&scaron;e u ureu, koja kasnije krvotokom dospeva u mleko. Ishrana, odnosno sadržaj sirovih proteina u obroku ima najveći uticaj na sadržaj uree u mleku. Sadržaj uree sve vi&scaron;e se koristi kao parametar pomoću kojeg je moguće pratiti unos sirovih proteina i energije obrokom kod mlečnih krava. Pored ishrane na sadržaj uree u mleku utiču i drugi faktori, kao &scaron;to su sezona, prinos mleka, stadijum laktacije, paritet i dr.<br />Plodnost krava je veoma važna osobina mlečnih goveda na koju utiče veliki broj faktora. Dosada&scaron;nja istraživanja u ovoj oblasti ukazuju na moguću vezu između plodnosti krava i sadržaja uree u mleku.<br />Određivanje sadržaja uree u mleku primenom infracrvene spektrofotometrije u analizi mleka prilikom redovne mesečne kontrole mlečnosti krava pružilo je nove mogućnosti u praćenju kvaliteta mleka.<br />Cilj ovog istraživanja bio je da se utvrdi:<br />1. variranje sadržaja uree u mleku unutar stada,<br />2. uticaj paragenetskih faktora (farma, sistem proizvodnje, sistem držanja, sezona, redni broj laktacije i stadijum laktacije) na sadržaj uree u mleku,<br />3. korelacija sadržaja uree sa parametrima mlečnosti krava i brojem somatskih ćelija i<br />4. korelacija između sadržaja uree u mleku i dužine servis perioda.</p><p>U okviru ovog istraživanja analizirano je 46.315 uzoraka mleka, u okviru redovne kontrole mlečnosti, sa 11 farmi sa područija Vojvodine. Od kojih je jedna sa organskom, a 10 farmi sa konvencionalnom proizvodnjom. Određivanje sadržaja uree u mleku vr&scaron;eno je na aparatu MilcoScan.<br />Prosečan sadržaj uree u mleku (25,18 mg/dl) bio je u okviru optimalnih vrednosti sadržaja uree u mleku. Koeficijent varijacije za sadržaj uree (34,15%) bio je znatno veći od koeficijenta varijacije za ostale sastojke mleka.<br />Dobijeni rezultati u okviru ove doktorske disertacije ukazuju da najveći uticaj na sadržaj uree u mleku ima menadžment farme. Utvrđen je i statitički značajan uticaj ostalih ispitivanih paragenetskih faktora.<br />Sistem držanja statistički značajno utiče na sadržaj uree u mleku. Niži sadržaj uree u mleku bio je kod krava u slobodnom sistemu držanja.<br />Kao posledica specifičnosti organske proizvodnje konstatovane su statistički značajne razlike između organske i konvencionalne proizvodnje mleka. Razlike se ogledaju u smanjenom sadržaju uree u mleku i prinosu mleka, a povećanom sadržaju mlečne masti i proteina. Broj somatskih ćelija u mleku iz organske proizvodnje je znatno manji u odnosu na broj istih u mleku iz konvencionalne proizvodnje mleka, dok je servis period u posmatranom periodu bio duži kod krava u organskoj proizvodnji u odnosu na krave u konvencionalnoj proizvodnji mleka.<br />Posmatrano po sezonama kontrole najniži sadržaj uree u mleku bio je u jesenjem periodu (22,19 mg/dl), a najveći u letnjem period (27,11 mg/dl). Razlike u sadržaju uree po laktacijama su male, ali statistčki značajne.<br />Konstatovan je statistički značajan uticaj stadijuma laktacije na sadržaj uree u mleku. Najniži sadržaj bio je na početku laktacije (23,05 mg/dl), a maksimalan sadržaj uree u mleku od 26,40 mg/dl, bio je od 121 do 180 dana laktacije.<br />Utvrđene su pozitivne, statistički visoko značajne, korelacije između sadržaja uree i pojedinih sastojaka mleka, kao i između sadržaja uree i prinosa mleka. Korelacija između sadržaja uree i broja somatskih ćelija u mleku je negativna i visoko statistički značajna.<br />U okviru ovog istraživanja utvrđena je pozitivna i statistički visoko značajna korelacija između sadržaja uree u mleku i dužine servis perioda.<br />Dobijeni rezultati, tumačeni u kontekstu drugih istraživanja na ovu temu ukazuju na veliki značaj praćenja sadržaja uree u mleku, koji može doprineti pobolj&scaron;anju menadžmenta ishrane i reprodukcije na farmama mlečnih krava, a sve u cilju smanjenja tro&scaron;kova na farmama, kroz smanjenje tro&scaron;kova ishrane i tro&scaron;kova veterinarskih usluga.</p> / <p>Urea as a part of non-protein fraction of nitrogen in milk represents the final product of protein metabolism in the rumen of ruminants. By portal bloodstream toxic ammonia comes into liver where it is transformed into urea, which comes later into milk by bloodstream. Nutrition and contents of crude proteins in the diet have the greatest influence on the milk urea content. The content of milk urea has been increasingly used as a parameter by which it is possible to monitor balance of energy and crude proteins in the feed of dairy cows.<br />Apart from feeding, milk urea content can be influenced by some other factors as season, milk yield, stage of lactation, parity etc.<br />A great number of factors influence cow&rsquo;s fertility. Previous research in this field has indicated a possible relationship between the fertility of cows and urea content in milk.<br />Determination of urea content in milk, by infrared spectrophotometric, on monthly bases, for milk recording porpoises, offers new opportunities for milk quality monitor.<br />The aim of this study was to determine:<br />1. Variations of milk urea content within the herd,<br />2. The influence of paragenetic factors (a farm, production and housing system, season, parity and stage of lactation) on the milk urea content,<br />3. The correlation with milk urea content and parameters of milk yield as well as the somatic cells count,<br />4. The correlation between the milk urea content and the length of open days.<br />This research includes 46,315 samples of milk which were analysed as regular milk recording samples from 11 farms in Vojvodina. There are 10 farms with</p><p>conventional and only one farm with organic production.<br />Determination of the milk urea contents was carried out by MilkoScan FT+.<br />In the analyzed milk samples the average milk urea content (25.18 mg / dl) was within the optimum values. The coefficient of variation for the milk urea content (34.15%) was significantly higher than the coefficient of variation for the other ingredients in milk.<br />The obtained results in this PhD thesis indicate that the management of a farm has the greatest influence on the content of milk urea. Statistically some significant influence of other examined paragenetic factors was found.<br />According to statistics the housing system significantly influences the milk urea content. Lower milk urea content was found in cow&#39;s milk in free-stall system.<br />As the result of specific organic production some statistically significant differences between organic and conventional milk production were found. The differences shows lower milk urea content and milk yield, as well as higher milk fat and protein content. The somatic cells count in organic milk production is significantly lower than their number in milk from conventional milk production. The open days during observation period were longer in cows from an organic dairy farm then the cows from conventional dairy farms.<br />According to season and stage of lactation an important influence on milk urea was found. The seasonal observation shows the lowest milk urea content in autumn period (22.19 mg / dl) and the highest in summer period (27.11 mg / dl). The lowest content was at the beginning of lactation (23.05 mg / dl) and the maximum milk urea content (26.40 mg /dl) was from 121 to 180 days of lactation. Differences in the milk urea content between lactation were small, but statistically significant.<br />There are positive and statistically significant, correlations between the milk urea content and some ingredients of milk, as well as between milk urea content and milk yield. The correlation between the milk urea content and somatic cells count in milk is negative and high statistically significant.<br />It is shown in this research that high milk urea content has negative impact on the length of open days.<br />Obtained results, interpreted in the context of other studies on the same subject indicate the great importance of the observation of the milk urea content. It can contribute to improving the feeding and reproduction on dairy farms, and lead to reduction of both feeding and veterinary costs.</p>

A positive phenotypic correlation between milk protein percentage and reproductive performance in dairy cattle, especially during early lactation has been recently reported. The objective of this study was to quantify the relationship between milk protein percentage and different measures of fertility in Irish, seasonal calving, dairy cattle using data from experiments comparing strains of Holstein-Friesian cows under different feeding systems. The relationships between body condition score, milk production and fertility were also investigated. The data used in this study consisted of 584 lactation records over a 5-yr period. Principal component analysis and logistic regression was used to study the relationship between milk protein percentage and fertility performance of the cow. Greater milk protein percentage during the first 60 days post-calving was associated with better reproductive performance. The probability of a cow being submitted in the first 21 days of the breeding season increased with increased milk protein percentage during early lactation. Similarly, the probability of a cow becoming pregnant to its first service or to the whole breeding season also increased. Cows were classified as either high or low milk protein percentage based on their protein percentage over the whole lactation. Cows in the high milk protein group had a 7% greater conception rate compared to cows in the low protein percentage group. In conclusion, cows with higher protein percentage, especially during early lactation are submitted earlier in the breeding season, and have a higher conception rate. Physiologically, the shortage of glucose caused by negative energy balance restricts the synthesis of milk protein in the udder. On the other side, negative energy balance also causes the reduction of IGF-I, LH and oestradiol, which consequently delay the ovarian follicular development and finally reduces fertility. Therefore, there is a biological explanation for the association between milk protein percentage and fertility performance.