Koncentracija uree u kravljem mlijeku poznata je i često se koristi za procjenu izbalansiranosti obroka energijom i proteinima. Podataka o koncentraciji uree u kozjem mlijeku zbog njegove manje ekonomske važnosti, sezonske poliestričnosti i različitog načina uzgoja i držanja koza relativno je malo. Osnovna je pretpostavka ovoga doktorskog rada da će utvrđena koncentracija uree definirati optimalan udio sirovih proteina (SP) u krmnoj smjesi (KS) nužnih za najmanja odstupanja kvalitativnih i kvantitativnih svojstava mlijeka. Za utvrđivanje te povezanosti istraživanje je obuhvatilo 72 alpina koze, podijeljene u tri skupine sa 24 slučajno odabrana grla. Svaka je skupina obuhvatila koze od 1. do 4. laktacije koje su hranjenje krmnom smjesom sa 14%, 16% i 18% SP. Uzorci mlijeka uzimani su jedanput mjesečno AT metodom tijekom jedne laktacije. Pojedinačni uzorci mlijeka za svaku kozu koristiti su se za utvrđivanje fizikalnih svojstava (pH, °SH, točka ledišta), kemijskog sastava (osnovni, omjer mliječne masti i proteina, NPN i urea) i fiziološkog broja somatskih stanica. U skupnim uzorcima mlijeka od svake skupine koza određena su svojstva zgrušavanja (vrijeme zgrušavanja, brzina formiranja gruša i čvrstoća gruša). Statistička analiza podataka provedena je procedurom MIXED, modelom s ponovljenim mjerenjima korištenjem softverskog paketa SAS V8 (SAS STAT, 2015.). Alpina koze hranjene krmnom smjesom sa 16% SP prosječno su dnevno tijekom laktacije proizvele 3,33 kg mlijeka sa 3,16% mliječne masti, 2,85% proteina, a prosječna koncentracija uree u mlijeku bila je 41,24 mg/100 mL. Povećanje udjela SP sa 14% na 16% u krmnoj smjesi povećalo je dnevnu količinu mlijeka za 0,4 kg (p<0,001) i koncentraciju uree u mlijeku od 35,01 na 41,24 mg/100 mL (p<0,001). Na temelju izračunate točke infleksije dnevne količine mlijeka i udjela proteina u mlijeku koncentracija uree u mlijeku alpina koza bila je između 40,00 i 45,00 mg/100 mL i ne viša od 45,00 mg/100 mL. Koncentracija uree između 40 i 45 mg/100 mL može se smatrati optimalnom i dobar je pokazatelj izbalansiranog obroka za alpina koze, uravnoteženih metaboličkih mikrobnih procesa u buragu i najmanjeg odstupanja u dnevnoj količini, kemijskom sastavu i preradbenim svojstvima mlijeka. Udio sirovih proteina u krmnoj smjesi viši od 16% povećava koncentraciju uree u mlijeku na više od 45 mg/100 mL, a nema učinak na povećanje dnevne količine i kemijski sastav mlijeka.
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Milk urea concentration is a useful indicator of energy and protein balanced diet and it is used in systematic control of cow milk, while in sheep and goat milk it is not determined/controled. Main determinents of urea formation in milk are the amount of crude protein intake (CP) and the ratio of the protein and energy portion in diet. Goat diet with excessive amount of crude protein and energy unbalanced diet cause the excess of nitrogenous substances in rumen along with release of ammonia and rise in concentration of urea in blood and milk, which adversely affects the production, coagulation propreties, environmental pollution and reproductive capabilities of goats. However, besides diet, there are other factors that influence milk urea concentarion: breed, stage of lactation, parity, milking time, season, body mass, litter size, production and chemical composition of milk. Numerous studies conducted on cow milk included the influence of listed sources of variability, while there are only few studies available for goat milk. Starting form the fact that milk urea concentration mostly depend on crude protein (CP) intake in diet, assumption of the study was to determine optimum ratio of crude protein in feed mixture (KS) by determining the urea concetration in goat milk. Also, assumption of the study was, that the milk urea concentration will affect milk quality for further processing. Therefore the goals of the study were to determine: environmental factors of urea concentration variation, indicators of milk production and quality, coagulation propreties of milk depending on urea concentration and finally to suggest recommendable range of values for milk urea concentration in Alpina goats. The study included a total of 72 goats, divided into 3 groups with 24 randomly selected goats at age from 1 to 5 years without visible signs of mastitis and defined diet with different ratio of crude protein in feed mixture (14%, 16% and 18%). During the lactation, regular monthly controls of goat milk yield were conducted (AT method), individual milk samples for analysis of physico-chemical composition and hygiene quality of milk were collected, while for determination of coagulation properties of milk, group samples from each group were collected. The proportion of total solids, milk fat, protein, casein, lactose, total solids non fat, milk urea concentrations, pH value, titration acidity, freezing point, total number of microorganisms (UBB) and number of somatic cells (BSS) were determined in goat milk samples. Coagulation propreties were determined with formagraph. Statistical data processing was performed using the SAS V8 software package (SAS STAT, 2015). Based on the results of the descriptive statistical data, it was found that during lactation, the goat produced daily 3.41 kg of milk with 3.14% milk fat, 2.85% of protein, and the average urea concentration in milk was 41.43 mg/100 mL. The increase in the proportion of crude protein in feed mixture was accompanied by an increase in daily milk production, daily protein production and milk urea concentration, as well as a decrease in the proportion of total solids, milk fat, protein and casein (p<0.05, p<0.01; p<0.001). The highest pH value of milk (6.56) was found in goats fed with feed mixture that contained 16% of crude protein (CP). The lactation stage significantly influenced almost all milk production and quality indicators, with the exception of milk latent. Daily milk yield and milk urea concentration during lactation show a similar trend, and the ratio of milk fat and protein the opposite trend. The peek of lactation production goats reached on 120th day of lactation (4.27 kg), followed by a gradual drop in milk yield. Highest milk urea concetration (41.94–48.55 mg/100 mL) was established between 120th and 180th day of lactation. The highest pH value of milk (6.56) was established in the middle and the titration acidity (7.15°SH) at the end of lactation. The highest (p<0.001) geometric mean value of the number of somatic cells in milk (BSS) was established in the early stage of lactation (724x103/ mL), in April (794x103/ mL), while BSS in the rest of the lactation was similar. The results showed a significant difference in certain lactations in daily milk yield and most of the milk components, except ratio of protein, casein and non protein nytrogen (NPN). Thus, the highest daily milk production and milk urea concentration is found in 4th lactation. By contrast, the highest average levels of total solids and milk fat were found in milk of goats in 1st lactation. With the increase in the order of lactation, a gradual decrease in the milk fever point (-0.5368ºC in 1st lactation to -0.5578ºC in 4th lactation) was established. The best hygienic quality of milk was found in goats in the 2nd and 3rd lactation. In milk of three-breasted goat, a richer chemical composition was found as well as a significantly higher milk urea concentrations compared to single-breasted goat. Average rennet coagulation time (r) of goat milk was 7.97 min, curd-firming time (k20) was 3.59 min, while the curd firmness (a30) was 19.50 mm. The ratio of crude protein (CP) in feed mixture did not significantly affect the milk coagulation properties. Changes in the milk coagulation properties, depending on the stage of lactation, were determined only for the curd firmness. Positive coefficients of correlation were determined between the milk urea concentration and the: daily milk yield (r=0.34; p<0.001), daily produced protein (r=0.30, p<0.001), crude protein ratio in feed mixture (r=0.50; p<0.001) and milk titration acidity (r=0.15; p<0.01), while for all other indicators of milk production and quality, negative correlation coefficients were determined. The daily milk yield as well as most chemical composition parameters, changed significantly depending on the milk urea concentration. At urea concentrations of 40.01-45.00 mg/100 mL there was a significant increase in daily milk yield and protein yield, while protein and milk fat content decreased significantly in comparison with the previous classes. Since no statistically significant increase in daily milk yield and protein yield was observed at urea concentration >45 mg/100 mL, as well as a significant decrease in milk fat and protein content compared to the previous urea class (40-45 mg/100 mL), from point of production efficiency it can be concluded that the optimal concentration of urea in Alpine goat milk is about 40 mg/100 ml. Goats with milk urea concentrations higher than 40.00 mg/100 mL were fed in the same proportion (19.92% versus 24.9%) of feed mixture with 16% CP and 18% CP, hence, it can be concluded that more crude protein in feed mixture will not result in higher production and better milk quality, and may have a negative effect on the fertility of goats and the environment as well as on the viability of production.