Results tested by the contrast method Dagnelie, 1994. In

O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 45 Table 2 a Composition and estimated nutritive value of diets in trial 2 Medium PDIE content Low PDIE content PDIN deficit 5 No PDIN PDIN deficit 5 PDIN deficit 5 21 10 g kg DM 21 21 deficit 10 g kg DM 20 g kg DM Composition of the DM Corn silage 77.3 83.3 72.8 87.4 Wheat straw 3.2 – – – Soybean meal 17.9 11.5 5.0 11.0 Formaldehyde treated mixed meal – 3.5 6.4 – Sugarbeet pulp – – 14.2 – Minerals 1.6 1.7 1.6 1.6 21 Chemical composition g kg DM CP 139 126 111 109 CF 171 169 173 174 ADF 198 197 173 207 NDF 389 388 339 407 Ash 64 63 63 61 b DT 0.65 0.63 0.57 0.67 a Nutritive value kg DM c UFL 0.90 0.91 0.92 0.91 d PDIN 94 86 77 72 d PDIE 96 95 97 84 e PDIA 45 44 43 33 a Calculated from the analyses of forage and concentrates. b DT: theoretical degradability in the rumen of the proteins of the total mixed ration. c UFL: feed unit for lactation net energy. d PDIN, PDIE: Protein digestible in the small intestine when rumen fermentable nitrogen or energy, respectively, are limiting. e PDIA: By-pass protein digestible in the small intestine. 21 for the last 2 weeks of each experimental period rumen-degradable PDIN deficit of 10 or 20 g kg DM, all three with a medium PDIE level. were tested for each measured variable. The effects of the metabolizable protein supply level and of the magnitude of rumen-degradable PDIN deficit were

3. Results tested by the contrast method Dagnelie, 1994. In

trial 1, the PDIE supply level was studied by 3.1. Effect of PDIE level comparing the diets with a high or medium PDIE content, both with a rumen-degradable PDIN deficit. 3.1.1. Trial 1 The rumen-degradable PDIN deficit was studied by DMI was significantly higher for the diet with the comparing the diets with balanced rumen ferment- 21 high PDIE level 1 0.8 kg DM day , P , 0.05 able energy and N or a rumen-degradable PDIN 21 Table 3. Consequently net energy intake 1 0.6 deficit of 10 g kg DM, both with a medium PDIE 21 21 UFL cow day , P , 0.05 and PDI intake in- level. In trial 2, the PDIE supply level was studied creased by as much 1 286 g PDIE and 1 299 g by comparing the diets with a medium or low PDIE 21 21 PDIN cow day , P , 0.01. The average daily content, both with a deficit rumen-degradable PDIN 21 gain of the cows was positive for all treatments and deficit of 10 g kg DM. The level of rumen-degrad- not significantly affected by the PDIE supply level able PDIN was studied by comparing the diets with Table 3. balanced rumen fermentable energy and N and a 46 O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 Table 3 Effect of PDIE level of the diet on intake, energy or nitrogen requirements covered and N utilization all the diets were calculated to have a 21 PDIN deficit of 10 g kg DM Trial 1 Trial 2 High Medium Significance S.E. Medium Low Significance S.E. PDIE PDIE of PDIE PDIE PDIE of PDIE level level level level level level 21 DMI kg day 21.3 20.5 , 0.05 1.0 20.2 19.2 , 0.05 1.1 a 21 Net energy intake UFL day 19.2 18.6 , 0.05 0.9 18.7 17.8 0.01 1.3 21 N intake g day 470 411 , 0.01 22 412 341 , 0.01 27 b 21 PDIE intake g day 2300 2014 , 0.01 107 1939 1643 , 0.01 131 b 21 PDIN intake g day 2087 1788 , 0.01 97 1757 1417 , 0.01 114 21 Average daily gain g day 287 471 NS 483 357 107 , 0.10 436 Energy supplies requirements 101 101 NS 5 111 108 0.10 6 PDIE supplies requirements 124 112 , 0.01 4 118 104 , 0.01 6 PDIN supplies requirements 112 100 , 0.01 4 107 90 , 0.01 6 Milk protein N yield N intake 29.9 32.5 , 0.01 1.2 31.9 36.4 , 0.01 2.9 Milk NPN yield N intake 1.84 1.86 NS 0.14 1.50 1.61 , 0.05 0.19 a UFL: feed unit for lactation net energy. b PDIE, PDIN: Protein digestible in the small intestine when rumen fermentable energy or nitrogen, respectively, are limiting. Milk yield increased significantly for the cows fed Total N, protein N and casein N contents in the the diet with the high PDIE level 1 1.3 milk were not significantly affected by the type of 21 21 kg cow day , P , 0.01, but the PDIE level diet. Soluble N and NPN contents increased sig- affected neither milk fat content, nor milk true nificantly for the diet with the high PDIE level 21 protein content. Consequently, fat and protein yields 1 38 and 1 22 mg l , respectively, P , 0.01, as increased significantly for the diet with the high did the proportion of NPN in total N 5.6 com- 21 PDIE level 1 40 g day , P , 0.10 and 1 42 pared to 5.2, P , 0.01 Table 4. 21 g day , P , 0.01, respectively Table 4. The percentage of cow energy requirements cov- Table 4 21 Effect of PDIE level of the diet on milk yield and composition all the diets were calculated to have a PDIN deficit of 10 g kg DM Trial 1 Trial 2 High Medium Significance S.E. Medium Low Significance S.E. PDIE PDIE of PDIE PDIE PDIE of PDIE level level level level level level 21 Milk yield kg day 29.3 28.0 , 0.01 1.3 24.9 23.5 0.05 2.2 21 Fat corrected milk kg day 30.3 29.2 , 0.05 1.6 25.8 24.7 NS 2.5 21 Fat content g kg 42.3 42.6 NS 1.7 41.4 43.3 , 0.05 2.1 21 Fat yield g day 1232 1192 , 0.10 72 1056 1023 NS 109 21 True protein content g kg 32.1 32.1 NS 0.7 32.2 32.2 NS 1.0 21 Protein yield g day 942 900 , 0.01 41 796 751 0.05 74 21 Total N content g l 5.11 5.06 NS 1.16 5.35 5.31 NS 206 21 Protein N content g l 4.82 4.80 NS 1.17 5.11 5.02 NS 302 21 Casein N content g l 4.09 4.08 NS 1.13 – – – – 21 Soluble N content g l 1.02 0.98 , 0.01 0.33 – – – – 21 NPN content mg l 286 264 , 0.01 12 246 233 NS 27 NPN N total 5.6 5.2 , 0.01 0.3 4.6 4.4 NS 0.5 21 Urea content mg l – – – – 182 160 NS 47 O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 47 ered was similar for the two diets and close to 100 PDIE level 107 but some supplies were clearly Table 3. The percentages of PDI requirements shown to be below the requirements for the low covered increased significantly P , 0.01 for the PDIE level 90. diet with the high PDIE level. Some supplies of The ratio of milk protein N yield to N intake PDIE were clearly shown to be higher than the increased significantly for the diet with the low PDIE requirements for the two diets 124 and 112 for the level 36.4 compared to 31.9, P , 0.01. The ratio high and medium PDIE levels, respectively. PDIN of milk NPN yield to N intake also increased supplies were particularly in excess for the diet with significantly for the diet with the low PDIE level the high PDIE level 112 and 100, respectively. 1.61 compared to 1.50, P , 0.05 Table 3. The ratio of N output in milk proteins to N intake increased significantly for the diet with the medium 3.2. Effect of difference between rumen-degradable PDIE level 32.5 compared to 29.9, P , 0.01, but energy and N the ratio of milk NPN yield to N intake was similar for the two diets Table 3. 3.2.1. Trial 1 3.1.2. Trial 2 Dry matter, net energy and PDIE intake were not DMI increased significantly for the diet with the significantly affected by the difference between 21 medium PDIE level 1 1.0 kg DM day , P , rumen-degradable energy and N of the diets Table 0.05. Consequently net energy intake increased 5. N and PDIN intake were significantly higher for 21 21 1 0.9 UFL cow day , P , 0.01 and PDI intake the diet with balanced supplies of rumen-degradable 21 21 increased by as much for the diet with the medium energy and N 1 73 and 1 249 g cow day , PDIE level 1 296 g PDIE and 1 340 g respectively, P , 0.01. 21 21 PDIN cow day , P , 0.01 Table 3. Average Neither milk yield, milk composition fat and true daily gain for the cows was positive. It tended to be protein contents, fat or protein yields, nor average greater for those fed the diet with the medium PDIE daily gain of the cows were significantly affected by 21 level 1 250 g day , P , 0.10. the rumen-degradable PDIN deficit of the diets Milk yield increased for the cows fed the diet with Table 6. Milk protein N and milk casein N contents 21 21 the medium PDIE level 1 1.4 kg cow day , P , were not significantly different for the two diets. But 21 0.05 and milk fat content decreased 21.9 g kg , total N, soluble N and NPN contents in the milk, as P , 0.05. Consequently fat yield and fat-corrected well as the proportion of NPN in total N, increased milk were not significantly different for the two diets significantly for the diet with balanced supplies of Table 4. The PDIE level of the diet did not rumen-degradable energy and N 1 109, 1 68 and 21 21 significantly affect milk protein content, but protein 1 53 mg cow day and 1 0.9, respectively, yield was significantly greater for the diet with the P , 0.01. 21 medium PDIE level 1 45 g day , P , 0.05. The percentage of cow energy requirements cov- The PDIE level of the diet did not significantly ered was close to 100 for the two diets and the affect total N, protein N, NPN or urea contents in the percentage of PDIE requirements covered was close milk, even if NPN and milk urea contents increased to 110 Table 5. The percentage of PDIN require- slightly for the diet with the medium PDIE level ments covered showed some supplies which were in 21 1 13 and 1 22 mg l , respectively Table 4. excess for the balanced diet while it decreased Energy supplies were considerably over 100 of significantly for the diet with a rumen-degradable requirements for the two diets Table 3. The PDIN deficit P , 0.01. percentages of PDI requirements covered increased The ratio of N output in the milk proteins to N significantly P , 0.01 for the diet with the medium intake decreased significantly for the balanced diet PDIE level: For PDIE, supplies were in excess for 28.6 compared to 32.5, P , 0.01. Moreover, the the medium PDIE level and close to 100 of ratio of milk NPN yield to N intake increased requirements for the low PDIE level. For PDIN, the significantly for the balanced diet 1.94 compared to percentage covered was positive for the medium 1.86, P , 0.05, Table 5. 48 O . Colin -Schoellen et al . Livestock Production Science 67 2000 41 – 53 Table 5 Effect of the difference between rumen-degradable energy and N of the diet on intake, energy or nitrogen requirements covered and N utilization all the diets were calculated to 21 have a medium PDIE content 95–98 g kg DM Trial 1 Trial 2 21 21 21 Difference Difference Significance S.E. Difference Difference Difference 0 vs. 10 g kg 0 vs. 20 g kg 10 vs. 20 g kg S.E. 5 0 5 10 g of rumen 5 0 5 10 g 5 20 g DM of PDIN DM of PDIN DM of PDIN 21 21 21 PDIN kg DM degradable PDIN kg PDIN kg deficit deficit deficit PDIN deficit DM DM 21 DMI kg day 21.0 20.6 NS 1.0 20.3 20.2 19.1 NS 0.01 0.01 1.1 a 21 Net energy intake UFL day 18.8 18.6 NS 0.9 18.5 18.7 17.8 NS , 0.10 , 0.05 1.3 21 N intake g day 484 411 , 0.01 22 454 412 346 , 0.01 , 0.01 , 0.01 27 b 21 PDIE intake g day 2058 2014 NS 107 1964 1939 1876 NS , 0.05 NS 131 b 21 PDIN intake g day 2037 1788 , 0.01 97 1924 1757 1492 , 0.01 , 0.01 , 0.01 114 21 Average daily gain g day 435 471 NS 483 315 357 101 NS , 0.10 NS 436 Energy supplies requirements 101 101 NS 5 110 111 106 NS , 0.10 , 0.05 6 PDIE supplies requirements 112 112 NS 4 118 118 118 NS NS NS 6 PDIN supplies requirements 111 100 , 0.01 4 116 107 94 , 0.01 , 0.01 , 0.01 6 Milk protein N yield N intake 28.6 32.5 , 0.01 1.2 29.6 31.9 36.2 0.01 , 0.01 , 0.01 2.9 Milk NPN yield N intake 1.94 1.86 , 0.10 0.14 1.57 1.50 1.53 NS NS NS 0.19 a UFL: feed unit for lactation net energy. b PDIE, PDIN: Protein digestible in the small intestine when rumen fermentable energy or nitrogen, respectively, are limiting. O . Colin -Schoellen et al . Livestock Production Science 67 2000 41 – 53 49 Table 6 Effect of the difference between rumen-degradable energy and N of the diet on milk yield and composition all the diets were calculated to have a medium PDIE content 95–98 21 g kg DM Trial 1 Trial 2 21 21 21 Difference Difference Significance S.E. Difference Difference Difference 0 vs. 10 g kg 0 vs. 20 g kg 10 vs. 20 g kg S.E. 21 21 21 5 0 5 10 g kg of rumen 5 0 5 10 g kg 5 20 g kg DM of PDIN DM of PDIN DM of PDIN DM degradable DM DM deficit deficit deficit PDIN deficit 21 Milk yield kg day 28.6 28.0 NS 1.3 25.0 24.9 24.4 NS NS NS 2.2 21 Fat corrected milk kg day 29.3 29.2 NS 1.6 25.7 25.8 25.2 NS NS NS 2.5 21 Fat content g kg 41.8 42.6 NS 1.7 41.1 41.4 44.0 NS , 0.10 NS 2.1 21 Fat yield g day 1187 1192 NS 72 1048 1056 1031 NS NS NS 109 21 True protein content g kg 32.3 32.1 NS 0.7 33.7 32.2 31.5 NS , 0.01 , 0.05 1.0 21 Protein yield g day 919 900 NS 41 811 796 764 NS , 0.05 NS 74 21 Total N content g l 5.17 5.06 , 0.01 1.16 5.52 5.35 5.20 , 0.01 , 0.01 , 0.05 206 21 Protein N content g l 4.86 4.80 NS 1.17 5.24 5.11 4.98 NS , 0.01 NS 302 21 Casein N content g l 4.12 4.08 NS 1.13 – – – – – – – 21 Soluble N content g l 1.05 0.98 , 0.01 0.33 – – – – – – – 21 NPN content mg l 317 264 , 0.01 12 286 246 218 , 0.01 , 0.01 , 0.01 27 NPN N total 6.2 5.2 , 0.01 0.3 5.2 4.6 4.2 , 0.01 , 0.01 , 0.01 0.5 21 Urea content mg l – – – – 270 182 116 , 0.01 , 0.01 , 0.01 47 50 O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 3.2.2. Trial 2 decreased significantly P , 0.01 as the difference 21 A rumen-degradable PDIN deficit of 10 g kg between rumen-degradable N and energy increased DM did not significantly affect dry matter intake. Table 6. Milk protein N also decreased as the But DMI decreased significantly for the diet with a deficit of rumen-degradable PDIN increased but the 21 rumen-degradable PDIN deficit of 20 g kg DM difference was only significant between the diet with 21 21 21 21.2 kg DM cow day compared to the bal- a rumen-degradable PDIN deficit of 20 g kg DM 21 anced diet. Consequently, the net energy intake and the balanced diet 2256 mg l , P , 0.01. decreased for the diet with a rumen-degradable PDIN The percentages of cow energy and PDIE require- 21 21 21 deficit of 20 g kg DM 20.7 UFL cow day , ments covered were positive and similar between the P , 0.10, Table 5. The PDIE intake was not differ- three diets Table 5. The percentage of PDIN ent between the diets with a difference between requirements covered showed some supplies in ex- rumen-degradable N and energy of 0 or 10 g PDIN cess for the balanced diet and the diet with a rumen- 21 kg DM, but decreased significantly for the diet with degradable PDIN deficit of 10 g kg DM, but the 21 a rumen-degradable PDIN deficit of 20 g kg DM supplies of PDIN did not cover the requirements of when compared to the balanced diet 288 g the cows for the diet with a rumen-degradable PDIN 21 21 21 PDIE cow day , P , 0.05. As expected, PDIN deficit of 20 g kg DM Table 5. intake increased significantly for the diet with a The ratio of milk protein N yield to N intake 21 rumen-degradable PDIN deficit of 10 g kg DM increased significantly with the increased deficit of compared to the diet with a rumen-degradable PDIN rumen-degradable PDIN 29.6, 31.9 and 36.2, 21 deficit of 20 g kg DM 1 265 g respectively, for a difference between rumen-degrad- 21 21 21 PDIN cow day , P , 0.01, and for the balanced able N and energy of 0, 10 and 20 g PDIN kg DM, diet compared to the two unbalanced diets 167 and P , 0.01. The ratio of milk NPN yield to N intake 432 g PDIN more, respectively, than diets with a was not affected by the rumen-degradable PDIN 21 rumen-degradable PDIN deficit of 10 and 20 g kg deficit Table 5. DM, P , 0.01. The average daily gain of the cows was positive. It was lower for the cows fed the diet 21 with a rumen-degradable PDIN deficit of 20 g kg 4. Discussion 21 21 DM 2214 g day , NS and 2 256 g day , P , 0.10 compared to the balanced diet and the diet with 4.1. Effect of the PDIE level 21 a rumen-degradable PDIN deficit of 10 g kg DM. Milk yield was not significantly affected by the The effect of the PDIE level was studied by difference between rumen-degradable N and energy. comparing each of the diets with a difference Milk fat content tended to increase for the diet with a between rumen-degradable N and energy of 10 g 21 21 rumen-degradable PDIN deficit of 20 g kg DM PDIN kg DM. Using a diet with a similar com- 21 compared to the balanced diet 1 2.9 g kg , P , position for the two trials 95 g PDIE and 85 g 21 0.10 Table 6, but fat yield and fat-corrected milk PDIN kg DM made it possible to discuss the were not significantly different between the three effects of the three PDIE levels of the diets tested diets. Milk true protein content decreased signifi- during the two trials: 84, 95–98 and 108 g 21 cantly for the diet with a rumen-degradable PDIN PDIE kg DM. DMI increased with the PDIE level 21 deficit of 20 g kg DM and the difference was of the diet. The increase was slightly greater between 21 21 particularly great for the balanced diet 22.2 g kg , 84 and 95 g PDIE kg DM 1 1.0 kg 21 21 P , 0.01. Consequently, milk protein yield de- DM cow day than between 98 and 108 g 21 creased significantly for the diet with a rumen-de- PDIE kg DM 1 0.8 kg DM. These results were 21 gradable PDIN deficit of 20 g kg DM 47 consistent with those of Oldham and Emmans 1988 21 21 g cow day lower than for balanced diet, P , and Sutton 1989. They have been confirmed more 0.05 Table 6. recently by Sutton et al. 1996, Weigel et al. 1997 ´ ´ Total N, NPN and urea contents in the milk, as and Verite and Delaby 1998, however, several well as the proportion of NPN in milk nitrogen, authors have expressed some doubts. Kaim et al. O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 51 21 1987 considered that the effect of CP content in the significant decrease in fat content 21.9 g kg diet on intake may depend on the composition of the which, however, did not vary between the medium diet: in diets with a high proportion of concentrate and high PDIE levels. Consequently, fat yield was 85, the increase in CP content may led to a not significantly affected in the first case but in- decrease in intake. Hof et al. 1994, using conven- creased significantly in the second. At the same time, tional diets, did not show any variation in intake average daily gain increased significantly in the first when the supplies of intestinal digestible protein case and varied slightly in the second. These results varied between 80 and 120 of animal requirements. may be due to the different orientation of the lipid Milk yield increased significantly with the PDIE metabolism. level of the diet. This increase was similar, on the As was the case for true protein content, total N, one hand, between the low and medium levels and, protein N and casein N contents in the milk were not on the other, between the medium and high levels affected by the PDIE content of the diet. NPN and 21 21 1 1.4 and 1.3 kg day cow , respectively. Most milk urea contents increased with the increase in the authors, using conventional diets, De Peters and PDIE content of the diet, but this increase was only Cant, 1992; Hof et al., 1994; Delaby et al., 1995, significant between the medium and high PDIE 1996; Moorby et al., 1996; Sutton et al., 1996 have supply levels. This increase in milk NPN content observed the same results, even if the milk yield with the increase in the CP content of the diet was ´ ´ difference was not always significant. Verite and also observed by Baker et al. 1995 and Sutton et al. Delaby 1998 showed that milk yield increases with 1996. These authors noted that a higher NPN the PDIE level of the diet and follows the rule of content could mainly be explained by an increase in decreasing yields. This increase reached 1.2 kg of milk urea content. Delaby et al. 1995 also observed milk for 10 g of PDIE for supplies of approximately an increase in milk urea content when, at pasture, a 21 95 g PDIE kg DM. Only Christensen et al. 1993 cereal concentrate was replaced by a formaldehyde- and Weigel et al. 1997, using total mixed rations, treated meal. Variations in milk urea content may be did not observe any variation in milk yield with an the result of a lack of balance between the supply of increase in CP content of the diet, but their trials fermentable nitrogen and energy available in the were conducted with only four and five cows, rumen on the one hand, and or of the lack of balance respectively. This higher milk yield may be ex- between CP or metabolizable protein supplies and ´ plained by higher energy supplies due to the increase animal requirements on the other Verite et al., 1995; of the dry matter intake 1 0.9 and 1 0.6 Broderick and Clayton, 1997; Hof et al., 1997; 21 21 ´ ´ UFL cow day , respectively. The utilization of Faverdin and Verite, 1998. In our trial, the increase the additional energy for milk production was close in the PDIE content of the diet led to an increase in 21 to the acceptable value 2.3 kg milk UFL : it was the percentage of PDIN requirements covered from 21 1.6 kg of milk UFL for the first increase level and 90 to 107 between the low and medium levels and 21 2.2 kg UFL for the second. from 100 to 112 between the medium and high The milk true protein content was not affected by levels. The increase in milk NPN or urea contents the PDIE content of the diet. However, due to the could doubtlessly be a result of higher N excretion, a milk yield increase, protein yield was significantly consequence of an excess supply of N in the diet. higher for the two PDIE content increase levels in For the first PDIE increase level, the smaller increase the diet. These results were consistent with those of in milk NPN content indicated a better utilization of Hof et al. 1994, Delaby et al. 1995,1996, Moorby N supplies. In fact, the higher N output in milk ´ ´ et al. 1996, Sutton et al. 1996 and Verite and proteins proportional to N intake was greater be- Delaby 1998. These authors also observed an tween the medium and low PDIE supply levels increase in protein yield, whether due to an increase 1 4.5 than between the high and medium PDIE in milk protein content or otherwise. supply levels 1 2.6. The ratio of urea N to NPN The results concerning the milk fat content were was 32 and 35, respectively for the low and more variable. The increase in the PDIE content of medium PDIE level. This slight difference between the diet between the low and medium levels led to a the two diets corroborated the better utilization of N 52 O . Colin-Schoellen et al. Livestock Production Science 67 2000 41 –53 21 supplies for low PDIE levels and therefore less N balanced diet, between 136 and 127 g N day for waste in urea. the diet with a rumen-degradable PDIN deficit of 10 21 21 g kg DM, and between 108 and 97 g N day for 4.2. Effect of difference between rumen-degradable the diet with a rumen-degradable PDIN deficit of 20 21 energy and N g kg DM. So, the increased difference between rumen-degradable N and energy led to a significant In the two trials, the difference between rumen- decrease in N excretion in dairy cows. 21 degradable N and energy of 10 g PDIN kg DM For the three diets with a difference between 21 i.e. 11 g PDIN UFL did not significantly affect rumen-degradable N and energy of 0, 10 and 20 g 21 either DMI, milk yield, milk composition, nor aver- PDIN kg DM, the ratio of urea N to NPN was 44, age daily gain. On the other hand, the increase in this 35 and 25, respectively. So, the proportion of urea 21 difference to 20 g PDIN kg DM led to a decrease in milk NPN decreased as the deficit of rumen- in DMI, milk protein content, and average daily gain degradable PDIN increased. These results clearly of the cows, and to an increase in milk fat content. show the importance of N recycling in the rumen ´ ´ So, the ability of dairy cows to balance a rumen Verite et al., 1987 to maintain performance for degradable PDIN deficit with regard to PDIE seemed diets with a rumen-degradable PDIN deficit. These to be limited. The acceptable deficit would be results supported N utilization in the diet, which was 21 doubtlessly between 10 and 20 g PDIN kg DM. better for the diets with a rumen-degradable PDIN 21 For values of up to 10 g PDIN kg DM of deficit deficit. The ratio of N output in milk proteins to N i.e. 9–14 of PDIE supplies, the correction of this intake increased significantly for the diet with a 21 deficit did not cause an improvement in the rearing rumen-degradable PDIN deficit of 20 g kg DM ´ ´ performances of the cows. Verite and Peyraud 1988 with regard to the other two diets and for the diet 21 considered that, for lactating cows, this deficit should with a rumen-degradable PDIN deficit of 10 g kg 21 ´ ´ not exceed 8 g PDIN UFL . Verite and Delaby DM with regard to the balanced diet. The greater the 1998 confirmed that a rumen-degradable PDIN difference between rumen-degradable N and energy, deficit of 8 was perfectly acceptable and that the the greater the increase in N fixation in milk proteins performances of the cows were not changed by proportional to nitrogen intake. correcting this deficit. But the increase in the differ- ence between rumen-degradable N and energy to 20 21 g PDIN kg DM was too great and could not be

5. Conclusion